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Chemistry 9th Class Chemistry

Chapter 3 Periodic Table 9th Class Chemistry Notes

Chapter 3 Periodic Table 9th Class Chemistry Notes

Question#1: Define Periodic table? What is the importance of periodic table?

Answer: Periodic Table:- Periodic table is the arrangement of elements in order of increasing atomic number in such a way that those with similar properties are placed one above other in the form of a table.

OR

Periodic table is the arrangement of elements in the form of a table in which elements with similar properties are placed in the same group.

Importance of periodic table: – The periodic table is the road map to elements. It tells us about atomic numbers, atomic masses, electronegativity, chemical properties as well as the relationship among various elements.

Question#2) Write a note on old forms of periodic table?      Or

Write a note on Dobereiner, Newland and Mendeleev’s periodic tables?

Dobereiners Triads: – A German chemist Doberneiner observed the relationship between atomic masses of several groups of three elements in which the central or middle element had atomic mass average of the other two elements.

Periodic Table 9th Class

Newlands Octaves; –  In 1984, Newland put forward his observations in the form of law of octaves. According to this law “there is a repetition in chemical properties of every eight element if they are arranged by increasing their atomic masses”.

 

Li      Be      B      C      N      O      F

7       9       11     12      14     16      19

 

Na     Mg      Al     Si      P       S      Cl

23      24      27     28      31      32     35.5

 

The properties of Sodium (Na) are similar to those of Lithium (Li). Similarly the properties of Chlorine ( Cl ) are similar to Fluorine(F).

Drawbacks of Newlands law of octaves:- The law of octaves failed because of two reasons.

  1. No space was left for undiscovered elements.
  2. At that time noble gases were also not known.

 

Mendeleev’s periodic table;- In 1869, a Russian chemist Mendeleev for the first time introduced the name periodic table. He arranged the known 63 elements in order of their increasing atomic masses. The vertical columns were called groups and the horizontal rows were called periods.

Elements having same properties are placed in the same group. This periodic table got the name Mendeleev’s periodic table due to name of presenter scientist and was based on Mendeleev’s law, this law states.

“Properties of elements are periodic functions of their atomic masses”.

Mendeleev Periodic Table

Merits of Mendeleev’s periodic table:-

 

Merits:-
1. Systematic arrangements of 63 elements in groups and periods.
2. Corrected doubtful masses of some elements like Be, Pd, Pt etc.
3. Predicted properties of elements which were not discovered -like scandium, gallium, germanium, which was corrected and left gaps for them.

Demerits of Mendeleev’s periodic table:-



  1. Controversial position of hydrogen.
    2. Position of isotopes was not correct.
    3. Two similar elements like Cu & Hg were grouped differently.
    4. Co with higher atomic no. was placed before lower atomic no. Ni which was contradicting arrangement of element in increasing atomic no.
    5. Did not provide space for noble gases which were discovered later.
  • No space left for isotopes.

 

Question#3) Explain Moseley’s periodic table?

Answer;- MODEREN PERIODIC TABLE    OR

LONG FORM OF PERIODIC TABLE        OR

MOSELEY’S PERIODIC TABLE:

In 1913 Henry Moseley discovered a new property of elements, the atomic number. He states that atomic number determines the position of elements. He arranged elements based on their increasing atomic number in periodic table.

Modern Periodic Table

Modern periodic law; – “properties of elements are periodic functions of their atomic number”.

According to Moseley atomic number is basic property of an element due to two aspects;

  • Atomic number increases regularly from element to element.
  • It is fixed for each and every element.

 

 

 

 

 

 Salient features of long form of periodic table;

1) This table consists of seven horizontal rows called periods.

  • First period consists of only two elements and is called short period.
  • Second and third period consists of 8 elements each and are called normal periods.
  • Fourth and fifth period consists of 18 elements each and are called long periods.
  • Sixth period has 32 elements and is called very long or longest period.
  • Seventh period has 23 elements and is incomplete. This is also called very long period.
  • Elements of a period show different properties.
  • Elements of a period show same number of shells.
  • There are 18 vertical columns in the periodic table called groups.
  • These groups are numbered 1 to 18 from left to right.
  • Elements of a group show same chemical properties due to same electronic configuration.
  • Elements are classified into four blocks depending upon the type of sub-shell that receives last electrons.

 

Question#4) Classify modern periodic table into groups, periods and Blocks?

Answer:-

Periods in Modern PERIODIC table; – The horizontal rows in periodic table are called periods. Atomic number in a period continuously increases. There are seven periods in modern periodic table.

  • First period or short period:- first period is called short period. This period consists of only two elements named Hydrogen(H) and Helium(He).
  • Second period or normal period:-it consists of 8 elements, named Lithium(Li), Beryllium(Be), Boron(B) , Carbon(C), Nitrogen(N) ,Oxygen (O), Fluorine(F), and Neon (N).
  • Third period: – this period is also called Normal period. It consists of 8 elements , sodium (Na) , Magnesium(M), Aluminum(Al) , Silicon(Si) , Phosphorous(P) , Sculpture or sulfur(S), Chlorine (Cl) and Argon (Ar).
  • Longs periods; –Fourth and fifth periods are called long periods, each consists of 18 elements.
  • Very long periods;-6th and 7th periods are called very long periods. 6th period contain 32 elements, while seventh period contain 23 elements and is incomplete period.
  • Lanthanides; – the series that starts after Lanthanum is called Lanthanide series. It contains 14 elements and is present in f block.
  • Actinides; – The series that starts after Actinium is called actinide series. It contains 14 elements and is present in f blocks.

 

 

Groups in modern periodic table;- There are 18 vertical columns in the periodic table called groups.

These groups are numbered 1 to 18 from left to right.

Elements of a group show same chemical properties due to same electronic configuration due to presence of same number of valence electrons.

GROUP 1 or Alkali metals;- it consists of hydrogen, lithium, sodium, potassium, rubidium, cesium and francium. All have 1 electron in outermost shell and thus all have general electronic configuration nS1..

Group 2 or Alkaline earth metals:- Group 2 consists of Beryllium, Magnesium, Calcium, Strontium, Barium and Radium. All OF them have 2 electrons in their valence shells having electronic configuration ns2.

Group 3 to 12 (Transition metals) :- This group is also called d-block elements. In these elements last electrons enters into d-sub-shell, so named d block elements.

Group 13 or Boron family: The boron family contains the semi-metal boron (B) and metals aluminum (Al), gallium (Ga),indium (In), and thallium (Tl). All of them have 3 electrons in valence shell and have general electronic configuration ns2, np1.

 

Group 14 Elements or Carbon family:- The carbon family, Group 14 in the p-block, contains carbon (C), silicon (Si), germanium(Ge), tin (Sn), lead (Pb), and flerovium (Fl). Each of these has 4 electrons in last shell. Each of these elements has only two electrons in its outermost p orbital: each has the electron configuration ns2np2.

 

 Group 15 Elements The nitrogen family:- includes the following compounds: nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). All of them have 5 electrons in outermost shells and so have general electronic configuration ns2np3. .

 

GROUP 16 The oxygen family,:-  also called the chalcogens, consists of the elements found in Group 16 of the periodic table and is considered among the main group elements. It consists of the elements oxygensulfur, seleniumtellurium and polonium. They have 6 electrons in outermost shell and have general electronic configuration ns2 ,np4

 

  Group 17 or halogen family:-  The Halogens are, Fluorine, Chlorine, Bromine, Iodine and Astatine. All of them have 7 electrons in outer most shell having general electronic configuration ns2, np5

GROUP 18 OR ZERO GROUP OR NOBLE ELEMENTS OR NOBLE GASES:-

All noble gases have the maximum number of electrons possible in their outer shell (2 for Helium, 8 for all others), making them stable. Helium, Neon, Argon, Krypton, Xenon and Radon. They all have general electronic configuration of ns2, np6..

POSITION OF HYDROGEN AND HELIUM IN PERIODIC TABLE;-

Hydrogen (H) and helium (He) are special elements. Hydrogen can have the electron traits of two groups: one and seven. For chemists, hydrogen is sometimes missing an electron like the members of group IA, and sometimes has an extra one as in group VIIA. When you study acids and bases you will regularly work with hydrogen cations (H+). A hydride is a hydrogen anion and has an extra electron (H-).

Helium (He) is different from all of the other elements. It is very stable with only two electrons in its outer orbital (valence shell). Even though it only has two, it is still grouped with the noble gases that have eight electrons in their outermost orbitals. The noble gases and helium are all “happy,” because their valence shell is full.

 

 

 

Blocks of modern periodic table:- Modern periodic table is divided into four blocks depending on type of sub shell in which the last electron present. Based on completion of sub shells, elements with similar sub shell electronic configuration are referred as a block of elements. There are four blocks in modern periodic table.

 

  • S Block Elements; –those elements in which last electrons is present in S sub shell are included in s block. Group 1 and 2 elements are present in this block.
  • P Block Elements:-those elements in which last electrons present in p sub shell are called p block elements. This block contains group 13 to 18 elements, except Helium.
  • d Block elements:-this group include all those elements that have last electrons in their outermost shell. Group 3 to 12 elements are present in this block.
  • f Block elements:- the elements in which last electron present in f sub shell are called f block elements. This block includes Lanthanide and Actinide series ad is present at the bottom of periodic table.

 

Question#5) Write a short note on periodic properties?

Answer:- periodic properties;- periodic properties of elements are those properties of elements that undergoes gradual decrease or increase from left to right in a period or from top to bottom in a group in periodic table. These properties include: atomic size, atomic radius, shielding effect, ionization energy, electron affinity, electronegativity, etc.

 

Question#6) write a short note on atomic radius and atomic size?

Answer;- Atomic Radius;- The half of the distance between nuclei of two adjacent same atoms is called atomic radius.

Or Atomic radius is generally stated as being the total distance from an atom’s nucleus to the outermost orbital of electron.

Atomic size: – the distance between the nuclei of two adjacent same atoms is called atomic size.

Example: – The distance between nuclei of two adjacent carbon atoms in its elemental form is 154 pm (Pico meter). It is called atomic size of carbon atom . Its half that is 77 pm is radius of carbon atom.

 

Trends in periodic table;-

 

Variation in a period; – From left to right in a period the atomic sizes gradually decreases because of the following reasons;

  • Increase in the effective nuclear charge.
  • Additions of electrons in the same shell.

 

Variation in a group: – From top to bottom in group atomic sizes increases due to following reasons;

  • Addition of new shells.
  • Decrease in effective nuclear charge.

 

Question#7) defines shielding effect and its trends in periodic table?

Answer; SHIELDING EFFECT:-Shielding effect or screening effect is the decrease in effective nuclear charge force of attraction of nucleus) on an electron that is caused by the repulsive forces of other electrons between it and nucleus.

 

Zeff = Z-S.

In the equation Zeff shows the effective nuclear charge.

Z Shows atomic number.

S Shows the number of inner electrons.

For example: for chlorine Zeff = 17-10 = 7.

While that for Sulfur is Zeff = 16-10 =6.

Variation in a period; in a period from left to right, the shielding effect decreases because of following reason;

♦ Electrons added to same outer shell. That’s why removal of electrons from sulphur is easy than chlorine.

 

Variation in a group:

From top to bottom in a group shielding effect increases due to following reason;

♦ The increase in inner electrons that repel the outer electrons by greater forces.

That is why it is easy to remove an electron from potassium (Z=19) than sodium (Z=11).

 

Question#8) what is Ionization energy? Write its trends in periodic table?

Answer;- Ionization Energy;- The amount of energy required to remove an electron from the valence shell of an isolated gaseous atom in its ground state is called ionization energy or ionization potential.

 

Na (g) + energy ———–> Na+ + 1e- .   ΔH = +496 KJ/.Mole

 

Unit; unit of ionization energy is kj/mol.

 

 

Explanation;- Electrons in an atom are held by electrostatic forces of attraction, that’s why for the removal of electrons energy is needed. Such type of energy is called ionization energy.

 

First Ionization Energy;- the energy required to remove first electron from an isolated gaseous atom in its ground state is called first ionization energy.

Na →Na+ +1e.   +496kj/mol

 

Second Ionization Energy;- the energy required to remove electron from an isolated mono-positive gaseous ion (M+) is called second ionization energy.

Example;

Mg → Mg++1e   1st I.E= +738KJ/MOL.

Mg+→ Mg+2+1e      2nd I.E = +1450KJ/MOL.

It is to be noted that first ionization < second ionization energy <Third ionization energy <Fourth ionization energy.

I1 <I2 <I3 <I4

 

Trends in periodic table:-

 

Variation in a period:-The Ionization energy value increases from left to right in a period. When we moves from left to right in a period;

  • Nuclear charge increases.
  • Electrons added or enter into same shell.

 

Variation in a group: – Ionization energy value decreases from top to bottom in a group. This is because of;

  • Nuclear charge decreases as shielding effect increases.
  • Atomic size increases thus hold of nucleus on valence electrons decreases.

 

Question#9) Write a short note on electron affinity?

Electron Affinity;- The amount of energy released when an electron is added to the valence shell of an isolated gaseous atom is called electron affinity.

Example; chlorine + 1e- → Cl    E.A= -349kj/mole.

 

Trends in periodic table:-

Variation of electron affinity in group:- the value of electron affinity of elements decreases down the group. This is due to following reasons.

  • The atomic size increases.
  • The nuclear charge decreases.

 

Variation in a period: – from left to right in a period electron affinity increases because of:

  • Increase in effective nuclear charge.
  • Size of the atoms decreases.

 

Question# 10) Define electronegativity? What is the trend of electronegativity in periodic table?

Answer: – Electronegativity:- The ability of an atom to attract the shared pair of electrons toward itself is called electronegativity. The value of electronegativity of elements described the ability of elements to compete for electrons. The most electronegative element in periodic table is Fluorine having electronegativity value of 4. The less electronegative element in periodic table is Cesium having electronegativity value of 0.7.

Example: – In HCl molecule the shared electron pair lies more towards Cl atom because Cl is more electronegative.

H  **Cl

 

Trends in Periodic table:-

Variation in a group: – Electronegativity value decreases from top to bottom in a group. This is because of the increase in size and decrease in nuclear charges.

 

Variation in a period: – Electronegativity value increases from left to right in a period, this is due to increase in nuclear charges and less shielding effect.

 

EXERCISE

 

Short Answers Questions.

 

Question#1) why noble gases are not reactive?

Answer: – Noble gases have their complete octet. They are stable and undergo no reactions.

Question#2) why cesium (atomic number 55) requires little energy to release its one electron present in the outermost shell?

Answer: – Cesium has less nuclear charge and larger size so attractive forces between nucleus and outer electrons are less. That’s why cesium requires little energy to release its one electron.

 

Question#3) how is periodicity of properties dependent upon number of protons in an atom?

Answer: – Number of protons increases in a period regularly, thus nuclear charge also increases in the same way and that’s why properties of atoms changes by the increase of protons.

 

Question#4) why shielding effect of electrons makes cation formation easy?

Answer: – Shielding effect decreases the attractive forces between nucleus and outer shell electrons. It means that it helps in removal of electrons. By the removal of electrons positive ions or cations are formed so shielding effect of electrons makes cation formation easy.

 

Question#5) what is the difference between Mendeleev’s periodic law and modern periodic law?

Answer:-

Mendeleev’s lawModern law
1)properties of elements are periodic functions of their atomic masses.Properties of elements are periodic functions of their atomic number.
2) This law is based on atomic number.This law is based on atomic mass.
3) Atomic mass is the basic property of this law.Atomic number is the basic property of this law.

 

 

Question#6) what do you mean by groups and periods in periodic table?

Answer: – The horizontal rows in periodic table are called periods while vertical columns in periodic table are called groups.

 

Question#7:- Why and how are elements arranged in fourth period?

Answer:- 8 elements are present in 4th period named sodium(Na), magnesium (Mg), aluminum(Al), silicon(Si), phosphorous (P), sulphur(S), chlorine(Cl) and argon(Ar) .

These all have 4 shells. These elements are arranged based on increasing atomic number.

Question#8) why the size of atoms does not decrease regularly in a period?

Answer: – with the addition of proton in nucleus of atom , there is also increase of one electron occur from left to right in a period. Due to this size of atoms does not decrease regularly in a period.

Question#9:- Give the trend of ionization energy in a period?

Answer:

Variation in a period:-The Ionization energy value increases from left to right in a period. When we moves from left to right in a period;

1) Nuclear charge increases.

2) Electrons added or enter into same shell.

 

LONG ANSWER QUESTIONS

 

Question#1. Explain the contributions of Mendeleev for the arrangement of elements in his Periodic Table?

Answer: Please see answer of question number 2.

Question#2. Show why in a ‘period’ the size of an atom decreases if one moves from left to right?

Answer: In a period from left to right the size of an atom gradually decreases because of following reasons.

(a) Increase in the effective nuclear charge: Due to increase in nuclear charge, the electrons in the shells are attracted at greater force by the nucleus. Due to this the distance between nuclei and outermost shell decreases. And that is why size of the atom also decreases.

(b) Additions of electrons in the same shell: Due to addition of electrons in the same shell electrostatic forces between the electrons and nucleus increases. And it is also responsible for the decrease in size of the atom.

Question#3. Describe the trends of electronegativity in a period and in a group.

Answer: Please see answer of question number 10.

Question#4. Discuss the important features of modern Periodic Table.

Answer: Please see answer of question number 3.

Question#5. What do you mean by blocks in a periodic table and why elements were placed in blocks?

Answer: Please see answer of question number 4.

Question#6. Discuss in detail the periods in Periodic Table?

Answer: Please see answer of question number 4.

Question#7. Why and how elements are arranged in a Periodic Table?

Answer: Reason for arrangement of elements in the periodic table   “OR”  Why elements are arranged in the periodic table : There are more than 100 elements discovered and research is in progress to discovered more elements. Properties of each elements are defficult to study. So it is the basic reason for arrangement of elements in the periodic table.

Method of arrangement of elements in the periodic table  “OR”  How elements are arranged in a Periodic Table: The tabular arrangement of elements according to atomic number as based on periodic law is called periodic table. So elements are arranged in periodic table according to increasing atomic number from left to right in horizontal rows.

When elements are arranged on this method, properties of elements were found repeating after regular interval. such that elements with similar properties and similar electronic configuration are placed in the same group. It was observed that after every eighth element ninth element had similar properties to the first element. It is due to the same number of electrons in the valence shell. For example: Sodium with atomic number 11 had similar propertties to Lithium with atomic number 3. It is due to the same number of 1 electron in the valence shell, and so both are placed in the same group. In a period an increase in atomic number each for each element from left to right shows a decrease in metallic character.

Question#8. What is ionization energy? Describe its trend in the Periodic Table?

Answer: Please see answer of question number 8.

Question#9. Define electron affinity, why it increases in a period and decreases in a group in the Periodic Table.

Answer: Please see answer of question number 9.

Question#10) Justify the statement, bigger size atoms have more shielding effect thus low ionization energy?

Answer: Bigger size atoms have more shielding and low ionization energy: Shielding effect is the decrease in attractive forces between the nucleus and outer electrons. Shielding effect increase with the increase of size of the atoms. It is because that with the addition of more inner shells and inner electrons the attractive forces between nucleus and outer electrons decreases. It means that the electrons present in the filled energy levels screen or shield the attraction of nucleus felt by the valence shell electrons. This shielding effect increases from top to bottom in a group because of increase in size of atom.

On the other hand Ionization energy which is the energy required to remove loosely bound electron from the valence shell of an isolated gaseous atom decreases with the increase of size and increase in shielding effect. because of decreases in effective nuclear charge, electron removal becomes easy. In a period the shielding effect decreases from left to right. Because of same number of inner electrons and increase in valence shell electrons and protons in the nucleus. This causes an increase in the effective nuclear charge and thus size of atoms decreases. Due to small size, low screening effect and high effective charge, the electron removal becomes difficult and thus ionization energy increases.

 

SOLVED TEST YOURSELVES

 

TEST YOURSELF 3.1

Question#1. What was the contribution of Dobereiner towards classification of elements?

Answer: Dobereiner arranged the elements into several groups of three elements called triads.

Question#2. How Newlands arranged the elements?

Answer: Newlands introduced the law of octaves. He noted that there was a repetition in properties of every eighth elements, if they were arranged on the based on their increasing atomic masses.

Question#3. Who introduced the name Periodic Table ?

Answer: The name periodic table was first introduced by Russian chemist Mendeleev.

Question#4. Why the improvement in Mendeleev’s periodic table was made?

Answer: Mendeleev periodic table was based on atomic masses. It was found that atomic number is more basic property than atomic mass because it is fixed for each and every element and it increases regularly. No two elements have the same atomic number, so improvement were made in Mendeleev,s periodic table and elements were arranged according to their increasing atomic number.

Question#5. State Mendeleev’s periodic law?

Answer: Mendeleev,s periodic law can be defined as : “ Properties of the elements are the periodic functions of their atomic masses”.

Question#6. Why and how elements are arranged in a period?

Answer: Elements are arranged in periods on the basis of their increasing atomic number. It was found that atomic number is fixed for each and every element, that’s why elements are arranged according to their increasing atomic number.

 

TEST YOURSELF 3.2

Question#1. How the properties of elements repeat after regular intervals?

Answer; When elements are arranged according to their increasing atomic number from left to to right in horizontal rows or periods. Properties of elements were found repeating after regular intervals. So that elements with similar properties and similar electronic configuration are placed in the same group.

Question#2. In which pattern modern periodic table was arranged?

Answer: In modern periodic table elements were arranged based on their increasing atomic number.

Question#3. How many elements are in first period and what are their names and symbols?

Answer: There are two elements in first period. Their names are Hydrogen and Helium.

Question#4. How many elements are placed in 4th period?

Answer: There are 18 elements in 4th period.

Question#5. From which element lanthanide series starts?

Answer: Lanthanide series starts from Lanthanum (La) with atomic number 57 so named Lanthanide.

Question#6. From which period actinides series starts?

Answer: The actinide series starts from period number 7. The first member is Actinium with atomic number 89, so named as actinide series.

Question#7. How many elements are in 3rd period, write their names and symbols?

Answer: The 3rd period consists of 8 elements. Names and their symbols are as follows:

Sodium (Na)Magnesium (Mg)Aluminum (Al)Silicon (Si)Phosphorus (P)Sulphur (S)Chlorine (Cl)Argon (Ar)

 

Question#8. How many periods are considered normal periods ?

Answer: Two periods, second and third periods are called normal periods.

Question#9. What do you mean by a group in a periodic table?

Answer: The vertical columns in the periodic table are called groups.

Question#10. What is the reason of arranging elements in a group?

Answer: The reason for arranging elements in the groups is same number of valence electrons and similar electronic configuration. Due to same electronic configuration elements in a group have same chemical properties.

Question#11. What do you mean by periodic function?

Answer: The repetition of properties of elements after regular intervals in the periodic table is called periodic function.

Question#12. Why the elements are called s or p block elements?

Answer: The elements of group 1 and 2 have their valence electrons in S subshell so they are called S block elements, while elements of group 13 to 14 have their valence electrons in P sub shell so they are called P block elements.

Question#13. Write down the names of elements of group 1 with their symbols?

Answer: Group consists of 7 elements named Hydrogen (H) , Lithium (Li) , Sodium (Na) , Potassium (K) , Rubidium (Rb) , Cesium (Cs) and Francium (Fr).

 

Question#14. How many members are in group 17, is there any liquid, what is its name ?

Answer: There are 5 members in group 17 named Fluorine (F) , Chlorine (Cl) , Bromine (Br) , Iodine (I) and Astatine (At). Bromine is a liquid in this group.

 

 

TEST YOURSELF 3.3

 

Question#1. Define atomic radius?

Answer:  Atomic Radius;- The half of the distance between nuclei of two adjacent same atoms is called atomic radius.

Or Atomic radius is generally stated as being the total distance from an atom’s nucleus to the outermost orbital of electron.

Question#2. What are SI units of atomic radius?

Answer: The SI unit of atomic radius is picometer (pm)

Question#3. Why the size of atoms decreases in a period?

Answer: The size of atoms in a period from left to right decreases due to addition of protons in the nucleus that results in the increase of effective nuclear charge. The addition of electrons in the same shell is also a main factor responsible for the decrease of size of atoms. As a result of these two forces, i,e increase in effective nuclear charge and addition of electrons in the same shells leads to contraction or decrease in size of the atoms.

Question#4. Define ionization energy.

Answer: Ionization Energy;- The amount of energy required to remove an electron from the valence shell of an isolated gaseous atom in its ground state is called ionization energy or ionization potential.

 

Na (g) + energy ———–> Na+ + 1e- .   ΔH = +496 KJ/.Mole

 

Unit; unit of ionization energy is kj/mol.

Question#5. Why the 2nd ionization energy of an elements is higher than first one?

Answer: With the removal of first electron, force of attraction between nucleus and remaining  outermost shells electrons increases so for the removal of second electron more energy is required and that’s why second ionization energy is always higher than the first ionization energy.

Question#6. What is the trend of ionization energy in a group?

Answer: Trend of Ionization energy in a group : – Ionization energy value decreases from top to bottom in a group. This is because of;

1) Nuclear charge decreases as shielding effect increases.

2) Atomic size increases thus hold of nucleus on valence electrons decreases.

Question#7. Why the ionization energy of sodium is less than that of magnesium?

Answer: Magnesium has 12 protons in its nucleus. It attracts the outermost electrons more tightly than sodium which has 11 protons in its nucleus. So electron removal from sodium is easy as compared to magnesium and so ionization energy of sodium is less than that of magnesium.

Question#8. Why is it difficult to remove an electron from halogens?

Answer: It is difficult to remove an electron from halogens, because halogens being non metals have high electronegativity , high effective nuclear charge and small sizes.

Question#9. What is shielding effect?

Answer: SHIELDING EFFECT:-Shielding effect or screening effect is the decrease in effective nuclear charge force of attraction of nucleus) on an electron that is caused by the repulsive forces of other electrons between it and nucleus.

Question#10. How does shielding effect decrease the forces of electrostatic attractions between nucleus and outer most electrons?

Answer: The inner electrons repel the outer electrons and thus reduced the attractive forces between nucleus and outer electrons. As a result the valence electrons experiences less nuclear charge than that of the actual.

 

Question#11. Why does the bigger size atoms have more shielding effect?

Answer: Bigger size atoms have more inner electrons to reduce nuclear charge so they have more shielding effect.

 

Question#12. Why does the trend of electron affinity and electronegativity is same in a period?

Answer: Electron affinity and electronegativity has same trends in a period because both depend on the same factor that is attraction of electrons by the nucleus.

Read Also 9th Class Chemistry All Chapter Notes

Question#13. Which element has the highest electronegativity?

Answer: Fluorine has highest electronegativity of 4.

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