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.
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.
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).
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.
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.
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.
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.
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.