Chapter No 1 Periodic Classification of Elements and Periodicity | FSC Part 2 Chemistry Notes
Question. What are the improvements made in the Mendeleev’s periodic table?
Answer. 1. If the elements are arranged in ascending order of their atomic numbers, their chemical properties repeat in a periodic manner.
2. The position of isotopes has been classified. All the isotopes of an element have the same atomic number and hence only one place is required for them.
3. Addition of group VIII (Rare earth elements for noble gases or 0 groups) was another significant improvement.
4. Division of groups in two subgroups A and B facilitated the separate placement of Alkali metals, (Li, Na، k) and coinage metals (Cu, A g, A u) which were presented in the same group in Mendeleev’s periodic table.
Question. How the classification of elements in different blocks helps in understanding their chemistry?
Answer. Blocks in the Periodic Table
Elements in the periodic table can also be classified into four blocks. This classification is based upon the valence orbital of the element involved in chemical bonding. According to this classification, elements of IA and IIA subgroups are called s-block elements because their valence electrons are available in s orbital. The elements of IIIA to Vlll A subgroups (except He) are known as p-block elements as their valence electrons are present in p orbital. Similarly in transition elements, electrons in d-orbital are responsible for their valence hence they are called d-block elements. For Lanthanides and Actinides valence electrons are present in f- orbital hence these elements are called f-block elements. This classification is quite useful in understanding the chemistry of elements and predicting their properties especially the concept of valency or oxidation state.
Question. How do you justify the position of hydrogen at the top of various groups?
Answer. POSITION Of HYDROGEN IN THE PERIODIC TABLE
Hydrogen is the first element of the periodic table. It resembles with alkali metals of group I-A, elements of group IV-A, and halogens of group VII-A. This makes is position in the periodic tables and anomalous.
Position of hydrogen with Alkali Metals (Group I-A)
1. Similarities of Hydrogen with Alkali Metals
Like alkali metals hydrogen has one electron in its outermost shell electron structure of hydrogen is 1 S (compliment) and that of alkali metals is ns (compliment).
Like alkali metals hydrogen loses 1 electron to form an unipositive ion.
Like alkali metal hydrogen shows plus one oxidation state and one valency.
Position of hydrogen with Halogens(Group VII-A)
Similarities of hydrogen with halogen
Hydrogen is a gas like most of the halogens.
Hydrogen is stable in the form of H2, molecule like f2, C12, Br2 and 12, molecules of halogens.
Hydrogen like halogens requires only one electron to complete outermost shell.
Question. Why the ionic radii of negative ions are larger than the size of their parent atoms?
Answer. Sizes of Ions
Ionic size plays a crucial role in determining the structure and stability of ionic solids. For example, the sizes of ions are important in determining both the lattice energy of the solid and the way in which the ions pack in a solid. Ionic size is also a major factor governing the properties of ions in solution. For example, a small difference in ionic size is often sufficient for one metal ion to be biologically important and another not to be.
The size of an ion depends on its nuclear charge, the number of electrons it possesses, and the orbitals in which the outer-shell electrons reside. Consider first the relative sizes of an ion and its parent atom. Positive ions are formed by removing one or more electrons from the outermost region of the atom. Thus, the formation of a cation not only vacates the most spatially extended orbitals, it also decreases the total electron-electron repulsions. As a consequence, cations are smaller than their parent atoms. The opposite is true of negative ions. When electrons are added to form an anion, the increased electron-electron repulsions cause the electrons to spread out more in space. Thus, anions are larger than their parent atoms.
Question. Why ionization energy decreases down the group and increases along a period?
Answer. Variation within our group
The ionization energy decreases down the group due to.
Increase in Atomic Size (Atomic Radius)
Due to the additions of a shell in every successive member of the group, the increased distance of the outermost shell decrease the influence of the nuclear charge and results in decrease in the value of ionization energy down a group.
With the increase of inner shell and number the electrons in them reduces the influence of nuclear charge on the outermost orbit resulting decrease in attraction. So the ionization energy decreases down the group due to shielding effect of an electron donor group.
Variation Across a Period
The ionization energy increases across a period due to the fact that the atomic size or atomic radius decrease across a period from left to right. As nuclear charge increases the outer shell electrons becomes more strongly bound to the nucleus and difficult to remove which results in increase in ionic equation energy across the period.
Question. Why the second value of electron affinity of an element is usually shown with a positive sign?
Answer. Electron Affinity
The electron affinity is the energy released or absorbed, when an electron is added to a gaseous atom to form a negative ion.
F (g) +e- → F- (g) E.A= -337 kJ mol-1
Energy is usually released when electronegative elements absorb the first
electron and E.A. in such cases is expressed in negative figures,
as in the case of halogens. When a second electron is added to a
uninegative ion, the incoming electron is repelled by the already
present negative charge and energy is absorbed in this process.
O(g) + e- → O- (g) E.A1 = -141 kJ mol-1
O-(g) + e- O2- (g) E.A2 = +780 kJ mol → -1
The absorbed energy is expressed as the electron affinity in positive figures. Electron affinity depends upon size of the atom, nuclear charge and vacancies in the outermost shell. Relatively smaller atoms with one or two vacancies in the outermost shell show large values of electron affinity.
Question: Why metallic character increases from top to bottom in a group of metals?
Answer. The size of metal atom down with a group increases due to the addition of a new orbit in every next element atom. Due to the increase in atomic size it is easier to remove the electrons from the outer most orbit to form a positive metal Ion. Hence metallic character increases down the group.
Question: Explain the variation in melting points along the short periods.
Answer. Melting And Boiling Points
Variation in a Period
Across the short periods, the melting and boiling points of elements increase with the number of valence electrons upto group IVA and then decrease upto the noble gases. The melting points of group IA elements are low because each atom in them provides only one electron to form a bond with other atom. Melting points of group IIA elements are considerably higher than those of group IA elements because each atom in them provides two binding electrons.
Since carbon has the maximum number of binding electrons, thus it has a very high melting point in diamond in which each carbon is bound to four other carbon atoms. In general, the elements which exist as giant covalent structures have very high melting points
Question: Why the oxidation state of noble gases is usually zero?
Answer. Since the outer most shell of noble gases completely filled i.e. it has either one pair of electrons (duplet) all four pairs of electrons (octet). So there is no vacancy for combination or bond formation this is why noble gases Sean zero oxidation state.
Question: Why diamond is a non-conductor and graphite is fairly a good conductor?
Answer. Diamond, is allotropic form of carbon is not conductor due to its tetrahedral structure. The electrons are found it tetrahedrally, so unable to move.
In graphite, another allotropic form of carbon, one of the valence electron can move freely, so it is good conductor of electricity and used as electrodes.
Chapter No 1 Periodic Classification of Elements and Periodicity | FSC Part 2 Chemistry Notes
Choose the correct option.
(i) Mendeleev in his periodic table, arranged the elements according to their atomic____________.
(ii) Vertical columns in modern periodic table are called_________and horizontal rows are called_______.
c. group, periods
(iii) Members of group VIlA are called _______ and alkali metals is the family name of________ group members.
(iv) Metals form___________ oxides and non-metals form_________ oxides.
(v) Hydrogen can be placed above the groups_______ of the periodic table.
Fill in the blanks,
(vi) Shielding effect is actually the_______________ due to electrons in between the
nucleus and the outermost shell.
(vii) Noble gases have the____________ values of ionization energy due to their complete
(viii) When a second electron is added to a uni-negative ion, the incoming electron is
__________ by the already present negative charge.
(ix) Due to having partly filled d-orbitals _____________metals usually show variable
(x) Melting and boiling points of halogens_________down the group.
6. decrease in attention