JEE Chemistry Classification of Elements and Periodicity in Properties Notes Set 03

Atomic and Ionic Size

1. Across a period the atomic radius decreases because the nuclear charge rises while electrons enter the same shell. Down a group it increases because new shells are added.
2. Effective nuclear charge Zeff = Z - sigma, where sigma is the screening (shielding) constant. Inner electrons shield well, and the order of shielding power is s > p > d > f.
3. A cation is smaller than its parent atom and an anion is larger. For isoelectronic species, the greater the nuclear charge, the smaller the radius, so N3- > O2- > F- > Na+ > Mg2+ > Al3+.
4. In the d-block the radius first falls then stays nearly constant, and elements of the 5th and 6th periods in a group (such as Zr and Hf) are about equal because of the lanthanide contraction.

Ionisation Enthalpy

The ionisation enthalpy is the energy needed to remove the outermost electron from an isolated gaseous atom.

1. It rises across a period and falls down a group. Successive values always rise: IE1 < IE2 < IE3 since each electron leaves a more positively charged ion.
2. A sudden large jump after the nth value shows that the element has n valence electrons.
3. Exceptions come from stable subshells: Be > B (filled 2s), N > O (half filled 2p), Mg > Al and P > S.
4. Second period order of IE1: Li < B < Be < C < O < N < F < Ne. In the 3rd period the first values for Si, P, S, Cl are 786, 1012, 999 and 1256 kJ per mole.

Electron Gain Enthalpy and Electronegativity

1. Electron gain enthalpy is the energy change when a gaseous atom takes up an electron. It is usually negative and becomes more negative across a period. Chlorine has the highest value, higher than fluorine because the small fluorine atom has strong electron repulsion.
2. Noble gases, Be, Mg and N have values near zero or positive because of stable configurations. The second electron gain is always endothermic since the anion repels the added electron.
3. Electronegativity is the tendency of a bonded atom to attract the shared pair. It rises across a period and falls down a group, with fluorine highest (4.0 on the Pauling scale).
4. A bond between atoms with a larger electronegativity difference has more ionic character, and bond length of A-B is often shorter than the sum of covalent radii when the difference is large.

Other Trends

1. Metallic character falls across a period and rises down a group, while non-metallic character shows the opposite trend.
2. Oxides change from basic on the left (Na2O, MgO) through amphoteric (Al2O3, ZnO) to acidic on the right (SO3, Cl2O7).
3. Diagonal relationships exist between Li and Mg, Be and Al, and B and Si because of similar charge to size ratio.
4. Valency of representative elements is the number of valence electrons or eight minus that number, whichever is smaller.