| period | Element(atmic radius in pm) |
|---|---|
| 2nd period | Li(152),Be(111),b(88),C(77),N(74),O(66),F(64) |
| 3rd period | Na (186), Mg (160), Al (143), Si (117), P(110), S(104), Cl(99) |
Atomic number of sodium (Na) is 11. Therefore sodium (Na) atom contains 11 protons and 11 electrons with outer electron as 3s1. On the other hand Na+ ion has 11 protons but only 10 electrons. The 3s shell of Na+ level has no electron in it. Hence its outer shell configuration is 2s22p6. As proton number is more than electrons in it, the nucleus of Na+ ion attracts outer shell electrons with strong nuclear force. As a result the Na+ ion shrinks in size. Therefore, the size of Na+ ion is less than ‘Na’ atom. In general the positive ion (cation) of an element has less size than its neutral atom Eg. Na (157pm) Na+ (98pm); K (203pm) K+(133pm); Mg (136pm) Mg2+(65pm); Al (125pm) Al3++(50pm) for neutral atoms covalent radiai are give.
Consider another example:Electronic configuration of chlorine (Cl) atom is 1s2 2s2 2p6 3s2 3p5 and the electronic configuration of chloride (Cl-)ion is 1s2 2s2 2p6 3s2 3p5 Both chlorine and chloride ion have 17 protons each but there are 17 electrons in chlorine atom, where as 18 electrons in chloride ion. Therefore, the nuclear attraction is less in Cl-ion when compared with chlorine atom.Therefore the size of the chlorine (Cl) atom is less size than Cl-)ion. In general negative ion (anion) of an element has bigger size than its neutral atom.
Chloride ion of Cl (99pm) Cl-(181pm); F (64pm) F-(133pm); O(73pm) O2- (140pm); N (75pm) N3- (171pm);