TUTORIAL 3.0 -PERIODIC TABLE

TUTORIAL 3.0

ANSWER QUESTION NO 4 AND 5

4. Removal and addition of electron results in change of atomic radii.
Explain the difference in radius between the ions and their respective
neutral atoms.

Species Na Na+ Cl Cl-
0.095 0.099 0.181
Radius/nm 0.156

• The number of protons is the same (11 protons).

• The number of electrons in Na+ ion is10 and in Na
atom is 11, but Na+ has less one shell (1s2 2s2 2p6)
compared to Na (1s22s22p6 3s1), screening effect of
Na+ ion reduces.

• Therefore the attraction between the nucleus and
the outermost (valence ) electrons in Na+ is
stronger.

• Size of Na+ ion is smaller than Na atom.

• The number of protons is the same (17 protons).

• The number of electrons in Cl- ion is 18 (1s2 2s2
2p6 3s2 3p6) and in Cl atom is 17 (1s22s22p6 3s2 3p6 )

• The electron-electron repulsions caused the
electrons to spread out and attraction between
valence electron and nucleus became weaker

• Size of Cl- ion is bigger than Cl atom.

5. Which ion has the smaller radius? Explain
(a) Mg2+ or Na+
(b) Na+ or O2-
(c) Cl- or Na+

a) Zeff = proton no – inner electron =12 - 2 = 10
Zeff = proton no – inner electron =11 - 2 = 9
12Mg2+: 1s2 2s2 2p6
11Na+ : 1s2 2s2 2p6

• Mg2+ and Na+ are isoelectronic species (same electronic configuration)

• But effective nuclear charge for Mg2+ stronger than Na+
• The attraction between the nucleus and the valence electrons in Mg2+ is

stronger than Na+.
• Size of Mg2+ is smaller than Na+

b) Zeff = proton no – inner electron =11 - 2 = 9
Zeff = proton no – inner electron = 8 - 2 = 6
11Na+ : 1s2 2s2 2p6
8O2- : 1s2 2s2 2p6

• O2- and Na+ are isoelectronic species (same electronic configuration)

• But effective nuclear charge for Na+ stronger than
• The attraction between the nucleus and the valence electrons in Na+ is

stronger than O2-.
• Size of Na+ is smaller than O2-

C)

11Na+ : 1s2 2s2 2p6
17Cl- :1s2 2s2 2p6 3s2 3p6

• Cl- has 3 electron shells high screening effect compared to Na+ has 2
electron shells

• The attraction between the nucleus and the valence electrons in Na+ is stronger
than Cl-.

• Size of Na+ is smaller than Cl-