Chemical periodicity part 4

Trend of Atomic Radius

Defined as half the internuclear distance between 2 similar atoms bonded only by a single bond.

Atomic radius decreases across the period
• Proton no./nuclear charge increases. Hence, drawing electrons closer to nucleus.
• Negligible increase in shielding effect as electrons are added to the valence shell
(no. of inner electron shells remain constant)

Trend of Ionic Radius

Defined as the radius of a spherical ion in an ionic compound.

Na, Mg, Al and Si

• Positive ions decreases from Na+, Mg2+, Al3+ to Si4+
• Nuclear charge increases as no. of proton increases
- Equal no. of electrons
- Negligible increase in screening effect as equal number of electron shells
- Hence, attraction force between the nucleus and the electrons in Si4+ ion is stronger than
that in Na+

P, S and Cl

Negative ions radius decreases from P, S to Cl
• Nuclear charge increases as no. of proton increases
- Equal no. of electrons
- Negligible increase in screening effect as equal no. of electron shells
- Therefore, the attraction force between the nucleus and the electrons in CL ion is
stronger than that in P ion.

Comparison between ionic radius and atomic radius

Size of positive ions : size of parent atoms

• Positive ions are smaller than parent atoms because:
- Cations are formed when a neutral atom loses electon/s. Thus cations have lesser
electrons than parent atoms.
- Cations and parent atoms have equal number of protons.
- nuclear charge is therefore shared among less no of electrons in cations.
- Hence the attraction force between the nucleus and the remaining electrons in cations
is stronger than that of parent atoms.


Size of negative ions : size of parent atoms

• Negative ions are larger than parent atoms
- Anions are formed when a neutral atom gains electron/s. Thus anions have more
electrons than parent atom.
- Anions and parent atoms have equal number of protons.
- Nuclear charge is therefore shared among more no of electrons in cations.
- Hence the attraction force between the nucleus and the remaining electrons in
cations is weaker than that of parent atoms.


Chemical formula of how these elements exist:

Na
Mg
Al
Si
P4
S8
Cl2