PERIODICITY AND RADII
All elements have been arranged in the periodic table, so as to reflect their atomic
number and chemical reactivity.
Each element in the periodic table is usually written with at least the following
information:
 Symbol
 Atomic number, Z
 Molar mass, M
Elements in the same row across the periodic table are members of the same
period. Elements in the same column down the periodic table are members of the
same group.
Periodicity and Periodic Trends
Periodicity refers to the similarities in characteristics and properties of elements
based on their position in the periodic table.
Across a period e.g. 11Na to 18Ar;
 The atomic number increases (because the number of protons increases)
 The atoms are smaller (because the effective nuclear attraction on the outer
electrons increases, pulling them in closer).
On moving down a group e.g. 3Li to 11Na to 19K the number of electron levels (shells)
increases, which results in:
 The effective nuclear attraction on the outer electrons is weaker (the outer
electrons are further away from the nucleus, hence the pull is weaker and there is
increased shielding of the outer electrons by the inner electrons)
 The atoms are larger (the number of electron levels increases and the effective
nuclear attraction on the outer electrons is weaker).
Atomic Radius
The actual size of the atoms is calculated from the average distances between
neighbouring atoms. This is called the atomic radius.
overlapping bond electrons
Nucleus
Atomic radius of a hydrogen atom
Distance = 2 x the atomic radius
of hydrogen
Radii of metal atoms are determined in a similar way.
Ionic Radius
For monatomic ions (ions containing only one atom), the ionic radius is calculated
using measurements from ionic crystals.
Positive monatomic ions
Group 1, 2 & 3 atoms lose all their outer valence shell electron(s) as they form
positive ions. This results in one less level of electrons so these positive ions are
always considerably smaller than the atom. e.g. 11Na 0.157 nm radius,
+
11Na 0.095 nm radius.
11Na
atom
11Na
2,8,1
+
ion
2,8
Group 4 – 15 metals can form positive ions without losing all their valence electrons.
These ions are still smaller than their corresponding atoms because:
 There are fewer outer shell electrons with less repulsion between them
 The same nuclear charge attracts a smaller number of electrons
 The electrons lost first are from an orbital in the outer shell that is further away
from the nucleus than the rest of the valence electrons.
Negative Monatomic Ions
Non-metal atoms gain electrons forming negative ions that are always larger than
the original atom because of the repulsion between the outer shell electrons and the
incoming electron or electrons, and the fact that the nuclear charge is now spread
over a greater number of electrons and the attraction is therefore weakened.
e.g.
chlorine atom
17Cl
0.099 nm
2,8,7
chloride ion
17Cl
0.181 nm
2,8,8
extra electron,
more repulsion,
increased radius