In Period 3 of the Periodic Table, the 3s and 3p orbitals are filling with electrons. Just as a
reminder, the shortened versions of the electronic structures for the eight elements are:
Na [Ne] 3s1
Mg [Ne] 3s2
Al [Ne] 3s2,3px1
Trends in Atomic Radius
We know that the number of shells in all the elements of a given periods remains the
same but the value of effective nuclear charge, increases from left to right. The increased
effective nuclear charge pulls the electron cloud of the atom nearer to the nucleus and thus the
size of the atoms and ions goes on decreasing from left to right. Thus in going from left to right
in a period of s-and p-block elements atomic and ionic radi decrease with the increase of atomic
number. This fact can be illustrated by considering the atomic (covalent) and ionic radii of the
elements as shown below.
Electro negativity is a measure of the tendency of an atom to attract a bonding pair of electrons. The Pauling scale is the most commonly used. Fluorine (the most electronegative element) is assigned a value of 4.0, and values range down to caesium and francium which are the least electronegative at 0.7.
The trend across Period 3 looks like this: In going from left to right in a period of s- and p-block elements, the electronegativity values increase. This increase can be explained on the basis of any of the following facts.
(i) On moving from left to right in a period, there is a decrease in the size of the atoms. Smaller atoms have greater
tendency to attract the electrons towards themselves i.e. smaller atoms have higher electronegativity values.
(ii) On moving from left to right in aperiod there is an increase of ionization energy and electron affinity of the elements. The atoms of the elements, which have higher value of ionization energies and electron affinities also have higher electro negativities.
The variation of electro negativity in a period and a group of representative elements (s- and p-block elements)
Notice that argon is not included. Electronegativity is about the tendency of an atom to attract a bonding pair of electrons. Since argon does not form covalent bonds, you obviously can’t assign it electronegativity.
The trend is explained in exactly the same way as the trend in atomic radii. As you go across the period, the bonding electrons are always in the same level – the 3-level. They are always being screened by the same inner electrons.
All that differs is the number of protons in the nucleus. As you go from sodium to chlorine, the number of protons steadily increases and so attracts the bonding pair more closely.