similar electronic configurations. Therefore, we expect all the elements
in a group should have similar chemical properties and there should be a
regular gradation in their physical properties from top to bottom.
Similarly, across the table, i.e., from left to right in any period elements
get an increase in the atomic number by one unit between any two successive
elements. Therefore, the electronic configuration of valence shell of any
two elements in a given period is not same. Due to this reason elements
along a period possess different chemical properties with regular gradation
in their physical properties from left to right. To understand this we will
take some properties of elements and discuss how they vary in groups and
in periods.
Properties of elements and their trends in Groups and in Periods
1. Valence:Valence (or) valency of an element was defined as the
combining power of an element with respect to hydrogen, oxygen or
indirectly any other element through hydrogen and oxygen.
Valence of an element with respect to hydrogen is the number of
hydrogen atoms with which one atom of that element chemically combines.
Valence of an element with respect to oxygen is twice the number of
oxygen atoms with which one atom of that element combines. Note that
the group number of an element gives the maximum valence of that element
as per old notation that is I to VIII groups but it is 18-group number as per
IUPAC numbering.
For example one atom of ‘Na’ chemically combines with one atom of
‘H’ to give NaH. Therefore, the valence of Na is 1. One atom of ‘Ca’
combines with one atom of ‘O’ to give CaO. So the valency of Ca is 2.
In general, the valence of an element with respect to hydrogen is its
traditional group number. If the element is in the group V or above, its
valence is 8– group number. For example, chlorine valence is 8-7 = 1.
In general, each period starts with valency 1 for 1st group elements,
increases upto 4 with respect to the group number and then decreases
from 4 to 3 to 2 to 1 to zero in the following groups (this is applicable
only for main group elements i.e., ‘s’ and ‘p’ block elements with respect
to hydrogen.
Now a days the valence of an elment is generally taken as the number
of valence shell (outer most shell) electrons in its atom. Oxidation number
concept almost is the latest subsititute to the valence concept in the modern
literature.
Atomic radius
Atomic radius of an element may be difined as the distance from the
centre of the nucleus of the atom to its outermost shell.