Trends in the periodic table : A

3. Position of elements in the periodic table
GROUP PERIOD
•Group number indicates the number of
valence electrons.
•Elements in one group have the same
number of valence electrons
•The number of occupied shells increases
by 1 as we move down the group.
•Period number indicates the number of
of occupied energy shells.
•Elements in one period have the same
number of energy shells.
• The number of valence electrons
increases by 1 as we move across the
period from left to right.
Group 1 Elec. Conf.
H (1) 1
Li (3) 2, 1
Na (11) 2, 8, 1
K (19) 2, 8, 8,1
P1 H1
1
He 2
2
P2 Li 3
2,1
Be 4
2,2
B 5
2,3
C 6
2,4
N 7
2,5
O 8
2,6
F 9
2,7
Ne 10
2,8

4. Valency :
The valency is determined by the number of valence electrons
Group Period
In a group the number of valence electron is
same. Hence the valency of all the elements
in a group is same.
The valency increases from 1 to 4 ( group 1,
2 ,3, 4 )and then decreases to 0 (goups 5 to
8 ) across a period.
Or
For groups 1,2,13,14 –
valency = valence electrons
For groups 15 to 18—
Valency = 8 - no. of valence electrons
G1 E.C. V G2 E.C. Val
H 1 1
Li 2,1 1 Be 2,2 2
Na 2,8, 1 1 Mg 2,8,2 2
K 2,8, 8, 1 1 Ca 2,8,8,2 2
Gr. 1 2 3 4 5 6 7 8
Na Mg Al Si P S Cl Ar
EC 2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8
VE 1 2 3 4 5 6 7 8
Val 1 2 3 4 3 2 1 0

5. Atomic size :It refers to the radius of an atom i.e. the distance
between the centre of the nucleus and the outermost shell.
Group Period
The atomic size increases down the
group. This is because
New shells are added as we move down
the group.
This increases the distance between the
nucleus and outermost electrons.
The atomic radius or size decreases across
the period. This is due to
Increase in the nuclear charge which
tends to pull the electrons closer to the
nucleus , thus reducing its size.

6. Metallic & non metallic character:
Metals are electropositive in nature.( electropositive nature is determined by
the tendency to lose electrons.)
Non metals electronegative in nature. (electronegative nature is determined
by tendency to gain electrons).
Group Period
Metallic character
increases down the
group.
atomic size
increases down the
group.
Force of attraction
between the nucleus
and valence electrons
decreases.
Hence atom can
lose outermost
electrons easily and
form positive ions.
Non metallic
character decreases
down the group.
Hence tendency to
attract electrons and
form negative ions
decreases.
Metallic character
decreases across the
period.
Atomic size
decreases across the
period.
Force of attraction
between the nucleus
and valence electrons
increases.
Hence tendency to
lose electrons
and form positive ions
decreases.
Non metallic
character increases
across the period.
Hence tendency to
accept electrons and
form negative ions
also increases.