Atomic Size (25)

Atomic Size
You have seen in the previous standards that size/volume is a fundamental property of matter. The size of an atom is indicated by its radius. atomic radius is the distance between the nucleus of the atom and its outermost shell. Atomic radius is expressed in the unit picometer (pm) which is smaller than nanometer (1 pm = 10^-12 m).

Element	O	B	C	N	Be	Li
Atomic Radius (pm)	66	88	77	74	111	152

1. By referring to the modern periodic table find out the periods to which the above elements belong.

→ The above elements belong to the first period.

2. Arrange the above elements in a decreasing order of their atomic radii.

Element	Li	Be	B	C	N	O
Atomic Radius (pm)	152	111	88	77	74	66

3. Does this arrangement match with the pattern of the second period of the modern periodic table?

→ Yes, this arrangement matches with the pattern of the second period of the modern periodic table.

4. Which of the above elements have the biggest and the smallest atom?

→ Biggest Atom: Cs
→ Smallest Atom: Li

5. What is the periodic trend observed in the variation of atomic radius while going from left to right within a period?

→ While going from left to right in a period, the atomic radius goes on decreasing.

You will find that atomic radius goes on decreasing while going from left to right within a period. The reason behind this is as follows. While going from left to right within a period, the atomic number increases one by one, meaning the positive charge on the nucleus increases by one unit at a time. However, the additional electron gets added to the same outermost shell. Due to the increased nuclear charge the electrons are pulled towards the nucleus to a greater extent and thereby the size of the atom decreases.

Element	K	Na	Rb	Cs	Li
Atomic Radius (pm)	231	186	244	264	152

1. By referring to the modern periodic table find out the groups to which above the elements belong.

→ The above elements belong to Group 1.

2. Arrange the above elements vertically downwards in an increasing order of atomic radii.

Element	Atomic Radius (pm)
Li	152
Na	186
K	231
Rb	244
Cs	262

3. Does this arrangement match with the pattern of the group 1 of the modern periodic table?

→ Yes, this arrangement matches with the pattern of the group 1 of the modern periodic table.

4. Which of the above elements have the biggest and the smallest atom?

→ Biggest Atom: Cs
→ Smallest Atom: Li

5. What is the periodic trend observed in the variation of atomic radii down a group?

→ While going down in a group, the atomic radius goes on increasing.

You will find that while going down a group the atomic size goes on increasing. This is because while going down a group a new shell is added. Therefore the distance between the outermost electron and the nucleus goes on increasing. As a result of this the atomic size increases in spite of the increased nuclear charge.

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