The position of certain elements in the Modern Periodic Table are shown below.

Group	1	2	3 to 12	13	14	15	16	17	18
Period 1	G								H
Period 2	A			I			B		C
Period 3		D			E				F

Using the above table answer the following questions giving reasons in each case:

(i) Which element will form only covalent compounds?

(ii) Which element is a non-metal with valency 2?

(iii) Which element is a metal with valency 2?

(iv) Out of H, C and F which has largest atomic size?

(v) To which family does H, C and F belong?

Definition of Atomic Size

Atomic size (or atomic radius) is defined as the distance between the center of the nucleus of an isolated atom and its outermost shell containing electrons.

Unit of Measurement

Atomic radius is extremely small and is measured in picometres ($\text{pm}$).

$$1\text{ pm} = 10^{-12}\text{ m}$$

---

Trends in Atomic Radius in the Modern Periodic Table

#### 1. Trend in a Period (From Left to Right)

• Observation: The atomic radius decreases as we move from left to right across a period.
• Reason: This is because the nuclear charge (number of protons in the nucleus) increases progressively, while the electrons are added to the same outermost shell. This increased positive charge pulls the outermost electrons closer to the nucleus, thereby reducing the size of the atom.

#### 2. Trend in a Group (From Top to Bottom)

• Observation: The atomic radius increases as we move down a group.
• Reason: This is because new electronic shells are added at each successive step down the group. This increases the physical distance between the outermost electrons and the nucleus. Although the nuclear charge also increases, the effect of adding new shells overcomes the increased nuclear pull, resulting in an overall increase in atomic size.