Define the following terms: (i) Valency (ii) Atomic size. How do the valency and the atomic size of the elements vary while going from left to right along a period in the modern periodic table?

(a) (i) Valency: Valency is the combining capacity of an atom. It is the number of electrons an atom can lose, gain, or share to achieve a stable electron configuration (usually a full outermost shell, like the noble gases). For example, the valency of sodium (Na) is 1 because it readily loses one electron to form a stable ion (Na+). Similarly, chlorine (Cl) has a valency of 1 because it readily gains one electron to form a stable ion (Cl-).

(ii) Atomic size: Atomic size refers to the distance between the nucleus and the outermost electron shell of an atom. It can be measured as atomic radius or covalent radius, depending on the method used. Generally, atomic size increases down a group (column) in the periodic table and decreases across a period (row).

(b) Variation of valency and atomic size across a period:

As we move from left to right across a period in the modern periodic table, the number of protons in the nucleus increases, while the number of electron shells remains constant. This increased nuclear charge attracts the electrons more strongly, pulling them closer to the nucleus. This results in:

1. Decrease in atomic size: The stronger nuclear pull reduces the distance between the nucleus and the outermost electrons, resulting in a decrease in atomic size.

2. Variation in valency: Across a period, elements typically exhibit a trend in valency. Elements on the left tend to have low valencies, losing electrons to form positive ions. The valency generally increases up to a maximum value (often 4) in the middle of the period. Then, as we approach the noble gases, the valency decreases, with elements gaining electrons to form negative ions. However, the exact valency shown can be influenced by other factors, making this trend not always perfectly consistent.