Complete the table of trends in periodic properties by -

(a) Filling in the blanks with 'decreases' or 'increases' across a period & down a group.

(b) The reasons for the 'decrease' or 'increase' based on the criteria given.

Trend			Reason for trend
Periodic property	Across a period	Down a group	in periodic property
(a) Atomic size			No. of shells • Nuclear charge
(b) Ionization potential	Increases		Atomic size • Nuclear charge
(c) Electron affinity			Atomic size • Nuclear charge
(d) Electronegativity		Decreases	Atomic size • Nuclear charge
(e) Metallic character			Atomic size • Ionisation potential

(a) Periodic properties

(b) Reasons for trends in periodic property

Atomic Size

1. Across a period — Decreases
   Reason — The atomic size decreases across a period because of increase in nuclear charge. The number of shells remain same across a period.
2. Down a group — Increases
   Reason — Atomic size increases due to increase in number of shells as new shells are added with increasing atomic number. Nuclear charge also increases but number of shells dominates over nuclear charge.

Ionisation potential

1. Across a period — Increases
   Reason — The atomic radii decreases because the electrons are added to same shell. The nuclear charge increases. Hence the outer electrons are more firmly held.
2. Down a group — Decreases
   Reason — Ionisation potential decreases because atomic radii increases as number of shells increases, outermost electrons can be removed easily. Nuclear charge also increases but number of shells dominates over nuclear charge.

Electron affinity

1. Across a period — Increases
   Reason — As nuclear charge increases and atomic size decreases, the nucleus exerts a stronger pull on an added electron. Atoms are closer to achieving a full octet, so they release more energy when gaining an electron.
2. Down a group — Decreases
   Reason — Atomic size increases, so the added electron experiences less nuclear attraction due to its greater distance and inner electrons reduce the attraction between the nucleus and outer electrons. Therefore, less energy is released.

Electronegativity

1. Across a period — Increases
   Reason — Nuclear charge increases while atomic radius decreases, making it easier for atoms to attract shared electrons in a bond.
2. Down a group — Decreases
   Reason — Even though nuclear charge increases, the atomic radius becomes larger, and the outer electrons are farther from the nucleus. This reduces the nucleus's ability to attract bonding electrons.

Metallic character

1. Across a period — Decreases
   Reason — Atomic radius decreases and nuclear charge increases, so atoms hold electrons more tightly and do not lose them easily reducing metallic behavior.
2. Down a group — Increases
   Reason — Atomic radius increases, and outer electrons are farther from the nucleus and held less tightly. This makes it easier for atoms to lose electrons, which is a typical property of metals.