Question

Define atomic size. Give its unit of measurement. In the modem periodic table what trend is observed in the atomic radius in a group and a period and why is it so?

Answer 1

Step 1: Definition of atomic size:

• Atomic size of any atom is the distance from the nucleus of that atom to its outermost shell.
• It is generally measured in angstroms ($\stackrel{\circ }{A}$).

Step 2: Trend in the Period:

• Atomic size decreases on moving left to right in the Period or row in the periodic table. The reason for the decrease in size is due to increase in the effective or net nuclear charge due to addition of electrons in the same energy level. Due to the increase in nuclear charge felt by valence electrons, the valence electrons move towards the nucleus and size decreases.
• Example, Carbon (C) and Fluorine (F) belong to Period 2 of the periodic table.
• In Period 2, Fluorine (F) is comparatively present on the right side of the periodic table. Since atomic size decreases on moving from left to right in the Period, Fluorine (F) has a lower atomic size than Carbon (C).

Step 3: Trend in the Group:

• The number of shells and effective nuclear charge both increase on moving from top to bottom in the Group. Due to a more significant increase in the number of shells, the distance between the outermost shell and nucleus increases, and therefore, the atomic size increases from top to bottom in any Group.
• Example, Fluorine (F) and Iodine (I) belong to Group 17 of the periodic table.
• Fluorine lies above Iodine (I) in Group 17. Since atomic size increases down the Group, the atomic size of Fluorine (F) is lower than Iodine (I).