(a) How many periods are there in the modern periodic table of elements? (b) How do atomic radius, valency and metallic character vary down a group? (c) How do the atomic size and metallic character of elements vary as we move from left to right in a period?

(a) There are 7 periods in the modern periodic table of elements.

(b) Variation down a group:

• Atomic radius: Increases down a group because new electron shells are progressively added, which increases the distance between the outermost electrons and the nucleus.
• Valency: Remains constant down a group because all elements in a group have the same number of valence electrons in their outermost shell.
• Metallic character: Increases down a group because the atomic size increases, which decreases the effective nuclear attraction on the valence electrons, making it easier for the atoms to lose electrons.

(c) Variation from left to right in a period:

• Atomic size (radius): Decreases because the nuclear charge increases (more protons are added) while the electrons are added to the same shell, pulling the outer electrons closer to the nucleus.
• Metallic character: Decreases because the effective nuclear charge increases, making it harder for atoms to lose electrons and easier to gain them.