Q.(a) What are the Isotopes of Hydrogen? [1]

(a) Hydrogen has three isotopes: protium, deuterium, tritium. (b) Hydrogen resembles alkali metals (loses its 1 electron to form H+) and also resembles halogens (needs 1 more electron to complete its shell, forms H-, exists as diatomic H2) — this dual behaviour is why its position in the periodic table is considered anomalous/debated.

(a) Isotopes of hydrogen:

Hydrogen has three isotopes, all with 1 proton but differing numbers of neutrons:

1. Protium (1H, or ¹H): 1 proton, 0 neutrons — the most common (~99.98%) form.
2. Deuterium (2H or D): 1 proton, 1 neutron (mass number 2).
3. Tritium (3H or T): 1 proton, 2 neutrons (mass number 3); it is radioactive.

(b) Position of hydrogen in the periodic table:

Hydrogen has the electronic configuration 1s1 — just a single electron in its only shell. This gives it an unusual, dual character that makes its placement in the periodic table debated:

• Resemblance to alkali metals (Group 1): Like Li, Na, K etc. (ns1 configuration), hydrogen has one electron in its outermost (and only) shell. It can lose this one electron to form a unipositive H+ ion, just as alkali metals lose their one valence electron to form M+ ions. It also combines with halogens and forms compounds similar in formula to alkali metal halides (HCl vs NaCl). For this reason hydrogen is conventionally placed at the top of Group 1. …