Hydrogen sits in group 1 but behaves like a halogen too, so where does it actually belong? Also which isotope is radioactive?

I've been colour coding the Hydrogen chapter and the very first heading has me stuck. Hydrogen's configuration is $1s^1$ so it looks like an alkali metal, it loses one electron to give $H^+$ , and it's placed in group 1. But it's also a diatomic gas like the halogens, needs one electron to complete the 1s shell, and forms $H^-$ in hydrides like NaH. So which group is it really? Is it just placed in group 1 "for convenience"?

I wrote down the numbers I could find. First ionisation enthalpy of hydrogen 1312 kJ/mol, Li is 520, Na 496, K 419. Electron gain enthalpy of hydrogen about −73 kJ/mol, chlorine about −349. These don't look like group 1 at all. So why is it there?

Second thing, isotopes. My notes say protium, deuterium, tritium, and then I got one mark deducted in a test for writing "deuterium is radioactive". Which one is? And how many neutrons does each have, I always lose track.

Also, do protons really float around free as $H^+$ in water or is that a simplification?

6 answers & replies

[+]Arjun Nair12thSample· 2mo ago

Your numbers are the answer. 1312 kJ/mol is way above every alkali metal, even Li (520), and it's closer to the halogens (Cl is about 1250). Hydrogen's electron is tightly held because it has no inner shells to shield it. Alkali metals have a low IE because the valence electron is shielded and far away.

So the honest answer: hydrogen resembles alkali metals in having $1s^1$ and forming $H^+$ , and resembles halogens in being diatomic, gaining an electron to form $H^-$ , and having IE and EA values in the same ballpark. It is in group 1 by convention because of the electronic configuration. But it's a unique element, and NCERT says it should be treated separately. In an exam I'd say "it resembles both and so doesn't fit anywhere perfectly".

[+][–]Ananya Reddy11thSample· 2mo agoAuthor
ok "unique element, treat separately" is exactly the sentence I'll use. Thank you arjun. Also tritium noted in red.

[+]Lakshmi TripathiDropperSample· 2mo ago

Isotopes table I keep in my notebook:

Name	Symbol	Neutrons	Radioactive?
Protium	$^1_1H$	0	No
Deuterium	$^2_1H$ (D)	1	No
Tritium	$^3_1H$ (T)	2	Yes, beta emitter

Protium is almost all of natural hydrogen (about 99.98%). Tritium has a half-life of 12.33 years. Neutron number = mass number minus 1, since Z is 1 for all.

[+]Sara Fernandes11thSample· 2mo ago

Maybe both deuterium and tritium are radioactive since they are the heavy ones and the nucleus is unstable with extra neutrons? That was my logic.

[+][–]Tanvi Deshmukh12thSample· 2mo ago
Sara, heavy doesn't mean unstable. Deuterium (one proton + one neutron) is a perfectly stable nucleus, it's even in heavy water. Tritium with two neutrons is the one that is unstable, and it decays by beta emission. Only tritium is radioactive among hydrogen isotopes, so the answer to "which is radioactive" is always tritium.

[+]Kavya NaikDropperSample· 2mo ago

Your last question: no, a bare proton doesn't float around in water. A proton is extremely small, so its charge density is huge, and it grabs a lone pair from a water molecule right away to form the hydronium ion, $H_3O^+$ (and further hydrated forms). Writing $H^+(aq)$ is shorthand. Same reason you won't find free $H^+$ in the lab, it only exists attached to something.

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