Halogens & Noble Gases

Groups 17 and 18 sit on the right-hand side of the periodic table and represent two extremes of chemical behaviour. The halogens are among the most reactive non-metals, eager to gain an electron and complete their outer shell. The noble gases, by contrast, are famously unreactive — their outer electron shells are already full.

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Group 17: The Halogens

The name "halogen" comes from the Greek hals (salt) and gennan (to produce) — meaning "salt-formers". These elements react with metals to form ionic salts.

Element	Symbol	Atomic Number	State at 25 °C	Colour
Fluorine	F	9	Gas	Pale yellow
Chlorine	Cl	17	Gas	Yellow-green
Bromine	Br	35	Liquid	Reddish-brown
Iodine	I	53	Solid	Dark purple/grey
Astatine	At	85	Solid (predicted)	Unknown (radioactive)
Tennessine	Ts	117	Unknown	Synthetic, superheavy

Key Properties