8 O

Oxygen (O)

Element 8 · Reactive nonmetal · Gas at 25 °C · Group 16, Period 2

Oxygen is the most abundant element in Earth's crust and about 21% of the atmosphere — the gas your cells use to release energy from food.

On the periodic table, oxygen sits in group 16 — the chalcogens (oxygen group) — in period 2. It is one of 7 elements classified as a reactive nonmetal, and at room temperature (25 °C) it is a gas — one of 11 elements that are gases at room temperature.

The reactive nonmetals are a diverse family that includes hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur, and selenium. They gain or share electrons in chemical reactions, forming the covalent bonds that hold molecules together. Most of the atoms in living things — including virtually all of you — belong to this category.

Properties of oxygen

Properties of Oxygen
Atomic number (Z)	8
Symbol	O
Atomic mass	15.999
Category	Reactive nonmetal
Group	16
Period	2
Block	p-block
Metal / nonmetal	Nonmetal
State at 25 °C	Gas
Electron configuration	[He] 2s2 2p4
Electronegativity (Pauling)	3.44
Melting point	-218.79 °C (54.36 K / -361.82 °F)
Boiling point	-182.95 °C (90.2 K / -297.31 °F)
Density	0.001429 g/cm³

Position in the periodic table

Oxygen shares group 16 — the chalcogens (oxygen group) with Sulfur (S), Selenium (Se), Tellurium (Te), Polonium (Po), Livermorium (Lv). Elements in the same group have the same number of outer electrons, so they tend to behave alike chemically — comparing them is one of the fastest ways to see periodic trends in action.

Within this dataset, it ranks #2 in electronegativity out of 99 elements with a pauling value.

Physical properties

Oxygen melts at -218.79 °C (54.36 K / -361.82 °F) and boils at -182.95 °C (90.2 K / -297.31 °F), giving it a liquid range of about 35.8 °C at standard pressure.

As a gas at standard conditions, its density is 0.001429 g/cm³ — measured at STP like the other gaseous elements here.