Q 01/05

What does oxygen's atomic number, 8, actually count?

A) Neutrons in its nucleus

B) Protons in its nucleus

C) Shells of electrons

D) Its stable isotopes

Answer · why

B) Protons in its nucleus

Protons in its nucleus: eight of them make an atom oxygen, whatever else it carries. Add or remove a single one and it becomes a different element ent...

Q 02/05

Roughly what fraction of Earth's atmosphere is oxygen gas, by volume?

A) About 2%

B) About 7%

C) About 21%

D) About 70%

Answer · why

C) About 21%

About 21% goes in with every breath, and most of it comes straight back out: exhaled air still holds around 16% oxygen, which is why mouth-to-mouth re...

Q 03/05

Oxygen is best known as a gas, but where is it the most abundant element of all?

A) Earth's core

B) The Sun

C) Earth's crust

D) Jupiter

Answer · why

C) Earth's crust

Earth's crust is nearly half oxygen by weight, locked up with silicon in rock and sand. The air and oceans together hold only a tiny share of the plan...

Q 04/05

Oxygen is the universe's third most abundant element; which is second?

A) Hydrogen

B) Helium

C) Carbon

D) Nitrogen

Answer · why

B) Helium

Helium and hydrogen were made in the Big Bang itself. Oxygen came later, forged inside stars by fusing helium and scattered when those stars died.

Q 05/05

What is the chemical formula of ozone?

A) O2

B) O3

C) CO2

D) H2O2

Answer · why

B) O3

O3 is ozone: three oxygen atoms in one molecule. Its name comes from the Greek for 'to smell', after the sharp odour it gives the air near electrical ...