Across
1
Gas essential for breathing
5
Group of similar cells
6
Nerve cell in the brain
7
Compound containing oxygen
11
Substance with pH below 7
Down
2
Unit of energy
3
Biological catalyst
4
Particle of light
8
Icy body with a tail near the sun
9
Second lightest element
10
Naturally occurring inorganic solid
12
Unit of force