Across
1
Particle of light
4
Biological catalyst
5
Substance with pH below 7
7
Center of an atom or cell
9
The Red Planet
10
Molten rock beneath Earth surface
11
Mass per unit volume
Down
2
Solid with repeating atomic pattern
3
Hottest planet in solar system
5
Smallest unit of an element
6
Second lightest element
8
Element found in all living things
12
Unit of energy