Across
1
Unit of energy
3
Element found in all living things
6
Compound containing oxygen
8
Group of similar cells
12
Second lightest element
Down
2
Particle of light
4
Mass per unit volume
5
Center of an atom or cell
7
Substance with pH below 7
9
Naturally occurring inorganic solid
10
Force that attracts objects together
11
Preserved remains of ancient life