Across
1
Particle of light
3
Center of an atom or cell
4
Icy body with a tail near the sun
5
Group of similar cells
7
Ball of burning gas in space
8
Mass per unit volume
11
Reddish electrical conductor
12
Fourth state of matter
Down
2
Gas essential for breathing
3
Nerve cell in the brain
4
Element found in all living things
6
Smallest unit of an element
9
Measure of matter in an object
10
Form of thermal energy