Across
1
Mass per unit volume
4
Icy body with a tail near the sun
5
Infectious microscopic agent
7
Second lightest element
8
Center of an atom or cell
Down
2
Positive particle in nucleus
3
Particle of light
6
Substance with pH below 7
9
Substance with pH above 7
10
Our home planet
11
Ball of burning gas in space
12
Curved glass that refracts light
13
Nerve cell in the brain