Across
1
Center of an atom or cell
5
Ball of burning gas in space
6
Gas essential for breathing
7
Curved glass that refracts light
8
The Red Planet
11
Unit of heredity
Down
2
Amplified beam of light
3
Positive particle in nucleus
4
Mass per unit volume
8
Natural satellite of Earth
9
Biological catalyst
10
Substance with pH below 7