Across
1
Group of similar cells
5
Gas essential for breathing
7
Second lightest element
8
Blocking of light by a celestial body
11
Amplified beam of light
Down
2
Unit of electric potential
3
Mass per unit volume
4
Reddish electrical conductor
6
Biological catalyst
9
Center of an atom or cell
10
Natural satellite of Earth
12
Push or pull on an object