Across
1
Unit of energy
4
Chemical element in table salt
6
Gas essential for breathing
7
Natural satellite of Earth
8
Group of similar cells
9
Substance with pH above 7
10
Unit of force
Down
2
Center of an atom or cell
3
Icy body with a tail near the sun
5
Mass per unit volume
7
Naturally occurring inorganic solid
11
Hottest planet in solar system
12
Unit of power
13
Second lightest element