Across
1
Biological catalyst
3
Ball of burning gas in space
7
Compound containing oxygen
8
Gas essential for breathing
9
Infectious microscopic agent
11
Positive particle in nucleus
12
Unit of energy
13
Second lightest element
Down
2
Group of similar cells
4
Natural satellite of Earth
5
Our home planet
6
Blocking of light by a celestial body
10
Smallest unit of an element