Across
1
Center of an atom or cell
4
Magnetic metallic element
6
Particle of light
9
Ball of burning gas in space
10
Curved glass that refracts light
11
Hottest planet in solar system
12
Form of thermal energy
Down
2
Gas essential for breathing
3
Unit of force
5
Tissue that contracts for movement
7
Mass per unit volume
8
Preserved remains of ancient life