Across
1
Form of thermal energy
3
Biological catalyst
4
Center of an atom or cell
9
The Red Planet
11
Gas essential for breathing
12
Path of a planet around the sun
15
Gas form of a liquid
Down
2
Naturally occurring inorganic solid
5
Particle of light
6
Basic unit of life
7
Curved glass that refracts light
8
Unit of energy
10
Force that attracts objects together
13
Glass that splits light into spectrum
14
Smallest unit of an element