The difference between crystalline and amorphous solids is determined by

regular repeating structure

Explanation:

Crystalline solids have an organized structure involving repeating units of a particular structural pattern known as the unit cell. This well organized structural arrangement is maintained throughout the solid. Amorphous substances do not have this well organized structural units hence they are not as strong as crystalline solids and lack a definite pattern of arrangement of particles in the solid.

1. a)c-14 has two more protons

2. d)they have different numbers of protons, neutrons, and valence electrons; they are all gases at room temperature.

3. c) iron and nickel.