What is the element which has the same oxidation number in its all compounds?
Beryllium, Chlorine, sulphur, nitrogen?

pure substance: iron,copper

impure substance: water,charcoal

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)

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.