1. A difference in the ______ number of atoms means the atoms are of different elements.

2. Combined atoms form a ______

3. Isotopes enable scientist to determine _______ of some rocks and fossils.

4. The ______ properties of an element determine how the element will change when it reacts with another element.

5. An example of _______ is water.

6. Table salt is an example of a ________.

I have the rest of the questions, but I'm stuck on these. Help?