1. Matter 2.1

2. Element A substance that cannot be broken down into simpler substances by ordinary chemical or physical means.

3. Atomic Number The number of protons in the nucleus of an atom.

4. Energy Level One of several distinct regions around the nucleus of an atom where electrons are located.

5. Isotope An atom with the same number of protons but different numbers of neutrons for a given element; an isotope’s mass number is different from that of the given element.

6. Mass Number The number of neutrons and protons in the nucleus of an atom.

7. Compound A substance formed by the chemical combination of two or more elements in definite proportions and usually having properties different from those of its constituent elements.

8. Chemical Bond A force that holds together atoms that form a compound.

9. Ion An atom or a molecule that possesses an electrical charge.

10. Ionic Bond A bond that forms between negative and positive ions.

11. Covalent Bond A bond that forms when atoms share electrons.

12. Metallic Bond A bond that forms when electrons are shared by metal ions.

13. Key Concept What is an element? An element is a substance that cannot be broken down into simpler substances by chemical or physical means.

14. Key Concept What particles make up atoms? An atom is the smallest particle of matter that contains the characteristics of an element.

15. Key Concept What are isotopes? Atoms with the same number of protons but different numbers of neutrons are isotopes of an element.

16. Key Concept What are compounds and why do they form? A compound is a substance that consists of two or more elements that are chemically combined in specific proportions.

17. Key Concept How do chemical bonds differ? When an atom’s outer-most energy level does not contain the maximum number of electrons, the atom is likely to form a chemical bond with one or more other atoms.