3.  The atomic number equals the number of protons.  The electrons in a neutral atom equal the number of protons.  The mass number equals the sum of the protons and neutrons.

4.  The charge indicates the number of electrons that have been lost or gained.  A positive charge indicates the number of electrons (which are negatively charged) that have been lost.  A negative charge indicates the number of electrons that have been gained.  This structure can be written as part of a chemical symbol.

6.  Isotopes ◦ Atoms of the same element that differ in the number of neutrons they contain are known as isotopes.  Because they have the same number of electrons, all isotopes of an element have the same chemical properties.

7.  The combinations of two or more elements in definite proportions form a chemical compound. The physical and chemical properties of a compound are usually very different from those of the elements from which it is formed.

8.  Atoms in compounds are bonded together by chemical bonds. ◦ Ionic bonds are formed when one or more electrons are transferred from one atom to another.

9.  Covalent bonds are formed when electrons are shared between atoms.

10.  The structure that results when atoms are joined together by covalent bonds is called a molecule.

11. When molecules are close together, a slight attraction can develop between the oppositely charged regions of nearby molecules.