1. Notes on Atoms Dense nucleus composed of protons & electrons Mostly empty space “Cloud” of electrons in orbitals around the nucleus Protons: positive electrical charge; Neutrons: negative electrical charge; Electrons: negative electrical charge Each orbital represents a specific energy level that its electrons possess. Orbitals / electrons closer to the nucleus: lower energy than those farther from the nucleus.

2. Notes on Atoms, cont’d. Protons give the atom its positive charge; electrons, its negative charge. Neutrons add mass to the atom, but contribute no charge. Atomic mass = sum of the protons + neutrons (the particles in the nucleus). The electrons also add a very small amount of mass, but we usually ignore this. Atomic number = the number of protons only. The atomic number is what identifies the atom as a particular element. Each element has a unique number of protons in its nucleus. The periodic table of the elements is arranged by increasing atomic number, as you move from left to right and down to the next row.

3. Notes on Atoms, cont’d. Atoms typically have an overall net charge = 0. This is because they have the same number of protons and electrons, so all the positive and negative charges cancel each other out. When an atom does not have a neutral charge, it is called an ION. This is because is has either lost or gained electrons. A positive ion has more protons than electrons; it has lost some of its electrons. A negative ion has more electrons than protons; it has gained some electrons. Remember, atoms become ions by gaining or losing electrons, never by gaining or losing protons. If the number of protons changes, the element’s identity has changed!

4. Notes on Atoms, cont’d. ISOTOPES are atoms of the same element (same number of protons), but have different numbers of neutrons. This means that their masses are different, but they are still the same element. They still have the same number of electrons as well. This is a typical example of how the elements appear in the periodic table. This one happens to be the element silicon, which is used extensively in the computer industry.

5. Notes on Atoms, cont’d. This is the periodic table used for STAAR testing in Texas. You don’t need to draw it.