This document discusses the structure of atoms using helium and xenon as examples. It notes that both are atoms but xenon has a greater mass because it contains more subatomic particles. The three main subatomic particles are identified as protons, neutrons, and electrons. A helium atom is drawn showing its structure of two protons, two neutrons, and two electrons. Isotopes are defined as atoms that have the same number of protons but different numbers of neutrons. Atomic number identifies the element based on its number of protons, while mass number includes both protons and neutrons. Atomic mass is calculated as a weighted average of the masses and abundances of an element's isotopes.

1. The Atom Unit III

2. Picture it…. Draw a picture of a Helium particle and a Xenon particle. Similarities: Both are atoms. Difference? Mass He: 4.0 g/mole Xe: 131 g/mole

3. Why??? Xenon has more stuff in it. So, what’s in an atom? Protons = positive charge =p+ Electrons = negative charge = e- Neutrons = neutral charge = n0 Now, draw a picture of the Helium atom, show how the subatomic particles are arranged.

4. The Helium Atom = proton = neutron = electron

5. Atomic Number (Z): Identifies the element. Number of protons. Whole number (Blue) on periodic table. For a neutral atom the # of protons is equal to the number of electrons.

6. ISOTOPES: Atoms that have the same number of protons but different number of neutrons. Mass Number (A): Unique for each isotope. Number of p+ + number of no Not found on periodic table. C-12 vs. C-14

7. Identify the # of subatomic particles

8. Atomic Mass: Also called atomic weight or molar mass. Weighted average mass for one mole of all isotopes. Decimal number found on P.T. Atomic Mass = (mass1)(relative abundance1) + (mass2)(relative abundance2) +………..

9. Example: Calculate the atomic mass of chlorine. The two isotopes have atomic masses and relative abundance of 34.97 amu (75.77%) and 36.97 amu (24.23%).