1. Atomic Structure Section 18.2

2. The Nucleus Nucleus- contains most of the mass of an atom – Protons & Neutrons contains most of an atom’s mass – Electrons- 1/2000 th of the mass of a proton Grams are not used to measure mass of atoms because atoms are too small – Amu is used instead – Amu = atomic mass unit Mass of proton = mass of neutron = 1amu

3. Atomic structure Amu – 1/12 th the mass of a carbon-12 atom, which contains 6 protons and 6 neutrons Protons- determine the identity of an atom – Every atom has a different # of protons Atomic number- tells us the number of protons Mass number- total number of protons & neutrons

4. Atomic Structure # of neutrons = mass # - atomic # Isotope- Atoms of the same element with different numbers of neutrons To indicate the identity of an isotope, write the element name – mass # – Carbon-12 Carbon-14 Average atomic mass- a weighted average of the masses of the isotopes of an element – Average atomic mass = (% x mass) + (% x mass) + (% x mass)….

5. Atomic Structure Electrons- determine physical and chemical properties of atoms Charges of atoms- indicates # of electrons gained or lost Cation- ion with a positive charge- lost electrons Anion- ion with a negative charge- gained electrons