Atomic Structure

Section 18.2

The Nucleus

Nucleus- contains most of the mass of an atom– Protons & Neutrons contains most of an atom’s mass– Electrons- 1/2000th 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

Atomic structure

Amu – 1/12th 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

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)….

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