Atomic Nature Ch11 All matter is made up of tiny particles called atoms and can combine to form molecules L9-1 • Atoms have 2 primary regions: – The nucleus – The electron cloud • The atomic nucleus contains protons & neutrons • The electrons are held to nucleus by the electric force (attractive) between the protons & the electrons • Protons and neutrons are held together in the nucleus by the strong nuclear force • Matter is in constant motion
Elements L9-2 •An element is made of only one type of atom •An element is the building block of other substances •Atoms of an element have the same # of protons: Atomic Number = # of Protons •Atoms of an element can have different # of neutrons: Atomic Mass = (# of Protons) + (# of Neutrons)
The Electron L9-3 Electrons are subatomic particles that define the outer boundaries of an atom • First discovered by J.J. Thomson (1897) • Small negatively charged “particle” – Mass of electron, me = 9.11x10-31 kg – Charge of electron = -1.60x10-19 C • Determines the chemical properties of a substance • Form a “cloud” around the nucleus of the atom • Establishes the volume of an atom • When an electron is freed from its nucleus, the atom becomes "ionized"
The Nucleus L9-4 • The nucleus is where the mass of an atom is concentrated. It contains protons and neutrons • Proton: positively charged heavy particle – ~2000 times as massive as an electron, mp = 1.673x10-27 kg – Same magnitude of charge as electron (+1.6x10-19 C) • Neutron: neutral heavy particle – Slightly heavier than a proton, mn = 1.675x10-27 kg – Neutrons help hold protons together in a nucleus – Different numbers of neutrons in an element are isotopes
The Periodic Table L9-5 • The master “menu” of matter • First proposed by Dmitri Mendeleev • All known elements are organized by chemical properties (electronic structure) • Columns are called Groups - # of valence electrons • Rows are called Periods - electron orbit levels • Currently ~120 elements have been identified • Only ~90+ occur naturally