Chapter 4
Atomic Structure
4.1 Studying Atoms
Ancient Greek Models of Atoms
Greeks came up with the idea of the atom several thousand years ago “atomos” = greek
word
Dalton’s Atomic Theory ( born 1766 in England)
All elements are composed of atoms
Atoms of the same element have the same mass,
Atoms of different elements have different masses
Compounds contain atoms of 2 or more elements
Atoms of different elements always combine in the same way
Not all correct, theory was revised
4.2 Structure of an Atom
Subatomic Particles
Atomic Number = number of protons
Each element has unique atomic number
Atomic number also = number of electrons
Number of protons = number of electrons
Protons (+) charge balanced by
electrons (–) charge
Mass Number = number of protons & neutron
Number of Neutrons = mass number – protons
Isotopes
Same
number of protons, but different
number of neutrons
 Elements can have several isotopes
4.3 Modern Atomic Theory
Bohr’s Model of the Atom
Better description of electrons
Electrons orbit around nucleus in energy levels like planets
1st Level = holds up to 2 electrons
2nd Level = holds up to 8 electrons
Electrons can move to different energy levels if atom gains or loses energy
Electron Cloud Model
Electrons move in a less predictable way
Electron cloud is area around nucleus where electrons are moving
Atomic Orbitals – space where an electron is most likely to be found