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Atomic Structure
CHEMISTRY 112
Early Models of the Atom
 An atom is the smallest particle of matter that
retains its identity in a chemical reaction
 Democritus was an early Greek philosopher who
suggested the existence of atoms
 He believed they were indivisible and indestructible
John Dalton and Atomic Theory
 From experimentation, Dalton was able to devise a set of
ideas supporting the idea of the atom
 Elements are composed of tiny indivisible particles called
atoms
 Atoms of the same elements are identical and one
element’s atom differ from another
 Atoms of different elements can come together to form
compounds
 Atoms of one element cannot change into atoms of
another
A Problem with Dalton
 We now know that the atom is divisible
 It is composed of subatomic particles
 Electrons are negatively charged particles and they are
found outside the nucleus in regions called orbitals
 Protons are positively charged and found in the nucleus
of an atom with neutrons, which have no charge
 There are even smaller particles but we do not study
them in introductory chemistry
The Size and Shape of an Atom
 The nucleus is found at the core of an atom
 It is very dense and positively charged
 The region around the nucleus is large and mostly
empty space
 Although all three subatomic particles are small, the
electron is 1/1840 the size of a proton/neutron
 Table 4.1, page 108
Atomic Number
 Elements are different from one another because
they contain different numbers of protons
 The atomic number = number of protons
 In atoms, the number of proton = number of electron
 Therefore, an atom is electrically neutral
Mass Number
 The mass number is the number of protons added to
the number of neutrons
 The number of neutrons can vary from one atom to
the next within one sample of an element
 Such atoms are called isotopes of an element and
have differing mass numbers – they are still,
however, chemically alike because their p+ and ecount are equivalent to one another
Isotopes
 There is a certain method used for naming and
writing isotopes