Atomic Theory

Atomic Theory
Bhavik Patel
Chemistry AP
Period: 3-4
Atomic Theory
• John Dalton and His Atomic Theory
• In 1803, John Dalton (1766–1844)
forcefully revived the idea of atoms.
Dalton linked the existence of elements,
which cannot be decomposed chemically,
to the idea of atoms, which are indivisible.
John Dalton’s Theory
• Compounds, which can be broken down into
two or more new substances, must contain
two or more different kinds of atoms. Dalton
went further to say that each kind of atom
must have its own properties, including
mass. This idea allowed his theory to account
quantitatively for the masses of different
elements that combine chemically to form
Dalton’s Theory
• 1. All matter is composed of tiny
indivisible particles called atoms.
• 2. Atoms of the same element are
identical. The atoms of any one element
are different from those of any other
Dalton’s Theory
• 3. Atoms of different elements can
combine with one another in simple whole
number ratios to form compounds.
e.g. H2O
• 4. Chemical reactions occur when atoms
are separated, joined or rearranged. Atoms
of one element are not changed into atoms
of another by a chemical reaction.
• Electrons are subatomic particles found
rotating around the nucleus of every
atom. The number of electrons outside the
nucleus is always the same as the number
of protons in the nucleus. This makes the
atom electrically neutral.
• The electrons which move around the
nucleus of an atom have different
quantities of energy. Those restricted to
being closest to the nucleus have the least
energy while those with the ability to be
furthest away from the nucleus have the
most energy.
Electron Arrangement
• The electrons can therefore be regarded as
occupying different spherical volumes of
different sizes, these levels usually being
referred to as shells. It is worth noting that
within each shell the electrons are not
orbiting the nucleus at a fixed distance
but can travel anywhere within the
spherical shape of that shell.
• Protons are stable subatomic particles
found in the nucleus of every atom.
• The number of protons in the nucleus is
the proton number of that atom, which
tells which element the atom belongs to.
• The proton number determines the relative
position of the element on the periodic
Proton: Discovery
• Two scientists, W. Wien in 1898, and
J.J.Thomson in 1910, concluded that a positive
particle existed and had a mass equal to that of
a hydrogen atom.
• In 1919, E.Rutherford demonstrated that
particles were emitted when nitrogen was
bombarded with alpha particles. In 1920 he
proposed that these hydrogen nuclei were
fundamental particles and he named them
• Neutrons are subatomic particles found in
the nucleus of every atom. The number of
neutrons in the nucleus does not follow a
regular pattern. The number of neutrons
can vary even for atoms of the same
• Neutrons have no charge.
• Discovery:
Neutrons were the last of the three main
subatomic particles to be discovered, being
first recorded in 1932.
Concept Map
Atomic Structure
Imagine This!
• If an atom were the size of a football
stadium, the nucleus would be the size of a
• If people were the size of atoms, all the
people living on the earth today would fit
on the head of a pin!