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2.2 Atomic Theory
(Section 6.2 pg 153-156)
• The idea of the atom is an ancient one, initially
proposed by the Greeks.
• Atomic Theory (the human understanding of
atoms) has gone through many ‘revisions’ since:
– John Dalton revised it in the early 1800s. He claimed all
matter is made of atoms that are indestructible and
unique to each element.
• Atomic Theory (the human understanding of
atoms) has gone through many ‘revisions’ since:
– In 1898, J.J. Thomson discovered that the atom has tiny
negatively charged particles called electrons.
• Atomic Theory (the human understanding of
atoms) has gone through many ‘revisions’ since:
– Ernest Rutherford discovered that the atom also has
positive particles called protons, that they were
contained in a small, dense area in the middle (the
nucleus), and that the nucleus accounted for nearly all
the weight of an atom.
• Atomic Theory (the human understanding of
atoms) has gone through many ‘revisions’ since:
– Niels Bohr proposed the idea of ‘electron shells’; i.e. the
concept that electrons orbited around the nucleus.
• Although Bohr’s ideas have been developed further,
we can still use Bohr’s model to help understand
the bonding behaviour and other properties of
atoms.
• Here are some key concepts:
– Every atom is composed of three types of subatomic
particles (proton, electron, neutron)
– Proton = found in the nucleus, positively charged with
an AMU (atomic mass unit) of 1.
– Neutron = found in the nucleus, no charge (neutral) with
an AMU of 1.
– Electron = orbits the nucleus in an electron shell and has
an AMU of 1/1800.
• Each element has a unique number of protons in its
nucleus; the # of protons is called an element’s
atomic number.
• E.g. H has the atomic #1; O has the atomic #8.
• Although the atomic number (# of protons) for each
element is the same, individual atoms in a sample
may have different # of neutrons.
• The mass number of an atom is the total number of
protons and neutrons in the nucleus.
• The mass number is written
after the element name e.g.
oxygen-18 represents O with a
mass # of 18.
• Different mass numbers don’t
affect the physical or chemical
properties of elements!
• The atomic mass of an
element is the average
mass of the atoms of the
element and reflects the
abundance of the
different mass numbers
of the element (Fig.2
p.153).
• When we draw the atom according to Bohr we call
it a Bohr Diagram. There are certain rules to follow
when drawing a Bohr Diagram (modified from p.155
in text):
• Look at what symbol the element has and write this in the
centre. Write the atomic # in front of it at the bottom.
• Determine the # of shells needed. Find what row (period) it is
in. This will be the # of electron shells. Draw them in.
• The atomic # represents the # of electrons (e) needed. Start
to fill the shells. The first shell 1 set of paired electrons (2 e).
The first shell is ‘full’ when it has 2 e.
• Fill the rest of the shells with your remaining e, obeying the
following rules:
 Start by placing single e around the shell, then pair up any
remaining e.
 The 2nd shell can take up to 8 e (4 pairs)
 The 3rd shell can take up to 8 e (4 pairs)
 The 4th shell can take up to 18 e (9 pairs)
• Never add more electrons than you started with (never add
more than the atomic #).
• The 5 methods for writing a Bohr diagram shown
below are all correct ways to represent O (Fig.6
p.155).