Download 6.2 Atomic theory - Lighthouse Christian Academy

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).
• 6.2. pg 156 2-7 ( assignment)