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Electrons and the Atom
SCH3U/4C
MS. RICHARDSON
Refining the Atomic Model
 Bohr’s
work provides information about
energy levels at which electrons can exist
 We
can also determine how many
electrons exist in each energy level:
# electrons in a shell = 2n2
Where n is the number of the shell
Electrons of the Atom
 When
a shell contains the maximum number
of electrons, it is said to be filled.
 Electrons in the outer shell of an atom are
known as valence electrons and the shell is
the valence shell.
 The valence electrons are the only electrons
involved in forming chemical bonds
Lewis Diagrams and Electrons
 Lewis
diagrams are simplified models used
to represent atoms.
 To
draw the Lewis diagram for an atom,
we write the chemical symbol and place
the valence electrons around the symbol
Lewis
Diagrams
(continued)
Always place
electrons at “N, S,
E, W” before
doubling up.
Lewis Diagrams (continued)
 Pairing
dots when drawing Lewis
diagrams for an atom with more than four
valence electrons is important.
 These
electron pairs are less likely to
participate in bonding than
unpaired/lone electrons
The Nucleus

Rutherford discovered that the nucleus
occupies the centre of the atom as well
as many other features of the nucleus
Protons & Neutrons
 The
nucleus is where the mass of the atom
is located, comprised of the protons and
neutrons.
Neutrons & Isotopes
 The
atomic number represents the number
of protons in an atom
 This also indicates the number of electrons
for a neutral atom
 The number of neutrons in the nucleus of
atoms of the same element can vary, as
seen with isotopes
Isotopes
 Isotopes
are atoms of the same element
that have different number of neutrons
 This
results in a different mass number for
the isotope
 Eg:
Carbon-12 and Carbon-14
6 neutrons
8 neutrons
Isotope Notation
 When
representing isotopes, we still use
standard atomic notation
 We
can also represent them by naming
the element and indicating the mass
number
 Eg:
carbon-14, nitrogen-15
Determining Number of Neutrons
 To
find the number of neutrons in an
element/isotope, we can use the formula:
#
of neutrons = atomic mass – atomic number