The Atom, Bohr’s Theory and
Elements
Bohr’s Theory of the Atom
 When looking at the light of different elements, we
notice they are not the same.
 Thus, each element emits a very specific pattern of
wavelength.
 By noticing these patterns we create the emission
spectrum – a pattern of wavelengths emitted by
different elements.
Bohr’s Theory of the Atom
 Rutherford was very close to finalizing what the atom
looked like. It was up to one of his students, Niels Bohr
that took Rutherford’s model and perfected it.
 Bohr examined the way light was emitted from different
elements. He noticed that if he excited an electron and
made it move a different light was produced.
 He then concluded that he was moving electrons from one
orbit to another orbit and thus deduced there are multiple
orbits that surround the nucleus of an atom.
Light Emission
+
Electron Excitation
n=1
n=2
n=3
Bohr’s Theory of the Atom
 Bohr Theory on Electron Orbits:
 Electrons are located in defined shells, which are located
a certain distance from the nucleus.
 Electrons cannot exist between the shells.
 Electrons can gain energy to move up to a higher shell,
or they lose energy to move down to a lower shell.
 Electrons are more stable (have less energy) when they
are closer to the nucleus.
Bohr’s Atomic Theory
 Bohr’s idea helped us understand the modern periodic
table.
 The first row of the periodic table has 2 electrons in the first
orbit.
 The second and third row of the periodic has 8 electrons in
the second and third orbit.
 The fourth and fifth row of the periodic table has 18 electrons
in the fourth and fifth orbit.
 The sixth and seventh row of the periodic table has 32
electrons in the sixth and seventh orbit.
 This isn’t quite accurate, but it was what Bohr concluded
and he was VERY close to what we really have. (You will
learn more about this in grade 10 and 11 Chemistry)
Bohr’s Atom
 Potassium
example.
 Potassium has
an atomic
number of 19
 Observe there
are 19
electrons…
ohhhh
ahhhhh!
Defining an Element
 An element has three important factors
 Atomic number – which states how many protons are in
an element.
 The Mass number – is the total number of protons and
neutrons in an atom.
 The Chemical symbol – is the symbol we use to denote
the chemical – Cl is Chlorine, C is Carbon, H is hydrogen
and so forth.
Defining an Element
 Information about an element includes the mass number,
the atomic number and the chemical symbol.
 A chemical is written with the chemical name and after it
will be the mass number. Example Chlorine - 35
 Standard Notation will tell us that
Chlorine - 35
has 17 protons, 18 neutrons
17 electrons and
a mass of 35 g.
This is written in Standard Notation
Defining an Element
 Elements tend to have a few different atomic masses,
but have the same amount of protons and electrons.
 Example Carbon-12 and Carbon-13.
 Hmmm what is going on here?
 Carbon 12 – has 6 proton, 6 neutrons and 6 electrons
and a mass of 12.00. Standard Notation - 126 C
 Carbon 13 – has 6 protons, 7 neutrons and 6 electrons
and a mass of 13. Standard Notation - 136 C
Isotopes
 Even though Carbon-12 and Carbon-13 have different
masses because of an extra neutron, THEY HAVE THE
SAME CHEMICAL PROPERTIES.
 These elements are called Isotopes. A lot of element
on the periodic table tend to have quite a few
isotopes.
Ion’s
 When we charge and atom we call it an ion.
 When there are more electrons than protons it has a
negative charge and will have a notation of 1-, 2-, 3and so forth.
 When there are more protons than electrons it has a
positive charge will have a notation of 1+, 2+, 3+ and
so forth.
 If you see just a ‘–’ or a ‘+’ symbol that represents 1- or
1+.
Ion’s
 Standard Notation for
a Chlorine Ion that has
a negative one charge
 Standard Notation for
a Magnesium Ion has a
positive 2 charge.
Periodic Table
 The negative ions
are on the right
side of the zigzag
line of the periodic
table up to the
noble gases.
 The positive ions
are on the left side
of the zigzag line
of the periodic
table.
7.3
The element on the Periodic Table
 On the periodic table each element has listed: the
atomic number, the density at standard temperature
and pressure, the most common ion charge, symbol
of the element, name of the element and the atomic
mass.
Atomic Mass
 The atomic mass is the average of the mass numbers
of particular isotopes of an element.
 You can determine the mass number of the element
by rounding the atomic mass number to the nearest
whole number.
 Aluminum’s atomic mass is 26.98 (this is the average
weight of all its isotopes) and but if we round it to 27
we get its mass.
Calculating the Number of Electrons
 If we look at Aluminum – it has an atomic number of
13. Thus it is supposed to have 13 protons, 13
neutrons and 13 electrons.
 But since Aluminum has an ionic charge of 3+ it must
have three more protons than electrons.
 Since electrons are easier to move than protons, Al3+
will have 13 protons, 13 neutrons and 10 electrons.
Complete the Chart
Element
Atomic
Number
Number of
Electrons in
the Atom
Ion Charge
Number of
Electrons in
the Ion
Aluminum
13
13
3+
10
Oxygen
8
8
2-
10
Sodium
11
11
+1
10
Phosphorus
15
15
3-
18
Assignment
 Pg 212 – Questions – 3, 5 (a, c, f, h), 6 (b, d, f), 7, 8, 10
 Pg 217 – Questions 1 – 5, 7 – 10, 12, 14, 16, 17