How Many Neutrons?
• It is not strictly true to say that elements
consist of one type of atom.
• Which sub-atomic particle must always be
of the same number in atoms of an
element?
Remember: The number of
protons defines
the element
• Atoms of the same element always have
the same number of protons, but they may
have different
numbers of
neutrons.
• Atoms that differ in this way are called
isotopes.
Write this down!
Isotopes are atomic forms of
the same element, with the
same number of protons but
differing numbers of
neutrons.
• Isotopes are virtually identical in their
chemical reactions. WHY?
• The uncharged neutrons make no
difference to chemical properties but do
affect physical properties such as melting
point and density.
Isotopes: Carbon
• Natural samples of elements are often a
mixture of isotopes. About 1% of natural
carbon is carbon-13.
12
6
13
C
99%
6
6
6
1%
Protons
Electrons
Neutrons
6
6
7
6
C
Isotopes: Hydrogen
• Hydrogen exists as 3 isotopes although
Hydrogen-1 makes up the vast majority of the
naturally occurring element.
1
1
H
Protons
Electrons
Neutrons
Hydrogen
2
1
H
Protons
Electrons
Neutrons
(Deuterium)
3
1
H
Protons
Electrons
Neutrons
(Tritium)
Isotopes: Chlorine
• About 75% of natural chlorine is 35Cl the rest is
37Cl.
37
35
Cl
17
Protons
Electrons
Neutrons
17
17
18
75%
Cl
17
25%
Protons
Electrons
Neutrons
17
17
20
Activity
Isotopes of Oxygen
• Almost all of natural oxygen is
but
about 0.2% is 18O.
• Produce a Table showing the particles
in each isotope.
16O
Oxygen-16
16
O
8
Oxygen-18
Protons
8
Protons
8
Electrons
8
Electrons
8
Neutrons
8
Neutrons
10
Isotopes and Relative Atomic Mass
• Many natural elements are a mixture of
isotopes, so when we react them we are
using a mixture of atoms with different mass
numbers.
• The relative atomic mass given in the periodic
table takes account of this.
E.g.. For 100 atoms of chlorine:
Mass of 75 atoms of Chlorine 35: 75 x 35 =2625
Mass of 25 atoms of Chlorine 37: 25 x 37 =925
Total
= 3550
Average (divide by 100)
= 35.5
Activity
Relative Atomic Mass
• Bromine contains 50.5% bromine-79 and
49.5% bromine-81.
• Calculate the relative atomic mass of natural
bromine showing your working.
Egg. For 100 atoms of bromine:
Mass of 50.5 atoms of bromine 79: 50.5 x 79 =3989.5
Mass of 49.5 atoms of bromine 81: 49.5 x 81 =4009.5
Total
= 7999
Average (divide by 100)
= 80
1. Which of the following is not a subatomic particle?
A.
B.
C.
D.
Proton.
Isotope.
Neutron.
Electron.
2. The element Cobalt has a relative atomic
mass of 59 and an atomic number of 27.
Which of these is a true statement about
each neutral cobalt atom?
A.
B.
C.
D.
It contains 59 neutrons.
It contains 27 electrons.
It contains 32 protons.
It contains equal numbers of neutrons and
electrons.
3. The Periodic Table displays iron as shown
below. This indicates that Fe atoms:
56
A.
B.
C.
D.
Fe
contain 56 neutrons.
26
contain 30 electrons.
contain 26 protons.
contains more protons than neutrons.
4. Bromine consists of a mixture of two
isotopes: Bromine-79 and Bromine-81
• Which of the following is true:
80
Br
A.
B.
C.
D.
35
Both isotopes contain 35 protons.
Bromine 79 contains 46 neutrons.
Bromine 81 contains 44 neutrons.
Bromine-81 is more reactive than bromine79.
5. Natural boron consists of approx. 20%
boron-10 and 80% boron-11.
• What will the relative atomic mass of natural
boron will be?
?
A.
B.
C.
D.
10.0
11.0
10.5
10.8
B
5