Grade 9 Science: 6.7 - Atomic Number and Atomic Mass
Key terms – proton, neutron, electron, atomic number, mass number, isotope
According to the Bohr-Rutherford theory of the atom:

All atoms have a central nucleus containing protons and neutrons
o Protons have a positive charge
o Neutrons have no charge
o Protons and neutrons have approximately the same mass
o Electrons orbit around the outside of the nucleus
o Electrons have a negative charge
 Atoms have a neutral charge. Therefore, the number of protons and electrons in
an atom must be the same (see diagram)
The only difference between atoms of different elements is the number of each
particle they have. So, while an atom of oxygen has 8 protons, 8 neutrons and 8
electrons, an atom of sodium has 11 protons, 12 neutrons and 11 electrons.
This information for each atom is given in the periodic table.
Using Germanium as an example, the periodic table will have the symbol of the
element (usually one or two letters) and two very important numbers. The top
number is the atomic number which is the number of protons in the atom.
The second number, the one at the bottom, is the mass number. The mass
number is the total number of protons and neutrons in the atom. The
number of electrons are not included in the mass number because they have
basically no mass.
If you know the atomic number and the atomic mass, you can figure out the
number of protons, neutrons and electrons in any atom.
Example:
An atom of copper has an atomic number or 29 and a mass number of 64. Give the
number of each particle.
Answer:
Since the atomic number tells you the number of protons, there are 29 protons. An
atom always has the same number of protons and electrons, so there must be 29
electrons as well. The mass number is 64 and it tells you the sum of the protons and
the neutrons. Since there are 29 protons, the number of neutrons must be
64 – 29 = 35 neutrons
When Mendeleev was constructing the periodic table, he noticed that the
characteristics of an element depended on the number of protons it had; not the
number of neutrons. So some elements have more than one isotope. That is,
different atoms of a certain element may have different masses and this is
achieved by having different numbers of neutrons. For example, most carbon
atoms have a mass number of 12 which means they have 6 neutrons. However,
some Carbon atoms have a mass of 14. This means that they have 8 neutrons.
Questions:
1. An atom of Fluorine has an atomic number of 9 and mass number of 19.
Give the number of
a. Protons ___9___
b. Neutrons ___10___
c. Electrons ___9___
2. Complete table 2 on page 240 in text book.
Element
Element
Atomic
Mass
# Protons
# Neutrons
# Electrons
Name
Symbol
Number
Number
Magnesium
Mg
12
24
12
12
12
Aluminum
Al
13
27
13
14
13
Phosphorus
P
15
31
15
16
15
Tin
Sn
50
119
50
69
50
Silver
Ag
47
108
47
61
47
Text Questions (page 240 1-5)
1. Yes. An element is defined by the number of protons it has
2. No. Some elements have different isotopes. Example, Carbon-12 (6
protons, 6 neutrons) and Carbon-14 (6 protons, 8 neutrons)
3. a – True
b – False
….cannot be smaller…
c - True (if there are no neutrons)
d - False
… does not always ….
e - True
f - True
4. 31
5. 2, 8, 8,