Understanding Atoms and the
Periodic Table of elements
CHAPTER 6 SNC1P –
M. COUTURIER
The atom
 Atoms are made up of three sub-atomic
particles: protons, neutrons and electrons.
 Protons are positively charged particles which
reside in the nucleus of an atom. It is the proton
that defines the element.
 Neutrons are neutral particles which also reside in
the nucleus of an atom. The neutron defines the
isotope.
 Electrons are negatively charge particles which
orbit the nucleus of the atom. The electron
defines the ion or the atoms charge.
The atom
The atomic number
 Recall that the number of protons defines the
element. This number is known as the
atomic number, which is located on the top
left of the element box within the PTE.
 Knowing the atomic number means you know the
element. Ex. Lithium has an atomic number of 3
and therefore has 3 protons.
 Can you identify which element has 35 protons?
Iodine’s Atomic Number is 53
The atomic mass
 The atomic mass is in essence the mass of a
single atom. The mass is combined mass of
the protons, neutrons and electrons. The
atomic mass of Iodine is 126.90 μ. It is found
below the element symbol.
 What is the atomic mass of Calcium?
The mass number
 The mass number is the summation of the
protons and neutrons within the atom’s
nucleus. The mass number is essential when
trying to determine the number of neutrons
(assuming you are given the atomic number)
and the isotope you are dealing with.
Example: Carbon-12 and Carbon-14 are
isotopes of carbon. The number 12 an 14 are
their mass numbers respectively.
The mass number
 In the case of Carbon-12, we can deduce that:
 Carbon-12’s mass number is 12 (given).
 Carbon has 6 protons (implied).
 Carbon-12 has 6 neutrons (12-6=6)
 In the case of Carbon-14, we can deduce that :
 Carbon-14’s mass number is 14 (given)
 Carbon has 6 protons (implied)
 Carbon-14 has 8 neutrons (14-6=8)
Mass Number
 So in essence, we can find the number of
neutrons if we are given the mass number
because:
 Mass Number = Atomic Number + Neutrons
 or
 Neutrons = Mass Number – Atomic Number
 Recall that the Atomic Number is the number of
protons
Charge Number
 The charge number is the electrical difference
between protons and electrons. Since
protons are positive and electrons are
negative, each particle cancels out the next.
 Charge Number = #protons + #electons
 Example: Na, or sodium always has 11 protons. If
the atom has 11 electrons, then the charge
number is 0, hence the atom is neutral. Since
 Charge Number  11 + (-11) = 0
Charge Number
 More examples:
 In the unlikely event that Na would have 12
electrons, then the charge number would be -1
which means the atom is negatively charged.
Recall that the protons never change.
 Charge Number = 11 + (-12) = -1.
 The more likely outcome is that Na would have 10
electrons, hence the charge number would be +1
which means the atom is positively charged.
 Charge Number = 11 + (-10) = +1.
Standard Atomic Notation
 To maximize the amount of information
given, we present the data in the form of the
standard atomic notation.
 The element symbol is placed in the centre
 The atomic number is given on the bottom left
 The mass number is given on the top left
 The charge number is given on the top right (if no
number appears then the net charge is 0).
Standard Atomic Notation
Standard Atomic Notation
 From the previous diagram we can state that:
 Sodium is the element (Na)
 Sodium has 11 protons (atomic number)
 Sodium-23 (isotope) has 12 neutrons because
(23- 11=12)
 There is a +1 charge, hence there are only 10
electrons because (+1 = +11 – 10).
Standard Atomic Notation