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A closer look at the importance of protons, electrons
and isotopes
Table of Contents
If you wish to focus on a single topic,
skip to the appropriate slide
Review: What is an Atom?
Slide 3
Atomic Number
Slide 4
Mass Number
Slide 8
How to Write a Complete Atomic Symbol
Slide 16
Ions
Slide 17
Review: What is an atom?
•An atom contains protons and neutrons, located inside
the nucleus, and electrons, located outside the nucleus.
• We have identified the charge on each particle as being
positive (protons), negative (electrons) or neutral
(neutrons).
• In a neutral atom, the number of protons and the
number of electrons must be equal.
(all about protons)
Atomic Number
The identity of an atom is
defined by the number of
PROTONS in the atom.
This number is referred to as the Atomic Number.
Atomic Number
Starting at the beginning:
An atom with one proton has an atomic number of one.
This atom is identified as Hydrogen.
An atom with two protons has an atomic number of two.
This atom is identified as Helium.
An atom with three protons has an atomic number of three.
This atom is identified as Lithium.
We could keep going. What would be the atomic number of an atom with four protons?
Atomic Number
Remember: An atom with one proton (an atomic
number of one) will ALWAYS be a Hydrogen
atom.
A Hydrogen atom will ALWAYS have one proton
(an atomic number of one).
The identity of an atom is defined by the number
of protons!
(Working with neutrons, too)
Mass Number
Let’s consider a hydrogen atom.
As we stated previously, Hydrogen has an atomic number of
one, meaning it has one proton. But what about the rest of
the atom? How many electrons does it have? How many
neutrons?
STOP and THINK: If we consider a neutral hydrogen atom,
how many electrons must it have?
Mass Number
If we remember that protons have a charge of +1 and
electrons have a charge of -1, we can determine the number
of each in a neutral atom.
Hydrogen has one proton, for a total charge of +1.
Therefore, to be neutral, it must have one electron.
But what about the number of neutrons?
At this point – we don’t know! There’s nothing in the
atomic number, or the fact that it’s a neutral atom, to tell us
how many neutrons there are. This is why we need a mass
number.
Mass Number
The mass number for an atom
is defined by the number of
PROTONS and NEUTRONS in
the atom.
Mass Number
Let’s consider three options for our Hydrogen atom.
Option 1: It has zero neutrons
Option 2: It has one neutron
Option 3: It has two neutrons
In each case, we still have a Hydrogen atom – the atomic
number hasn’t changed.
The mass number, however, would be different.
Option 1: 1 proton + 0 neutrons gives a mass number of 1
Option 2: 1 proton + 1 neutron gives a mass number of 2
Option 3: 1 proton + 2 neutrons gives a mass number of 3
Mass Number
Atoms with the same number of
protons but differing numbers of
neutrons are called ISOTOPES.
They have the same atomic number, but a different
mass number.
Mass Number
In the case of Hydrogen, all three of these isotopes
are naturally occurring. However, they don’t occur
with equal frequency – 1H is the most common!
The same applies to many other types of atoms.
Carbon, for example, has three naturally occurring
isotopes (12C, 13C, 14C). How many protons, neutrons,
and electrons are present in a neutral atom of each
isotope?
Mass Number
12C
has:
6 protons
6 neutrons
6 electrons
13C
has:
6 protons
7 neutrons
6 electrons
14C
has:
6 protons
8 neutrons
6 electrons
Writing
Full
Atomic
Symbols
We can write a full atomic symbol to give information regarding
the atomic number and mass number of an atom.
1
1
H
2
1
H
Mass Number
Atomic Number
Displayed at the lower left,
often omitted, since this can be
determined from the symbol
Displayed at the upper left
3
1
H
Atomic Symbol
As determined from the periodic
table. The first letter in the symbol
is always a capital. The second
letter (when present) should be
lowercase.
(And finally, electrons)
Ions
To this point, we’ve been considering neutral atoms.
However, atoms are not always neutral!
When an atom has an unequal
number of protons and electrons,
the result is an atom with an
electrical charge, known as an ION.
Ions
When an atom has more protons
than electrons, it has a positive
charge. This type of ion is called a
CATION.
Ions
When an atom has more electrons
than protons, it has a positive
charge. This type of ion is called
an ANION.
Ions
Let’s go back to Hydrogen.
STOP and THINK:
What is the atomic number for Hydrogen?
How many protons are in a Hydrogen atom?
How many electrons are in a neutral Hydrogen atom?
Ions
A neutral Hydrogen atom has an atomic number of 1, contains
one proton and one electron.
(If you came up with a different answer,
you may wish to go back and review a bit!)
The next step is to consider a non-neutral atom.
STOP and THINK: What would happen if Hydrogen were
to lose its electron?
1. How many protons does it have now? Electrons?
2. What would be the charge on the Hydrogen
atom now?
Ions
1. Well, we didn’t say anything about losing a proton,
so the Hydrogen atom still has one proton. We started
with one electron, and lost one electron, so now the
atom has zero electrons.
With different numbers of protons and electrons, this
atom is now called an ION.
2. The charge on each proton is +1, and there is one
proton. Each electron has a charge of -1, but there
aren’t any electrons. If we add those up, we get a total
charge of +1 on this ion
Ions
To include a charge when writing the full atomic symbol
for an ion, place the value (1,2,3, etc) and sign (+ or -) of
the charge in the upper right hand corner.
1
1
H
1+
The value and the sign of
the charge are shown in the
upper right corner. If the
charge has a value of 1, it is
possible to display only the
sign (H+ rather than H1+)