Starter
 On MWB:
 Draw and label a diagram of an atom of helium with its
sub-atomic particles (with relative masses and charges)
Learning outcomes:
 Define the terms mass number and atomic number
(Grade C)
 Define an isotope and give examples (Grade B)
 Explain why the isotopes of the same element have
identical chemical properties (Grade A)
Specification reference 3.1.1
Atomic number, Z
 Atoms consist of a tiny nucleus made up of protons and
neutrons.
 This nucleus is surrounded by electrons.
 The number of protons in the nucleus is known as the
atomic number or the proton number Z.
 The atomic number defines the chemical identity of the
element e.g. All sodium atoms have 11 protons
Mass number, A
 The total number of protons and neutrons in the nucleus
is called the mass number A.
 It is the nucleons that make up the mass of the atom
because electrons weigh virtually nothing.
 Mass number A = no. protons + no. neutrons
What’s the number?
Isotopes
 What are isotopes?
 Isotopes are atoms of the same element that contain
different numbers of neutrons.
 Is the reactivity of an isotope different to another isotope
of the same element?
 The reactivity of different isotopes of an element is
identical because they have the same number of electrons
 Are the physical properties of an isotope different to
another isotope of the same element?
 The different masses of the atoms means that physical
properties of isotopes are slightly different.
Isotopes of chlorine
About 75% of naturally-occurring chlorine is chlorine-35 (35Cl) and 25% is chlorine-37
(37Cl).
17 protons
17 protons
18 neutrons
20 neutrons
17 electrons
17 electrons
Isotopes of carbon
 Can you complete the table for the different isotopes of
carbon?
Isotope
Carbon-12
Carbon-13
Carbon-14
Protons
Neutrons
Extension – Who
was Willard Libby
and why is he
important?
Explain briefly
how his technique
can be used
Summary questions
1.
Isotopes of hydrogen have been given their own
names. Hydrogen-2 is known as deuterium and
hydrogen-3 tritium. State how many p, n & e the
atoms of the following have.
a.
b.
Deuterium
Tritium
and 157Z Identify which of these atoms
(not their real symbols) is a pair of isotopes.
3. For each element in question 2, state:
2.
31 W, 14 X, 16 Y
15
7
8
a.
b.
c.
The number of protons
The mass number
The number of neutrons
Arrangement of electrons
 Describe how electrons are arranged in an atom (Grade
C)
 Describe how the structure of an atom developed from
Dalton to Schrödinger
 Recognise that the electron can behave as a particle, a
wave, or a cloud of charge
Electron arrangement
 Read the page on the link attached
Electron shells
 The electron shell model is still used at A-level
particularly for describing the bonding of simple
compounds
 Electron shells
 First shell holds up to two electrons
 Second shell up to eight electrons
 Third shell holds up to eighteen electrons
Electron diagrams
 Draw the electron arrangement for the following atoms:
 Carbon
 Sulfur
 Sodium
 Draw the electron arrangement for the following ions:
 Na+
 O2 Mg2+
Summary questions
1.
Draw the electron arrangement diagrams of atoms
that have the following numbers of electrons
a.
b.
c.
3
9
14
2. State, in shorthand, the electron arrangement of atoms
with:
a.
b.
c.
4 electrons
13 electrons
18 electrons
3. Identify which of the following are atoms, positive ions,
or negative ions. Give the size of the charge on each
ion, including its sign. Use the Periodic Table to identify
the elements A-E
Number of
protons
Number of
electrons
A
12
10
B
2
2
C
17
18
D
10
10
E
3
2