KS4: ATOMS
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Atoms – the building blocks
• All substances are made
from very tiny particles
called atoms.
• John Dalton had ideas about
the existence of atoms about
200 years ago but only
recently have special
microscopes been invented
that can “see” them.
Water
Elements – different types of atom
• Elements are the simplest
substances. There are
about 100 different
elements. Two of John
Daltons ideas were:
– Elements contain different
types of atom.
– Each element has only one
type of atom.
O
S
N
Fe
K
C
Atoms – How Small?
N
• Atoms are about 1/100,000,000 cm
across. They are small.
• If we magnified the size of a football by
the same amount it would stretch from
the UK to the USA!
X 3,000,000,000
• To make an atom the size of a football
we would have to magnify it to around
3,000,000,000 times its normal size
Atoms – How Heavy?
Si
O
O
Billions of these atoms join to form
each tiny grain of sand
• A single grain of sand contains
billions of atoms of silicon and
oxygen.
• It follows that each atom must have
an extremely small mass.
• However, it is still possible to find out
the relative masses of atoms.
“Weighing” atoms: The mass spectrometer
The more mass the atom has, the more
nearly it travels in a straight line.
Atoms are given
a + charge
The deflected
ions are
detected on a
+ screen or film.
+
+
+ + ++ + +
These charged
atoms (ions) are
accelerated past a
magnet
++
+ +
+
+
+
+
The magnet deflects this
ion. The lighter the ion
the more it is deflected
Relative Atomic Mass 1
• The deflection in the mass spectrometer varies
with the mass of the atom.
• However, this does not tell us the mass in
grams.
• What it tells us is the relative masses of atoms
– or relative atomic mass (RAM)
• The element carbon is the atom against which
the mass of all other atoms are compared.
Carbon is given a RAM value of 12.
C
Relative atomic mass = 12
Relative Atomic Mass 2
• The lightest atom is hydrogen. It has one
twelfth the mass of carbon and so has a RAM
of 1.
• Magnesium is twice as heavy as carbon. It
has a RAM of 24.
C
H H
HH H H
H H H
H HH
H H
HH H H
H H H
H HH
1 x 12
C
=
12 x 1
C C
C
C
C
C
Mg
Mg
Mg
24 x 1
=
12 x 2
Activity
Relative Atomic Mass
• The Table shows the mass of various atoms
relative to carbon.
• Calculate their relative atomic mass.
Element
Symbol
Times as heavy as carbon
R.A.M
Helium
He
one third
4
Beryllium
Be
three quarters
9
Molybdenum
Mo
eight
96
Krypton
Kr
seven
84
Oxygen
O
one and one third
16
Silver
Ag
nine
108
Calcium
Ca
three and one third
40
Even Smaller Particles!
• For some time people thought atoms were
the smallest particles and that they could
not be broken into anything smaller.
• We now know that atoms are themselves
made from even smaller and simpler
particles.
• These particles are
– Protons
– Neutrons
– Electrons
How Are the Particles Arranged?
• Protons, neutrons and electrons are
NOT evenly distributed in atoms.
• The protons and neutrons exist in a
dense core called the nucleus.
• Around the outside are very thinly
spread electrons.
• These electrons exist in layers
called shells.
‘Shells’ of electrons
electrons are really very very tiny
so the atom is mostly empty
space.
The Nucleus
a dense core of
protons and
neutrons
containing
nearly all the
mass of the
atom
The Atom: Check It Out!
• Draw a labelled diagram
of the atom showing the
nucleus and labelling
protons, neutrons and
electrons
Properties of Sub-atomic Particles
• There are two properties of sub-atomic
particles that are especially important:
– Mass
– Electrical charge
Particle
Protons
Neutrons
Electrons
Charge Relative Mass
+1
1
0
1
-1
0.0005
Element atoms contain equal numbers of protons and
electrons and so have no overall charge
Atomic Number
• The atom of any particular element always
contains the same number of protons. E.g.
– Hydrogen atoms always contain 1 proton
– Carbon atoms always contain 6 protons
– Magnesium atoms always contain 12 protons
• The number of protons in an atom is known
as its atomic number.
• It is the smaller of two numbers shown in
most periodic tables
12
C
6
Activity
How Many Protons?
• Here is some data about elements
taken from the Periodic Table.
• What is the atomic number of the
elements?
23
56
119
19
27
Na
Fe
Sn
F
Al
11
26
50
9
13
11
26
50
9
13
How Many Protons?
• Note that any element has a definite and
fixed number of protons.
• If we change the number of protons in an
atom then this changes that atom into a
different element.
• Changes in the number of particles in the
nucleus (protons or neutrons) is very rare.
It only takes place in nuclear processes
such as radioactive decay, nuclear bombs
or nuclear reactors.
Mass Number
• The mass of each atom results almost entirely
from the number of protons and neutrons that
are present. (Remember that electrons have a
relatively tiny mass).
• The sum of the number of protons and
neutrons in an atom is the mass number.
Atom
Protons
Neutrons Mass Number
Hydrogen
1
0
1
Lithium
3
4
7
Aluminium
13
14
27
Activity
Mass Number
• Calculate the mass number of the atoms
shown.
MASS NUMBER = Number of protons + number of neutrons
Atom
Protons
Neutrons
Mass Number
Helium
2
2
4
Copper
29
35
64
Cobalt
27
32
59
Iodine
53
74
127
Germanium
32
41
73
Activity
How Many Neutrons
• Calculate the number of neutrons in these
atoms.
Number of Neutrons = mass number - atomic number
Atom
Mass
Atomic Number of
Number Number Neutrons
Helium
4
2
2
Fluorine
19
9
10
Strontium
88
38
50
Zirconium
91
40
51
Uranium
238
92
146
How Many Neutrons 2
• It is not strictly true to say that elements
consist of one type of atom.
• Whilst atoms of a given element always
have the same number of protons, they
may have different numbers of neutrons.
• Atoms that differ in this way are called
isotopes.
Remember: The number of
protons defines
the element
How Many Neutrons 3
• Isotopes are virtually identical in their chemical
reactions. (There may be slight differences in
speeds of reaction).
• This is because they have the same number of
protons and the same number of electrons.
• 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.
• This means that when we react atoms of an
element 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 (3 s.f.)
How Many Electrons
• The atoms of any element have an equal
number of protons and electrons.
Atom
Protons
Neutrons Electrons
Helium
2
2
2
Copper
29
35
29
Cobalt
27
32
27
Iodine
53
74
53
Germanium
32
41
32
..and so
overall
charge
equals
zero
Activity
How Many Electrons
• Fill in the blank columns
Atom
Protons Neutrons Electrons
Boron
5
6
Potassium
19
20
Chromium
24
28
Mercury
80
121
Argon
18
22
Atomic
number
Mass
Number
5
19
5
19
11
39
24
80
18
24
80
18
52
201
40
Note – atomic number is defined as the number of protons
rather than electrons because atoms can lose (or gain)
electrons but do not normally lose protons
Activity
Drag the statements at the top onto the correct
side of the table
Activity
Drag the statements at the top onto the correct
side of the table
Summary: The atom so far!
The nucleus.
• Dense: contains nearly all the mass of the
atom in a tiny space.
• Made up of protons and neutrons.
• Has a positive charge because of the protons.
The electrons.
• Exist thinly spread around the outside of the
atom.
• Very small and light.
• Negatively charged.
• Exist in layers called shells.
• Can be lost or gained in chemical reactions.
How Are Electrons Arranged?
• Electrons are not evenly spread.
• The exist in layers known as shells.
• The arrangement of electrons in these shells is
often called the electron configuration.
1st Shell
2nd Shell
3rd Shell
4th Shell
How Many Electrons per Shell?
• Each shell has a maximum number of
electrons that it can hold.
The maximum
1st Shell: 2 electrons
2nd Shell: 8 electrons
3rd Shell: 8 electrons
Which Shells Do Electrons go into?
• Opposites attract.
• Protons are + and electrons are – charged.
• Electrons will occupy the shells nearest the
nucleus unless these shells are already full.
1st Shell: Fills this first
2nd Shell: Fill this next
3rd Shell: And so on
Working Out Electron Arrangements
1. How many electrons do the element atoms
have? (This will equal the atomic number).
2. Keeping track of the total used, feed them into
the shells working outwards until you have used
them all up.
Drawing neat
diagrams helps you
keep track!
1st Shell: Fills this first
2nd Shell: Fill this next
The Electrons in Carbon
The Electrons in Neon
The Electrons in Silicon
Electrons in Phosphorus
The Electrons in Argon
The Electrons in Sodium
The Electrons in Fluorine
The Electrons in Aluminium
The Electrons in Nitrogen
The Electrons in Sulfur
The Electrons in Oxygen
The Electrons in Chlorine
The Electrons in Magnesium
Ions and Electron Structures
1. Ions are atoms that have either extra
electrons added or electrons removed. e.g.
Atoms
Protons
Electrons
Lose 1 electron
1+ ion
Gain 1 electron
Protons
Electrons
So in ions the
number of
electrons no
longer equals the
number
of protons
1- ion
Protons
Electrons
Positive Ions
•
These are ions formed by the atom losing one or
more electrons.
Lost 1 e-
•
•
•
+
They are called cations. This is because during
electrolysis
(Or
It is nearly they move towards the cathode.
Ions usually
always they are Lost
because
“pussytive”!).
have
2 e2+
metal
OUTER
atoms
that on the ions is equal to theelectron
The
charge
number of
lose
electrons
that the atom
has lost. shells that
Lost 3 e
electrons
3+
are either
completely
In equations the charge is usually shown
above
2+or).else
full
and to the right of the symbol. (E.g. Mg
empty
The Electrons in a Sodium Ion
In the sodium atom
Atomic number = number of protons
Number of electrons = 11
Na
23
= 11 Na
11
Na+
Bye!
Electron
lost
Electron arrangement:
2.8.1 (Incomplete Shell)
Electron arrangement:
2.8 (Full Shells)
The Electrons in a Magnesium Ion
In the magnesium atom
Atomic number = number of protons
Number of electrons = 12
24
= 12 Mg
12
Bye!
Mg2+
Mg
Bye!
Electron arrangement =
2.8.2 (Incomplete shell)
2 electrons
lost
Electron arrangement
2.8 (Full Shells)
Activity
The Lithium Ion
•How many electrons?
3
•How many electrons in the first shell?
2
•How many electrons in the second shell?
1
What electron arrangement?
7
Li
3
2.1
1st Shell = 2: full
2nd Shell = 1: not full
How many electrons to lose?
1
Li
Li+
New electron arrangement?
Include a diagram
2.(0)
Bye!
Activity
The Boron Ion
•How many electrons?
5
•How many electrons in the first shell?
2
•How many electrons in the second shell?
3
What electron arrangement?
11
B
5
2.3
1st Shell = 2: full
2nd Shell = 3: not full
How many electrons to lose?
New electron arrangement?
Bye!
Bye!
3
2.(0)
BB3+
Bye!
Negative Ions.
•
These are ions formed by the atom gaining one
or more electrons.
Gain 1 e
•
They are called anions. This is because during
It is nearly
Ions usually
electrolysis
they
move
towards
the
anode.
always
have
non-metal
OUTER
The
charge
number of
atoms
that on the ions is equal to theelectron
gain
electrons
that the atom has gained.shells that
Gain 3 eelectrons
3are either
completely
In equations the charge is usually shown
above
2- or
and to the right of the symbol. (E.g.. Ofull
). else
empty
Gain 2 e
•
•
-
2-
The Electrons in a Sulphide Ion.
In the sulphur atom
Atomic number = number of protons
Number of electrons = 16
32
= 16
S2-
S
2 electrons
gained
Electron arrangement:
2.8.6 (incomplete shell)
Electron arrangement
2.8.8 (Full shells)
S
16
The Electrons in a Fluoride Ion.
In the fluorine atom
Atomic number = number of protons
Number of electrons = 9
19
=9
F
F2-
F
1 electron
gained
Electron arrangement:
2.8.7 (incomplete shell)
Electron arrangement
2.8.8 (Full shells)
9
Activity
The Oxide Ion
8
•How many electrons?
•How many electrons in the first shell?
2
•How many electrons in the second shell?
6
What electron arrangement?
16
O
8
2.6
1st Shell = 2: full
2nd Shell = 6: not full
How many electrons to gain?
2
OO2-
New electron arrangement?
2.8
Activity
Drag the words at the top to their correct places
in the sentences
Activity
The following sentences are all mixed up. Drag and
drop the Words in red until they are in the correct places
Activity
Word Check
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
6. Which answer best describes the shell
arrangement of the electrons in a sodium
atom?
23
Na
11
A.
B.
C.
D.
2,8,1.
2,2,7.
2,8,8,3.
2,8,8,1.
7. Which answer best describes the shell
arrangement of the electrons in an
oxygen atom?
16
O
A.
B.
C.
D.
2,8,6.
2,8,8.
2,8.
2,6.
8
8. Which of these elements will have
electron shells that are either full or
empty (i.e. not partially full)?
A
14
N
B
7
C
40
Ca
20
40
Ar
18
D
27
Al
13
9. Which answer best describes the shell
arrangement of the electrons in an
oxide ion O2-?
16
O
A.
B.
C.
D.
2,8,6.
2,8,8.
2,8.
2,6.
8