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Learning worksheet on atomic structure

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MYP 3 Worksheet 2.6
LEARNING WORKSHEET ON ATOMIC STRUCTURE
No-one really knows what atoms are like.
Scientists have built up pictures to help them
explain what they observe and measure in
experiments.
These pictures are called
"models". The model we use to explain all we
need to know about atoms from a chemical
point of view is the Rutherford Model, shown
opposite.
cloud of electrons
(negatively charged)
very small nucleus
(positively charged)
THE THREE SUB-ATOMIC PARTICLES
There are three important particles which together make up an atom. Complete the table.
PARTICLE
WHERE
FOUND
proton
in the nucleus
neutron
in the nucleus
electron
around the nucleus
RELATIVE MASS
RELATIVE CHARGE
The electrons have such a small mass that we usually ignore them and think of the neutrons
and protons as providing the overall mass of an atom. The more neutrons and protons in
the nucleus, the heavier the atom is. Neutrons and protons are sometimes called
NUCLEONS, because they are found in the nucleus of the atom.
TWO IMPORTANT NUMBERS
Any individual atom can be identified by two numbers. These are the ATOMIC NUMBER
and the MASS (or NUCLEON) NUMBER. Both these numbers are whole numbers.
ATOMIC NUMBER This is given the symbol Z and it tells us the number of PROTONS.
It is also the number of electrons in the neutral atom.
It identifies which chemical element the atom is.
MASS NUMBER
This is given the symbol A and it tells us the TOTAL number of
PROTONS  NEUTRONS in the nucleus.
This gives us the relative mass of the individual atom.
When we want to describe a particular
atom we usually write these numbers
with the element symbol in the format
shown here for a magnesium atom. We
can see that there are 12 protons.
There must be 12 electrons too. This
atom has 12 neutrons (since 24-12 =12).
MASS NUMBER
(total no. of p + n)
24
ATOMIC NUMBER
(no. of p = no. of e)
12
Mg
element symbol
Page 1
MYP 3 Worksheet 2.6
It has a relative mass of 24.
Each of the boxes below shows an atom of a different element. Write down the name of
each element and the number of protons, electrons and neutrons the atoms contain.
14
N
7
name =
protons =
electrons =
neutrons =
Si
14
Ag
47
207
Fe
238
Pb
82
name =
protons =
electrons =
neutrons =
name =
protons =
electrons =
neutrons =
26
name =
protons =
electrons =
neutrons =
name =
protons =
electrons =
neutrons =
108
56
28
92
U
name =
protons =
electrons =
neutrons =
ISOTOPES
It is possible for atoms of the same chemical element to have different numbers of neutrons.
The number of protons and electrons is the same, so the atoms behave in exactly the same
way chemically. Different numbers of neutrons means there is a different relative mass,
1
H
1
name = hydrogen
protons = 1
electrons = 1
neutrons = 0
12
C
6
name = carbon
protons = 6
electrons = 6
neutrons = 6
2
H
1
name = hydrogen
protons = 1
electrons = 1
neutrons = 1
13
C
6
name = carbon
protons = 6
electrons = 6
neutrons = 7
3
H
1
name = hydrogen
protons = 1
electrons = 1
neutrons = 2
14
C
6
name = carbon
protons = 6
electrons = 6
neutrons = 8
e.g.:Learn this definition of isotopes:Isotopes are atoms of the same element, with the same number of
protons, but with a different number of neutrons and a different mass.
Page 2
MYP 3 Worksheet 2.6
Fill in the details of protons, electrons and neutrons for each of the pairs of isotopes below:16
name =
protons =
electrons =
neutrons =
O
8
0
Sn
Ca
20
50
name =
protons =
electrons =
neutrons =
name =
protons =
electrons =
neutrons =
44
18
120
Ca
20
O
8
name =
protons =
electrons =
neutrons =
118
40
50
name =
protons =
electrons =
neutrons =
Sn
name =
protons =
electrons =
neutrons =
ISOTOPES AND RELATIVE ATOMIC MASS
We saw on page 1 that the protons and neutrons in the nucleus of an atom make up
virtually all the mass of the atom (the electrons only contribute a tiny amount to the total).
The relative atomic mass of an element is obviously related to the total number of neutrons
and protons in the nucleus. Many of the chemical elements have two or more naturally
occurring stable isotopes. Any sample of the element will contain atoms of different mass.
The relative atomic mass is an average which takes into account the masses of the different
stable isotopes as well as the proportion of each one present, so it is not usually a whole
number. Most Periodic Tables show the relative atomic masses of each element rounded to
the nearest whole number, but chlorine (mass 35.5) and sometimes copper (mass 63.5) are
often exceptions.
How to Calculate Relative Atomic Mass
Using chlorine as an example: 75% of Cl atoms have mass 35 and 25% have mass 37 so the
weighted average is:(75  35) + (25  37) = 35.5
100
Relative atomic mass for Cl = 35.5
Do similar calculations for copper and silicon, giving your answers to 1 decimal place.
 70% of Cu atoms have mass 63 and 30% have mass 65.
………………………………………………………………………………………………….
………………………………………………………………………………………………….
 92% of Si atoms have mass 28, 5% have mass 29 and 3% have mass 30.
………………………………………………………………………………………………….
………………………………………………………………………………………………….
Page 3
MYP 3 Worksheet 2.6
WHAT ABOUT THE ELECTRONS?
There is evidence to suggest that the
electrons are arranged around the nucleus
in "shells", a little bit like the skins on an
onion. Each shell can hold only so many
electrons. When one shell is full, the
remaining electrons have to go into the next
shell. We represent the shells by drawing
circles of increasing radius around the
nucleus.
1st shell
(holds 2 electrons)
2nd shell
(holds 8 electrons)
3rd shell
(holds 8 electrons)
The electrons can be represented by dots or crosses on these circles, e.g.
H has only 1 electron
in the 1st shell
He has 2 electrons.
1st shell is now full
Mg has 12 electrons
O has 8 electrons
2 in the 1st shell
6 in the 2nd shell
Ne has 10 electrons
2 in the 1st shell
8 in the 2nd shell
(2 shells now full)
Draw similar diagrams below for the atoms indicated:B (Atomic Number 5)
F (Atomic Number 9)
2 in the 1st shell
8 in the 2nd shell
2 in the 3rd shell
Si (Atomic Number
14)
Electron configuration: This means the arrangement of electrons in shells and can be
written in shorthand as follows:He = 2
O = 2,6
Ne = 2,8
Mg = 2,8,2
Page 4
MYP 3 Worksheet 2.6
ELECTRON CONFIGURATIONS AND THE PERIODIC TABLE
For GCSE Chemistry you need to be able to give the FULL electron configurations for the elements up to and including Calcium (Atomic Number 20). The
Periodic Table below shows the first four Periods (rows). The Atomic Number is given for each element. Remember that this tells you the number of
electrons in the atom. Fill in the electron configurations for every element up to Calcium (Ca). The first three have been done for you.
Group
I
Period
1
Period
2
Period
3
Period
4
Group
II
Group
III
Group
IV
Group
V
Group
VI
Group
VII
H
1
Group
0
He
2
1
2
Li
3
Be
4
B
5
C
6
N
7
O
8
F
9
Ne
10
Na
11
Mg
12
Al
13
Si
14
P
15
S
16
Cl
17
Ar
18
Ga
Ge
As
Se
Br
Kr
2,1
K
19
Ca
20
TRANSITION METALS
Sc Ti V Cr Mn Fe Co Ni Cu Zn
REMEMBER THAT
Each PERIOD (row) corresponds to an electron shell. The Period Number tells you the number of shells occupied by electrons.
The GROUP number tells you the number of electrons in the OUTSIDE shell (except for the Noble Gases – Group 0).
Page 5
MYP 3 Worksheet 2.6
IONS
Atoms sometimes lose or gain electrons when they react with other atoms. This results in
imbalance between the number of protons (positive charges) and the number of electrons
(negative charges). The atom is no longer electrically neutral and so we call it an ION.
POSITIVE IONS
These have LOST electrons.
They have more protons than electrons.
IMPORTANT:
NEGATIVE IONS
These have GAINED electrons.
They have more electrons than protons.
The number of protons and neutrons in the nucleus does NOT change.
It is only the outer electrons that are involved in chemical reactions.
Examples of each type of ion are shown below. Notice that the charge is written on the top
right hand side of the element symbol.
23
24
+
Na
11
2+
16
O
8
Mg
12
magnesium ion
protons = 12
electrons = 10
neutrons = 12
sodium ion
protons = 11
electrons = 10
neutrons = 12
2-
oxide ion
protons = 8
electrons = 10
neutrons = 8
19
F
9
-
fluoride ion
protons = 9
electrons = 10
neutrons = 10
Fill in the table for the following atoms and ions, giving symbols as in the examples above.
ELEMENT
PROTONS
ELECTRONS NEUTRONS
Potassium
19
18
20
Calcium
20
18
20
Bromine
35
35
44
Tin
50
50
69
Chlorine
17
18
20
Aluminium
13
10
14
Copper
29
29
34
Nitrogen
7
10
7
53
54
74
Iodine
Page 6
ATOM OR
ION?
FULL
SYMBOL
1/--страниц
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