Download UNIT 4 ATOMIC THEORY 1. Atomic theory: Dalton`s model

UNIT 4
ATOMIC THEORY
1. Atomic theory:
Dalton’s model
Thomson’s model
Rutherford’s model
Bohr’s model
Electron cloud model
2. Particles inside the atom
Atomic number
Mass number
3. Ions
Cations
Anions
4. Isotopes
5. Relative atomic mass
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1. ATOMIC THEORY
Because we cannot see atoms, we use models to teach and learn about them.
The atomic theory has changed over time as new technologies have become available.
Scientific knowledge builds on past research and experimentation.
Dalton’s model.
Scientist
Information
John
Dalton said that:
All matter was made of atoms.
Atoms were too small to be seen, indivisible and
indestructible.
All atoms of a given element were identical.
Compounds were formed by the joining of two or more
elements
Dalton
Model
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Thomson’s model
Scientist
Information
J.J
Discovered the negative particles called electrons, and
Thompson
predicted that there also must be a positive charge to hold
Model
the electrons in place.
He said that atoms were made from a positively charged
substance with negatively charged electrons scattered
about, like raisins in a pudding.
Rutherford’s model
Rutherford’s experiment involved firing a stream of tiny positively charged particles at a thin sheet
of gold foil (2000 atoms thick)
Most of the positively charged “bullets” passed right through the gold atoms in the sheet of gold foil
without changing their course at all.
Some of the positively charged “bullets,” however, did bounce off away from the gold sheet as if they
had hit something solid.
Few particles bounced back.
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He knew that positive charges repel positive charges.
This could only mean that the gold atoms in the sheet were mostly open space. Atoms were not a
pudding filled with a positively charged material.
Rutherford concluded that an atom had a small, dense, positively charged center that repelled his
positively charged “bullets.”
He called the center of the atom the “nucleus”.
The nucleus is tiny compared to the atom as a whole.
Rutherford reasoned that all the positively charged particles were contained in the nucleus. The
negatively charged particles were scattered outside the nucleus around the atom’s edge.
After Rutherford had stated his model, Chadwick discovered that in the nucleus there were also
other particles called neutrons that have no charge.
Scientist
Ernest
Rutherford
Information
Model
Discovered the nucleus of an atom.
Named the positive particles in the nucleus “protons”.
He said that electrons were scattered in empty space
around the nucleus.
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Bohr’s model
Scientist
Information
Neils Bohr
Concluded that electrons are located in planet-like orbits
Model
around the nucleus in certain energy levels.
Electron cloud model
Scientist
Information
(Many Scientists!)
Nowadays we know that:
The Modern
Electrons do not orbit the nucleus in neat planet-like
Atomic Theory
orbits but move at high speeds in an electron cloud
Model
around the nucleus.
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2. PARTICLES INSIDE THE ATOM
An atom is the smallest piece of matter that still retains the properties of the element.
Three subatomic particles make up every atom:
Subatomic Particle
Charge
Location
Proton
Positive (+)
Nucleus
Neutron
No Charge (0)
Nucleus
Electron
Negative (-)
Shell-Electron Cloud
ATOMIC NUMBER:
It is the number of protons in an element.
It defines the element. (it never changes)
MASS NUMBER:
It is the sum of the number of protons and neutrons.
Knowing the atomic number and the mass number we can deduce the number of protons, electrons and
neutrons and the other way round.
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3.IONS
As the atom is neutral it has the same number of protons as electrons so if we know the atomic
number we know the number of protons and electrons of the atom.
Sometimes an atom can gain or lose electrons, so that they became charged atoms, then we call them
IONS
When a neutral atom gains electrons from another atom it becomes a negative ion called
ANION
When a neutral atom loses electrons it becomes a positive ion called CATION.
The charge of the ion depends on how many electrons it has gained or lost.
To represent ions the charge is written in the top right corner of the symbol of the element:
X+2
4.ISOTOPES
ISOTOPES are atoms with the same number of protons, but different number of neutrons.
They are atoms of the same element (same atomic number) with a different mass number.
Despite these differences, isotopes are chemically the same because they have identical numbers of
protons and electrons.
Alike= similar
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Examples:
Isotopes of Neon
Isotopes of Hydrogen
5. RELATIVE ATOMIC MASS
The relative atomic mass is the mass of an atom when compared to a standard atom. It is
represented by Ar. It has no unit.
Why do we use the relative atomic mass?
The mass of an atom is too small to be measured in grams.
We can only measure the mass of an atom by comparing its mass to the mass of another atom.
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At the beginning all the atoms of the elements were compared with the hydrogen atom because it was
the smallest.
At present, carbon-12 is used as the standard of comparison because:
It is solid (and can be easily handled)
Its mass can be more easily measured (it is very stable)
Carbon-12 is easily found as its compounds are a lot.
Carbon-12 isotopes are assigned a mass of exactly 12 units.
The RELATIVE ATOMIC MASS of an element is the mass of one atom of the element when
compared to 1/12 of the mass of an atom of carbon -12.
Examples:
An atom of oxygen is 16 times heavier than 1/12 of the mass of an atom of carbon-12. Its relative
atomic mass is 16.
An atom of sodium is 23 times heavier than 1/12 of the mass of an atom of carbon-12. Its relative
atomic mass is 23.
To calculate the relative atomic mass of an element that has several isotopes, multiply the relative
mass of each isotope by its natural abundance, expressed as a decimal, and then add all the products.
Atomic mass is the average of all the isotopes of an element.
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UNIT4
ACTIVITIES
A1-PARTICLES IN THE ATOM
Calculate the number of protons, neutrons and electrons of these elements:
Element
Z( atomic number)
A(Mass number)
Na, sodium
11
23
Li, lithium
3
7
K,potassium
19
39
Al, aluminium
13
27
Br, bromine
35
80
Ar,argon
18
40
Protons
Electrons
Neutrons
Protons
Electrons
Neutrons
A2- PARTICLES IN THE ATOM
Complete the following table:
Element
Z( atomic number)
A(Mass number)
Ca
F
20
9
Sc
Fe
P
20
10
45
21
26
30
31
15
95
Be
9
4
A3-
A4- IONS
a) How many electrons does an atom of Mg have to lose to become the cation Mg2+?
b) How many electrons does an atom of Cl have to gain to become the anion Cl-1?
c) Does an atom of Ca have to gain or lose electrons to form the ion Ca2+?
d) Does an atom of H have to gain protons or lose electrons to form the ion H+?
A5-IONS
Complete the table:
Ion
Z
A
Li+
3
7
O2-
8
16
Fe3+
26
56
Sr 2+
38
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I-
53
127
Protons
Electrons
neutrons
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Zn2+
30
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A6-ISOTOPES
Say which of the following atoms are isotopes of the same element:
Atom
Protons
Electrons
Neutrons
A
6
6
6
B
17
17
18
C
5
5
5
D
6
6
7
E
4
4
5
F
17
17
20
A7-ISOTOPES
Calculate the relative atomic mass of the element chlorine . You know that it has two isotopes
and the abundance of each one is:
Cl-35
75,5%
Cl-37
24,5%
A8-RELATIVE ATOMIC MASS
Practice in your group the next sentence using different elements in order to learn it:
An atom of oxygen is 16 times heavier than 1/12 of the mass of an atom of carbon-12. Its relative
atomic mass is 16.
You can practise with:
Hydrogen 1, Helium 4, Lithium 7, Beryllium 9, Boron 10, Carbon 12, Nitrogen 14, Oxygen 16,Fluorine
19, Neon 20.
Its relative
time heavier than 1/12 of the
An atom of
is
atomic mass
mass of an atom of carbon-12.
is
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Try to say it following this chart
An atom of
is
times...................................of the
mass of......................................
Its
...................
is
Then, try to say it following this chart
An atom of
is
times
its
A9-Bingo activity for the review of the unit.
Thomson
Rutherford
Dalton
Bhor
atomic
number
mass number
electrons
neutrons
anion
isotope
nucleus
ion
cation
shell
neutral atom
protons
You have to draw a grid with 9 squares, then choose nine words from the above and write one in each
square of the grid. The teacher reads one by one the following definitions randomly.
You cross the word that corresponds with the definition until you have all of them crossed. That is
full house!
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UNIT 2
EXERCISES
1
2
3
99
4
5
6
100
7
8
9
101
10
11
12
13
Using the information in the unit. Calculate the relative atomic mass of Neon
14
Using the information in the unit. Calculate the relative atomic mass of Hydrogen
102
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