“Atomic Structure”
Chemistry- power point adapted from
Charles Page High School
Stephen L. Cotton /R Wolfendale/ Derek
Stacy
Defining the Atom
 OBJECTIVES:
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Describe the structure of an atom
Outline development of atomic models
Atoms/ions-the difference
Using chemical formulas to identify ionic compounds
Distinguish between chemical and nuclear reactions
Compare protons, neutrons and electrons
Describe how Alpha and Beta particles and Gamma
radiation are released from unstable atoms
The Atom- SW pg69 + P 4+
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Everything is made of atoms-WOOD, GLASS, SKIN
They are so small you cannot see them even
with a microscope
There are 118 known types of atoms, 91 that are
natural on earth, the other 27 made in the lab
Atoms are listed smallest to largest on the
periodic table
Each square has the atom, its chemical name
and symbol
(students to receive periodic table to stick in book and blank one
to label later
Year 9 Science
Elements
Pure substances can be further classified as elements and
compounds.
Elements are the building blocks of all other substances.
Elements consist of atoms of the same type and are
represented separately on the periodic table.
Elements cannot be broken down further by chemical
processes.
E.g. calcium, oxygen, zinc, chlorine, carbon, iron, etc
4
Common elements
Phosphorus-P4
Buckyballcarbon 60
Oxygen-O2
Made of 60
Carbon atoms
in the shape of
a soccer ball
Year 9 Science
Molecules
Elements are made up of characteristic atoms and
they can join together to form discrete groups of
atoms called molecules. It is often in a grid or
crystal lattice like structure
Two hydrogen atoms can join with an oxygen atom to form
a water molecule.
H2O
7
Crystal Lattice

Silicon and oxygen atoms form crystal lattices to
make beach sand! Depending on how big the
sand grain-determines how many atoms
Compounds
Year 9 Science
Compounds are pure substances that can be broken down
further by chemical reactions.
E.g. water, sugar, polythene, propane, sodium chloride, etc,
Water (H2O) can be broken down into the elements
hydrogen and oxygen by the process of electrolysis.
water
electricity
→
hydrogen + oxygen
9
Year 9 Science
Compounds
Sodium chloride (NaCl) a compound, called a crystal
lattice, can be broken down into the elements sodium and
chlorine, also by electrolysis.
electricity
sodium chloride
→
10
sodium + chlorine
Year 9 Science
Compounds
Compounds can also be synthesised (man made) by
reacting elements together. A compound is defined as
having more than one type of atom.
When hydrogen is burned in oxygen water is produced.
hydrogen + oxygen
11
→
water
Compounds
Year 9 Science
Compounds have very different properties to those of the
elements that make them up.
Sodium (Na) : very active metal
Chlorine (Cl2): a poisonous gas
Sodium chloride (NaCl): our bodies need for cellular
processes.
+
sodium
→
chlorine
12
sodium chloride
Inside the Atom
Year 9 Science
Atoms
Protons and neutrons exist in the central part of the atom
called the nucleus.
Electrons orbit the nucleus in energy levels called shells.
Electrons cannot exist in the nucleus.
They are much smaller and move very fast
This speed of the electrons forms an electron cloud
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Subatomic Particles
Particle
Charge
Mass (g)
Location
Electron
(e-)
-1
9.11 x 10-28
Electron
cloud
Proton
(p+)
+1
1.67 x 10-24
Nucleus
Neutron
(no)
0
1.67 x 10-24
Nucleus
Year 9 Science
Atoms
The number of protons in the nucleus is
characteristic of the element and is called the
atomic number.
All hydrogen atoms have 1 proton. …
All uranium atoms have 92 protons. Etc.
16
Atomic Number
The number of protons in the nucleus of an
atom= Atomic Number –the number you see on
the periodic table above the symbol/name
Number of protons and neutrons in the atom’s
nucleus is the MASS number
Isotopes
 Frederick
Soddy (1877-1956)
proposed the idea of isotopes in
1912
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Isotopes are atoms of the same element
having different masses, due to varying
numbers of neutrons.

Soddy won the Nobel Prize in
Chemistry in 1921 for his work with
isotopes and radioactive materials.
Isotopes
Sometimes atoms of the same element have
different numbers of neutrons from one anothere.g. Helium has generally 2 protons/2 neutrons
Mass number = 4 in this case
However there is a HELIUM 3
2 PROTONS AND 1 NEUTRON –Mass= 3
When the atomic number is the same but mass
number different for the same element- it is called
an
Isotope
Isotopes cont:
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Almost all elements have two isotopes
Some have many more
Most common isotope is Hydrogen 1 = 1 proton in
nucleus and no neutron
Makes up 99.98 % of the hydrogen on earth-called
protium
Hydrogen 2 is deuterium, and Hydrogen 3 = tritium
It is the isotopes of hydrogen that are used in nuclear
power plants to generate power more efficiently
Isotopes are atoms of the same element having
different masses, due to varying numbers of
neutrons.
Isotope
Protons Electrons
Neutrons
Hydrogen–1
(protium)
1
1
0
Hydrogen-2
(deuterium)
1
1
1
1
1
2
Hydrogen-3
(tritium)
Nucleus
A look back then!Defining the Atom

The Greek philosopher Democritus (460
B.C. – 370 B.C.) was among the first to
suggest the existence of atoms (from
the Greek word “atomos”)
 He
believed that atoms were indivisible and
indestructible
 His ideas did agree with later scientific
theory, but did not explain chemical
behaviour, and was not based on the
scientific method – but just philosophy
Thomson’s Atomic Model
J. J. Thomson
Thomson believed that the electrons
were like plums embedded in a
positively charged “pudding,” thus it
was called the “plum pudding” model.
Dalton’s Atomic Theory (experiment based!)
John Dalton
(1766 – 1844)
1) All elements are composed of
tiny indivisible particles called
atoms
2) Atoms of the same element are
identical. Atoms of any one
element are different from
those of any other element.
3) Atoms of different elements combine in
simple whole-number ratios to form
chemical compounds
4) In chemical reactions, atoms are combined,
separated, or rearranged – but never
changed into atoms of another element.
Sizing up the Atom
 Elements are able to be subdivided into
smaller and smaller particles – these are
the atoms, and they still have properties
of that element
If you could line up 100,000,000
copper atoms in a single file, they
would be approximately 1 cm long
Despite their small size, individual
atoms are observable with instruments
such as scanning tunneling (electron)
microscopes
Section 4.2
Structure of the Nuclear Atom
 One
change to Dalton’s atomic
theory is that atoms are divisible
into subatomic particles:
 Electrons,
protons, and neutrons are
examples of these fundamental
particles
 There are many other types of
particles, but we will study these three
Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray
tube to deduce the presence of a negatively
charged particle: the electron
Modern Cathode Ray Tubes
Television
Computer Monitor
Cathode ray tubes pass electricity
through a gas that is contained at a
very low pressure.
Mass of the Electron
Mass of the
electron is
9.11 x 10-28 g
The oil drop apparatus
1916 – Robert Millikan determines the mass
of the electron: 1/1840 the mass of a
hydrogen atom; has one unit of negative
charge
Conclusions from the Study
of the Electron:
a) Number of electrons = number of
protons: This means atoms have a
neutral charge because = +’ve charges =
- ‘ve charges
b) Electrons have so little mass = they are
so small that atoms contain electron
clouds (all the electrons flying round the
nucleus) these can be huge -1000 X
wider than the nucleus
c) Most of an atom is therefore empty space
Conclusions from the Study
of the Electron:
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In New Zealand a scientist Earnest
Rutherford discovered that the nucleus
took up such a small space inside the
atom
He fired a beam of helium nuclei)alpha
particles, into a piece of gold foil – most
passed through to his surprise! Some
deflected back
Up till now most scientist thought atoms
were quite solid
Ernest Rutherford’s
Gold Foil Experiment - 1911
Alpha particles are helium nuclei The alpha particles were fired at a thin
sheet of gold foil
 Particles that hit on the detecting
screen (film) are recorded
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Electrons…..
The electrons are attracted to the nucleus by the protons in the
nucleus because of their opposite charges
Why don’t they “fall into the nucleus”?....they are trapped in
electron shells
These are a little like layers on an onion
Many of the shells in an atom are empty as obviously many atoms
do not have many electrons
Electrons have set numbers than can be held in each shell…i.e the
first shell and smallest holds two
The next can hold 8, the third is 18, 4th is 32 and so on…works on a
formula 2n squared
The number of electrons in each shell is the ELECTRON
CONFIGURATION
I.E Sodium is 2:8:1 as it has 11 electrons…..
Structure of the Nuclear Atom
 Practical
pg 11 (P)
Experimenting like Rutherford
Describe
the structure of
atoms, according to the
Rutherford atomic model.
Some revision- before Ions!
Complete Symbols
Element, the mass number and the
atomic number.
Superscript →
Mass
number
Subscript →
Atomic
number
X
Symbols
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Find each of these:
a) number of protons
b) number of
neutrons
c) number of
electrons
d) Atomic number
e) Mass Number
80
35
Br
Symbols
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If an element has an atomic
number of 34 and a mass
number of 78, what is the:
a) number of protons
b) number of neutrons
c) number of electrons
d) complete symbol
Symbols
 If an element has 91
protons and 140 neutrons
what is the
a) Atomic number
b) Mass number
c) number of electrons
d) complete symbol
Symbols
 If an element has 78
electrons and 117 neutrons
what is the
a) Atomic number
b) Mass number
c) number of protons
d) complete symbol