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ATOMS
Everything that surrounds us is composed of matter. Chemistry is the scientific study of the substances that
make up the universe. These substances may be found as ELEMENTS, COMPOUNDS or MIXTURES.
These substances may under go PHYSICAL or CHEMICAL changes.
SOME DEFINITIONS:
MATTER
is everything that has mass and occupies space.
ELEMENTS
are pure substances that cannot be split up into simpler substances by
chemical means. Each element has its own symbol which is found on the
periodic table.
COMPOUNDS
are substances that are formed when two or more elements are joined
together chemically so that their properties are changed.
MIXTURES
are found when two or more substances are mixed together, but not
chemically joined to each other.
PHYSICAL CHANGE
occurs when a phase change takes place (ie: solid  liquid  gas), or when
a substance changes shape, is crushed, broken into pieces, or heats up or
cools down.
CHEMICAL CHANGE
is the result of a chemical reaction when a new substance with new
properties is formed (eg: reddish copper metal + yellow chlorine gas 
blue/green copper chloride salt)
ELEMENTS
Altogether 118 different elements are currently known. Only 98 of these are naturally occurring and have
been found in the earth’s crust and atmosphere by scientists – some only in very small trace amounts. The
other 30 have been artificially made by scientists in laboratories and atomic reactors.
ATOMS
All matter exists in the form of extremely small particles called atoms. An atom is the smallest particle of an
element that carries the characteristics of that particular element.Over the years, scientists have produced
many models of the atom in an attempt to explain what they look like.
THE HISTORY OF THE ATOMIC MODEL
YEAR
SCIENTIST MODEL
400BC Democritus
Atomic
1803
Dalton
Billiard
Ball
1898
Thompson
Plum
pudding
1910
Rutherford
Nuclear
1913
Bohr
Planetary
ADVANCES
Matter is made up of indivisible particles
Experimental evidence of conservation of mass
All atoms of a given element are identical,
but different to the atoms of another element.
Compounds form from atoms of more than one
element and only whole atoms can combine.
In a chemical reaction, atoms can be
separated or combined but never destroyed
Evidence of two types of sub-atomic particles.
Negatively charged electrons (plums) were
embedded in a positively charged pudding
Positive protons concentrated in small nucleus.
Mass concentrated in nucleus.
Atom consists mainly of empty space
Volume due to motion of negative electrons.
Electrons found in fixed orbits.
Electrons with lower energy found in orbits
closer to nucleus.
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1932
Chadwick
Discovered neutrons
The present day atomic model has been developed far beyond our understanding at Year 8 level, so we
use a combination of the older models to develop our concept of the atom.
THE STRUCTURE OF THE ATOM
The atom consists of a central nucleus containing protons and neutrons. Orbiting around the nucleus are
electrons.
neutron
proton
electron shell
electron
THE STRUCTURE OF THE ATOM
Atoms are the building blocks for all matter. Each element is characterised by having atoms that are all the
same as each other, but different to the atoms found in any other element.
Atoms can be divided up further into smaller sub-atomic particles. There are three types: Protons,
electrons and neutrons. Over many years, scientists have gradually built up a model of what the atom
probably looks like. This model is constantly being improved upon as new discoveries are made. It is
important to realise that this model is most likely a pretty accurate theory, although it is possible that it will
be refined and developed further as more is discovered.
Protons:
 Found in the nucleus, tightly packed together with the neutrons
 Have a charge of +1
 The number of protons found = the Atomic Number. It identifies the element
Neutrons:
 Found in the nucleus, tightly packed together with the protons.
 Are neutral – ie: they have no charge.
 The number of (protons + neutrons) = the Atomic Mass. They are responsible for the mass of the
atom.
Electrons:
 Found orbiting the nucleus. The space in which they move will determine the size (volume) of the
atom.
 Have a charge of 1
 In a neutral atom, the number of protons = the number of electrons.
 The more energy an electron has, the further it will be found from the nucleus.
Note: the Atomic Number is always the smaller number, while the Atomic Mass is always the larger
number
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Fig 2.1.9
UNIT
2 .1
Common molecules
MOLECULES:
Molecules form between atoms of the same element (eg: O 2, H2, N2) , or between atoms of different
elements (eg: H2O, CO2, NH3 HCl). Some groups of atoms share electrons and form molecules. Others
lose or gain electrons, forming positive and negative ions which then attract each other and arrange
structures
called lattices.
Speciallattice
forces (eg:
called
atomic
themselves
into a crystal
salts
such as NaCl, NaOH, CuSO4, AgNO3).
bonds hold the atoms together. Water is perhaps the
most famous molecule—each water molecule is made
up of two hydrogen atoms and one oxygen atom,
hence the familiar chemical symbol for water—H2O.
A compound consists of a number of identical
molecules or a lattice containing different atoms
joined or ‘bonded’ together. A glass of the compound
water contains billions of water molecules. The
compound sodium chloride (table salt) consists of
grains made up of a lattice of sodium and chlorine
atoms held together by atomic bonds.
lattice
molecular
oxygen
atom
hydrogen
atom
water (H2O)
chloride atom
sodium
atom
sodium chloride (NaCl)
Two types of compound—molecular and lattice
Fig 2.1.10
Millions of compounds are possible since there are
so many ways the 100 or so different types of atoms
can be combined.
Compounds usually have quite different properties
(eg colour, texture, smell, density) than the elements
whose atoms they contain. For example, the
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CHEMICAL REACTIONS
A chemical reaction involves an interaction between the electrons of two or more atoms. At the end of a
chemical reaction, a new substance is formed with different properties to the properties of the reactants.
Eg:
Hydrogen (H2)
– gas at room temperature – highly flammable
Oxygen (O2)
– a gas at room temperature – supports combustion
Water (H2O)
- liquid at room temperature
- used to put out fires
Evidence that a chemical reaction has occurred:
 Heat is produced or absorbed
 A colour change is observed
 Bubbles/gas forms (odour may be detected)
 A solid is formed – may be a precipitate
 One metal deposits onto another (electroplating)
NB:
 Changes of state, temperature, size and shape are all physical changes, not chemical changes. Eg:
ice, liquid water and water vapour are all H2O and have the same chemical structure and properties.
Some Types of chemical reactions and examples we’ve studied:
SYNTHESIS / COMBINATION
Magnesium + oxygen  magnesium oxide
DECOMPOSITION
Hydrogen peroxide  water plus oxygen
PRECIPITATION / Exchange
Copper nitrate + sodium hydroxide  sodium nitrate + copper hydroxide 
(one of the products is insoluble)
Copper + silver nitrate  copper nitrate + silver
DISPLACEMENT
Iron + copper sulfate  iron sulphate + copper
COMBUSTION
Magnesium + oxygen  magnesium oxide
Iron + oxygen  iron oxide (rust)
CORROSION
Reactions may be Exothermic or Endothermic, Spontaneous or Non-spontaneous.
 Exothermic: Reactants  Products + Energy (eg: combustion reactions)
 Endothermic: Reactants + Energy  Products (eg: the thermal decomposition of potassium
permanganate)
 Spontaneous: happens without the input of extra energy at room temperature.
 Non-spontaneous: only happens with the input of extra energy.
Tests for the Three Common Gases:
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Carbon dioxide (CO2)
Carbon dioxide gas will turn lime water milky.
Reaction to prduce CO2:
calcium carbonate + hydrochloric acid  carbon dioxide + calcium chloride + water
CaCO3
+
2HCl

CO2
+
CaCl2
+ H2O
Hydrogen (H2)
Hydrogen gas will extinguish a flame with a squeaky
pop.
Reaction to produce H2:
Magnesium + hydrochloric acid  hydrogen + magnesium chloride
Mg
+
2HCl

H2
+
MgCl2
Oxygen (O2)
oxygen gas will re-ignite a glowing splint.
Reaction to produce O2:
(manganese dioxide)
Hydrogen peroxide
2H2O2
(MnO2)
oxygen + water
O2
+ H2O
(MnO2 is a catalyst and does not get used up in the reaction)
FACTORS AFFECTING REACTION RATES:
Rates of reaction are affected by the following four factors:
Factor
Increased
Temperature
Concentation
Surface Area
Catalyst / Enzyme
Hotter speeds up
Cooler slows down
More concentrated  speeds up
Less concentrated  slows down
Many, smaller particles  speeds up
Fewer, larger particles  slows down
The presence of a catalyst or enzyme speeds up the reaction rate
Definitions:
CATALYST:
Decreased
Speeds up a chemical reaction, without being used up in the reaction. There
will be exactly the same amount of catalyst present at the end of the reaction as
there was before the reaction began.
Eg: iron filings (Fe) and manganese dioxide (MnO2) are both catalysts that will speed
up the decomposition of hydrogen peroxide (H2O2):
(catalyst)
Hydrogen peroxide
2H2O2
(MnO2 or Fe)
ENZYME:
oxygen + water
O2 + H2O
Is a catalyst that is made by a living organism (ie: a plant, animal, bacteria or
fungus). We call it an organic catalyst.
Eg: enzymes found in animal liver or apple pieces will greatly increase the rate of
decomposition of hydrogen peroxide:
(Enzyme)
Hydrogen peroxide
2H2O2
oxygen + water
O2
+ H2O