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ATOMS, ELEMENTS, COMPOUNDS AND BONDING
o E.g. Carbon 12 and Carbon 14.
o Two types: non-radioactive isotopes and radioactive.
o Radioactive isotopes are unstable atoms that break down giving radiations
o Medical use: cancer treatment (radiotherapy) – rays kill cancer cells using
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cobalt-60
Industrial use: to check for leaks – radioisotopes (tracers) added to oil/gas. At leaks
radiation is detected using a Geiger counter.
Isotopes of the same element all have the same number of electrons, and
therefore all have the same number of electrons in their outer shells. This means
that isotopes of the same element all have the same chemical properties. The
chemical properties of elements depend on the number of electrons in the outer
shell of their atoms.
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The protons and neutrons exist in the centre of the atom in a dense region called the
nucleus. The electrons move around the nucleus and exist in electron shells at
increasing distances from the nucleus.
Examiner’s tip
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In some books, the two numbers may be reversed. It is a good idea to remember
that the nucleon number is always higher than the proton number (with the
exception of hydrogen, in which case both numbers are 1 in the most abundant
isotope).
Atoms are often represented as shown in Figure above. The proton number is the
number of protons in one atom of the element. Because atoms do not have a charge,
the number of protons in an atom is always equal to the number of electrons.
Elements in the Periodic Table are arranged in order of increasing proton number. This
means that as we move from one element to the next element, the atoms have one
extra proton in the nucleus and one extra electron. The extra electron goes into the
outer shell until the outer shell is full. The next shell then begins to fill up. Elements in
the same group all have the same number of electrons in the outer shell of their atoms.
This applies beyond the first 20 elements
Examples are:
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All Group (I) elements have 1 electron in the outer shell.
All Group (II) elements have 2 electrons in the outer shell.
All Group (VII) elements have 7 electrons in the outer shell.
All Group (0) elements have a full outer shell.
Most metallic elements have 1, 2 or 3 electrons in their outer shell.
Most non-metallic elements have 5, 6 or 7 electrons in their outer shell (or a full outer
shell in the case of Noble gases).
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BONDING
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Formulae of Ionic Compounds
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Some sample questions on how to determine the formula of an
ionic compound from a lattice diagram
Q11: The diagram below is a diagram of magnesium oxide. Determine its formula
from the diagram
Answer: MgO .
The ratio of magnesium to oxygen is 1 to 1.
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Key
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Pink = Magnesium ion
Yellow = Oxygen ion
ANSWER: Option c
Ratio of Cd:Cl is 1:2
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A shared pair of electrons is known as a single (covalent) bond.
Eg: H2
Double bonds (two shared pairs of electrons)
Eg: O2
Triple bonds (three shared pairs of electrons) also exist.
Eg: N2
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Atoms which form a covalent bond join together to form uncharged molecules. All the
atoms involved achieve a full outer shell of electrons.
Common errors•
In exam questions which ask why substances with simple molecular structures
have low melting points and boiling points, it is very commonly said that this is
because covalent bonds are weak. This is a bad error. All covalent bonds are
strong bonds.
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The correct answer is that intermolecular forces are weak which is why
substances with simple molecular substances are either solids with low melting
points such as iodine, liquids such as water or gases such as carbon dioxide.
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GRAPHITE
SILICON DIOXIDE(SiO2)
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DIAMOND
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Both diamond and graphite have giant covalent (macromolecular) structures, but
because there are differences in their structure and bonding, these lead to
differences in properties and uses.
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Silicon (IV) oxide (silicon dioxide)
Silicon (IV) oxide (silicon dioxide) has a giant covalent (macromolecular) structure.
Each silicon atom is covalently bonded to four oxygen atoms. The bonds are directed
tetrahedrally. Each oxygen atom is covalently bonded to two silicon atoms. All the
bonds in silicon (IV) oxide are strong covalent bonds. There are no mobile electrons
present. Because of its structure and bonding, silicon (IV) oxide is strong, hard, has a
high melting and boiling point and is a non-conductor of electricity. These properties
are like those of diamond which has a very similar structure and the same bonding.
COMPARING STRUCTURES OF DIAMOND AND SILICON (IV) OXIDE
The structure of diamond
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The giant covalent structure of diamond
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Carbon has an electronic arrangement of 2,4. In
diamond, each carbon shares electrons with four
other carbon atoms - forming four single bonds.
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In the diagram some carbon atoms only seem to be
forming two bonds (or even one bond), but that's
not really the case. We are only showing a small bit
of the whole structure.
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This is a giant covalent structure - it continues on
and on in three dimensions. It is not a molecule,
because the number of atoms joined up in a real
diamond is completely variable - depending on the
size of the crystal.
The physical properties of diamond
Diamond


Has a very high melting point (almost 4000°C). Very strong carboncarbon covalent bonds have to be broken throughout the structure
before melting occurs.
Is very hard. This is again due to the need to break very strong covalent
bonds operating in 3-dimensions.

Does not conduct electricity. All the electrons are held tightly between
the atoms, and aren't free to move.
The structure of silicon dioxide, SiO2
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Silicon dioxide is also known as silicon (IV) oxide.
The giant covalent structure of silicon dioxide
There are three different crystal forms of silicon
dioxide. The easiest one to remember and draw is
based on the diamond structure.
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Crystalline silicon has the same structure as
diamond. To turn it into silicon dioxide, all you need
to do is to modify the silicon structure by including
some oxygen atoms.
Notice that each silicon atom is bridged to its
neighbours by an oxygen atom. Don't forget that
this is just a tiny part of a giant structure extending
on all 3 dimensions.
The physical properties of silicon dioxide
Silicon dioxide

Has a high melting point - around 1700°C.
Very strong silicon-oxygen covalent bonds
have to be broken throughout the structure
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
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
before melting occurs.
Is hard. This is due to the need to break the
very strong covalent bonds.
Does not conduct electricity. There aren't
any delocalized electrons. All the electrons
are held tightly between the atoms, and
aren't free to move.
Is insoluble in water and organic solvents.
There are no possible attractions which
could occur between solvent molecules and
the silicon or oxygen atoms which could
overcome the covalent bonds in the giant
structure.
All metallic elements have giant metallic structures. They contain positive ions
surrounded by a mobile sea of electrons. Metals are held together by the strong forces
of attraction between positive ions and the mobile sea of electrons known as metallic
bonds.
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Examiner’s tip
In exam questions which ask for the meaning of metallic bonding, students
usually describe the giant metallic structure but do not mention the strong forces
of attraction between positive ions and the mobile sea of electrons, known as
metallic bonds.
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EXPLAINING CERTAIN PROPERTIES OF METALS
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PRACTICE QUESTIONS FOR BONDING DIAGRAMS:
Draw the dot cross diagrams for the following ionic compounds:
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Magnesium chloride
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Chemical Formula-
Calcium Oxide
Chemical Formula-
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Aluminium fluoride
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Chemical Formula-
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Aluminium oxide
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Chemical Formula-
Draw the dot cross diagrams for the following covalent molecules:
Oxygen
Chemical Formula-
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Nitrogen
Chemical Formula-
Chlorine
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Hydrogen Chloride
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Chemical Formula-
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Chemical Formula-
Methane
Chemical Formula-
Ammonia
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Chemical Formula-
Water
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Carbon Dioxide
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Chemical Formula-
Chemical Formula-
Ethene
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Chemical Formula-
Methanol
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Chemical Formula-