- Created by: mohammed786159ismail
- Created on: 14-12-18 13:29
The electrons in an atom occupy the lowest available energy levels
(innermost available shells). The electronic structure of an atom can be
represented by numbers or by a diagram. For example, the electronic
structure of sodium is 2,8,1 or
The periodic table
The elements in the periodic table are arranged in order of atomic
(proton) number and so that elements with similar properties are
in columns, known as groups. The table is called a periodic table
because similar properties occur at regular intervals.
Elements in the same group in the periodic table have the same
number of electrons in their outer shell (outer electrons) and this gives
them similar chemical properties.
Development of the periodic table
Before the discovery of protons, neutrons and electrons, scientists
attempted to classify the elements by arranging them in order of their
The early periodic tables were incomplete and some elements were
placed in inappropriate groups if the strict order of atomic weights
Mendeleev overcame some of the problems by leaving gaps for
elements that he thought had not been discovered and in some
places changed the order based on atomic weights.
Elements with properties predicted by Mendeleev were discovered
and filled the gaps. Knowledge of isotopes made it possible to explain
why the order based on atomic weights was not always correct.
Metals and non-metals
Elements that react to form positive ions are metals.
Elements that do not form positive ions are non-metals.
The majority of elements are metals. Metals are found to the left
and towards the bottom of the periodic table. Non-metals are found
towards the right and top of the periodic table.
There are three types of strong chemical bonds: ionic, covalent and
metallic. For ionic bonding the particles are oppositely charged ions.
For covalent bonding the particles are atoms which share pairs of
electrons. For metallic bonding the particles are atoms which share
Ionic bonding occurs in compounds formed from metals combined
Covalent bonding occurs in most non-metallic elements and in
compounds of non-metals.
Metallic bonding occurs in metallic elements and alloys.
When a metal atom reacts with a non-metal atom electrons in the
outer shell of the metal atom are transferred. Metal atoms lose
electrons to become positively charged ions. Non-metal atoms gain
electrons to become negatively charged ions. The ions produced by
metals in Groups 1 and 2 and by non-metals in Groups 6 and 7 have
the electronic structure of a noble gas (Group 0).
The electron transfer during the formation of an ionic compound can
be represented by a dot and cross diagram, eg for sodium chloride.Students should be able to draw dot and cross diagrams for ionic
compounds formed by metals in Groups 1 and 2 with non-metals in
Groups 6 and 7.
The charge on the ions produced by metals in Groups 1 and 2 and
by non-metals in Groups 6 and 7 relates to the group number of the
element in the periodic table.
Students should be able to work out the charge on the ions of metals
and non-metals from the group number of the element, limited to the
metals in Groups 1 and 2, and non-metals in Groups 6 and 7.
An ionic compound is a giant structure of ions. Ionic compounds are
held together by strong electrostatic forces of attraction between
oppositely charged ions. These forces act in all directions in the lattice
and this is called ionic bonding.
The structure of sodium chloride can be represented in the following
When atoms share pairs of electrons, they form covalent bonds.
These bonds between atoms are strong.
Covalently bonded substances may consist of small molecules.
Students should be able to recognise common substances that
consist of small molecules from their chemical formula.
Some covalently bonded substances have very large molecules, such
Some covalently bonded substances have giant covalent structures,
such as diamond and silicon dioxide.
Metals consist of giant structures of atoms arranged in a regular
The electrons in the outer shell of metal atoms are delocalised and
so are free to move through the whole structure. The sharing of
delocalised electrons gives rise to strong metallic bonds. The bonding
in metals may be represented in the following form:
The three states of matter
The three states of matter are solid, liquid and gas. Melting and
freezing take place at the melting point, boiling and condensing take
place at the boiling point.
The three states of matter can be represented by a simple model. In
this model, particles are represented by small solid spheres. Particle
theory can help to explain melting, boiling, freezing and condensing.
The amount of energy needed to change state from solid to liquid
and from liquid to gas depends on the strength of the forces between
the particles of the substance. The nature of the particles involved
depends on the type of bonding and the structure of the substance.
The stronger the forces between the particles the higher the melting
point and boiling point of the substance.
(HT only) Limitations of the simple model above include that in the
model there are no forces, that all particles are represented as spheres
and that the spheres are solid.
In chemical equations, the three states of matter are shown as (s), (l)
and (g), with (aq) for aqueous solutions
Properties of ionic compounds
Ionic compounds have regular structures (giant ionic lattices) in which
there are strong electrostatic forces of attraction in all directions
between oppositely charged ions.
These compounds have high melting points and high boiling points
because of the large amounts of energy needed to break the many
When melted or dissolved in water, ionic compounds conduct
electricity because the ions are free to move and so charge can flow.
Knowledge of the structures of specific ionic compounds other than
sodium chloride is not required.
Properties of small molecules
Substances that consist of small molecules are usually gases or
liquids that have relatively low melting points and boiling points.
These substances have only weak forces between the molecules
(intermolecular forces). It is these intermolecular forces that are
overcome, not the covalent bonds, when the substance melts or boils.
The intermolecular forces increase with the size of the molecules, so
larger molecules have higher melting and boiling points.
These substances do not conduct electricity because the molecules
do not have an overall electric charge.
Students should be able to use the idea that intermolecular forces are
weak compared with covalent bonds to explain the bulk properties of
Giant covalent structures
Substances that consist of giant covalent structures are solids with
very high melting points. All of the atoms in these structures are
linked to other atoms by strong covalent bonds. These bonds must
be overcome to melt or boil these substances. Diamond and graphite
(forms of carbon) and silicon dioxide (silica) are examples of giant
Students should be able to recognise giant covalent structures from
diagrams showing their bonding and structure.
In diamond, each carbon atom forms four covalent bonds with other
carbon atoms in a giant covalent structure, so diamond is very hard,
has a very high melting point and does not conduct electricity
In graphite, each carbon atom forms three covalent bonds with three
other carbon atoms, forming layers of hexagonal rings which have no
covalent bonds between the layers.
In graphite, one electron from each carbon atom is delocalised.
Students should be able to explain the properties of graphite in terms
of its structure and bonding.
Students should know that graphite is similar to metals in that it has
Graphene and fullerenes
Graphene is a single layer of graphite and has properties that make it
useful in electronics and composites.
Students should be able to explain the properties of graphene in
terms of its structure and bonding.
Fullerenes are molecules of carbon atoms with hollow shapes. The
structure of fullerenes is based on hexagonal rings of carbon atoms
but they may also contain rings with five or seven carbon atoms.