Summary notes for Cambridge AS level students- Chemical bonding
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Course
Chemistry
Institution
12th Grade
The summary notes of bonding provide the complete revision on the lesson with enough examples and explanation to achieve complete knowledge over the nature of chemical bonding and effects of chemical bonding.
Chemical bonding
Atoms can combine to form chemical compounds. In doing this, the atoms become more stable due to
their full outer shells.
The physical and chemical properties of compounds are determined by the way of chemical bonds and
intermolecular forces are held together in chemical compounds.
When atoms combine to form compounds, atoms may share , donate or gain electrons.
When elements combine in the form of ions , they can form ionic bonds with one another and are
known as ionic compounds. An ionic bond is a strong attractive force that exists between certain ions in
a substance.
When atoms share electrons , they form covalent bonds. In a covalent bond, two atoms share valence
electrons (outer-shell electrons), which are attracted to the positively charged cores( nucleus) of both
atoms, thus linking them.
The type of bonding, found in metals only, called metallic bonding. It is electrostatic attraction among
delocalised electrons in metallic lattice and positive metallic ions.
Ionic bonding
The complete transfer of one or more electrons from one atom to a different atom leads to the
formation of an ionic bond.
Ions are formed when atoms lose or gain electrons.
• Positive ions are formed when a metal atom loses one or more electrons.
• Negative ions are formed when an atom gains one or more electrons. Non-metal atoms usually gain
electrons and form negative ions.
• As a result of this, the metal and non-metal atoms usually end up with outer electron shells
which are complete – they have an electron configuration of a noble gas.
• The charge on the ion depends on the number of electrons lost or gained.
• The ions are packed together in a regular arrangement called a ionic lattice. Within the
lattice,oppositely charged ions attract one another strongly by the electrostatic attraction.
Ionic bonding can be represented by dot – cross diagrams
,Complete revision notes for Cambridge – AS level
Dot cross diagrams to show ionic bonds
Example 1
The dots and crosses are simply used to show which electrons come from the metal and which come
from the non-metal.
When drawing a dot-and-cross diagram for an ionic compound it is usually acceptable to draw the outer
electron shell of the metal ion without any electrons. This is because it has transferred these electrons to
the negative
The dot-and-cross diagram for sodium chloride.
A dot-and-cross diagram shows:
• the outer electron shells only
• that the charge of the ion is spread evenly, by using square brackets
• the charge on each ion, written at the top right-hand corner of the square brackets.
• the sodium ion has the electronic structure [2,8]+, the same as that of neon.
• the chloride ion has the electronic structure [2,8,8]−, the same as that of argon. When metals react with
non-metals, the ions produced form ionic crystals.
An ionic crystal is a giant lattice containing of positive and negative ions packed together in a
regular pattern.
Ionic crystal/ ionic lattice of sodium chloride
Example 2
Each calcium atom has two electrons in its outer shell and these can be transferred to two chlorine
atoms. By losing two electrons, each calcium atom achieves the electron configuration [2,8,8] . The
, Complete revision notes for Cambridge – AS level
two chlorine atoms each gain one electron to achieve the electron configuration [2,8,8]. [2,8,8] is the
electron configuration of argon; it is a ‘noble-gas configuration’.
Example 3
When magnesium reacts with oxygen to form magnesium oxide. The two electrons in the outer shell of
magnesium atom are transferred to the outer orbital of an oxygen atom. By losing two electrons,
each magnesium ion achieves the electron configuration of [2,8] , Mg 2+. By gaining two electrons,
each oxygen ion achieves the electron configuration of [2,8], O 2-. [2,8] is the electron configuration of
neon; it is a ‘noble-gas configuration’.
Properties of ionic compounds due to the bonding
• They have high melting points — the ions are held together by a strong attraction.
• They are hard, brittle crystalline substances have high melting and boiling temperatures as they
have strong attraction, and needs more energy to melt the crystals.
• They are often soluble in water but not in non-polar solvents . The ions are pulled apart by
polar molecules like water, but not by non-polar molecules.
• Ionic compounds can’t be shaped . The repulsion between these ions would be very strong, so
ionic compounds are brittle (they break when they’re stretched or hammered). This supports the
lattice model.
• They do not conduct electricity when solid, because their ions cannot move away from fixed
positions in the giant lattice. They conduct electricity when they are melted or dissolved in
water, because the charged ions are then free to move( but not in solid state).
Covalent bonding
A covalent bond is the strong electrostatic attraction between two nuclei and the shared pair of
electrons between them.
• Covalent bonds form between non-metals.
• Electrons are shared between the two outer shells in order to form a full outer shell( reach the
structure of the noble gas).
• A shared pair of electrons is called a single covalent bond, or a bond pair.
• Multiple electron pairs can be shared to produce multiple covalent bonds.
• A single covalent bond is represented by a single line between the atoms. For example, Cl -Cl.
• Some atoms can bond together by sharing two pairs of electrons. We call this a double covalent
bond. A double covalent bond is represented by a double line between the atoms. For example,
O = O.
• Atoms can also bond together by sharing three pairs of electrons. We call this a triple covalent
bond.
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