What is a chemical bond?
A chemical bond is a mutual attraction between two atoms resulting from the simultaneous
attraction between their nuclei and the outer electrons
Types of chemical bonds
3.2.1 Covalent bonds
A covalent bond forms when two atoms come close together and their half-filled orbitals overlap,
and the electrons are found in orbitals around both atoms. Simply put, this is when atoms share
electrons to achieve a noble gas state. We use Lewis structures to show how the atoms share
electrons. A covalent bond is the sharing of electrons between non-metals.
Covalent bonds can be single, double, or triple, depending on the number of half-filled orbitals that
overlap. Atoms in a single bond share two electrons, atoms in a double bond share four electrons,
and atoms in a triple bond share six electrons.
Ionic bonds
An ionic bond is formed when there is a transfer of electrons between two atoms. This happens
when the difference in electronegativity between the two atoms is greater than 1,7, which is often
the case between a metal and a nonmetal. Sodium chloride (NaCl) is a common ionic compound,
also known as table salt. Let’s look at how sodium and chlorine form an ionic bond to produce
sodium chloride. Sodium has 1 valence electron that it can donate to chlorine, which has 7 valence
electrons. After donating this electron, both atoms have a full valence shell, but let’s look at the
charge on each ion after the electron transfer. The nature of the charge on the sodium ion is now
positive, and on the chlorine ion, it is negative. We know that unlike charges attract each other, so
the positive sodium ion now attracts the negative chlorine ion. It is this attractive force that creates
the ionic bond between the two atoms. This attraction between cations and anions forms a crystal
lattice where ionic bonds hold the atoms together. We do not find individual sodium atoms
attracted to individual chlorine atoms but, rather, a crystal lattice of ions packed together in a 1: 1
ratio. It is mostly referred to as a compound.
Metallic bonds
Metallic bonds occur between metal atoms. Metallic bonds are very different from the covalent and
ionic bonds that we have been looking at so far. These bonds occur when metal nuclei are packed
together with the valence electrons that are free to move among all the nuclei. These free valence
electrons hold the metallic structure together.
Properties of metallic compounds
• Metals can conduct electricity well due to their sea of delocalised electrons that are free to
move.
• Metals conduct heat well because the nuclei are packed together closely, allowing energy to
be transferred easily.
• Metals are dense because of the tightly-packed nuclei.
• Metals mostly have high melting points because the nuclei that are tightly packed together
indicate strong forces between the nuclei, making us need more energy to overcome the
forces between the particles.
,3.3 Valence electrons
Valence electrons are the electrons in the outermost electron shell of an atom. The number of
electrons in an atom's outermost valence shell governs its bonding behaviour. Elements in Groups 1
- 3 have the same number that corresponds to the group number. Elements in Group 13 have 3
valence electrons. Group 14 has 4 valence electrons, Group 15 has 5 valence electrons. See the
pattern? So, Groups 6 and 17 will have 6 and 7, respectively.
What are Lewis diagrams?
Gilbert Newton Lewis published his important work in 1916, in which he proposed that a chemical
bond is made up of two atoms sharing a pair of electrons. Knowing an atom's Lewis structure allows
you to predict how, and how many bonds it will form. This understanding will eventually allow us to
understand molecule shapes and chemical characteristics.
We know that covalent bonds are the sharing of electrons between nonmetals, but how do we know
how many bonds are formed? This is the beauty of the Lewis notation. Lewis notation is a way to
represent molecules with atoms, using their valence electrons. We use the chemical symbol to
represent the nucleus and core electrons, and we use dots or crosses to represent the valence
electrons.
Rules for drawing a Lewis diagram for an atom
How do we draw a Lewis diagram? We are going to start by looking at an element and, in the
following pages, get more complicated diagrams of molecules.
Let’s start!
Step 1: Write down the symbol for the element (remember that the first letter is a capital and the
second letter, if there is one, is lowercase).
Step 2: Find the number of valence electrons by looking at the periodic table. (Download the
periodic table here.)
Step 3: Represent each electron with a dot or cross. Place one dot or cross on each of the four sides
of the element symbol.
Step 4: Add the rest of the dots or crosses to make pairs around the atom, until you have used all
the valence electrons.
We will work through some worked examples on the next page.
Drawing Lewis diagrams for molecules with single bonds
One important characteristic is that most molecules have an even number of electrons. GN Lewis
proposed that electrons from different atoms form pairs and that covalent bonding is the sharing of
a pair of electrons. We can use Lewis diagrams to represent molecules.
Let’s consider some terminology, first. The pair of electrons between the elements is called a shared
pair, or a bonding pair of electrons. The other pairs of electrons are called lone pairs.
,A bonding pair is a pair of electrons that is shared between two atoms in a covalent bond.
A lone pair is a pair of electrons in the valence shell of an atom that is not shared with another atom.
A single bond occurs when there is only one bonding pair of electrons.
A stable arrangement is attended when the atom is surrounded by 8 electrons. This octet can be
made up of its electrons and some electrons which are shared. Thus, an atom continues to form
bonds until an octet of electrons is made.
A double bond is formed when atoms share 2 pairs of electrons.
A dative covalent bond is the description of covalent bonding that occurs between two atoms, in
which both electrons shared in the bond come from the same atom.
To summarise what we have learnt so far:
• Any electron will attempt to pair up with another electron. As a result, atoms with at least 1
unpaired electron can establish bonds with any other atom with an unpaired electron. This is
not only limited to 2 atoms.
• An atom with an electron pair will not ordinarily share that pair, to create a bond. A lone pair
is a name given to this electron pair.
• An atom can create numerous bonds to another atom if it has more than 1 unpaired
electron. Double and triple bonds are created in this fashion.
• An atom with no electrons and an atom with a lone pair can create a dative covalent
connection.
Lone pair refers to paired valence electrons that do not become involved in the chemical bonding
between two atoms.
Bonding pair is a pair of electrons that are formed when a chemical bond is formed.
Bond length is the distance between the nuclei of two atoms when they bond.
Bond energy is the amount of energy that must be added to a system to break the bond that has
formed or, the energy that is released when a bond is formed.
Bond strength refers to how strongly one atom attracts and is held to another atom
Bond Energy
We saw, on the last page, that a bond is an electrostatic force of attraction that holds a molecule
together. Remember that as atoms approach each other, the energy decreases (energy is released
from the system). A bond is formed at position B when atoms have released a certain amount of
potential energy.
Figure 7: Graph showing the change in energy as bonding takes place.
, Key terms
Bond energy of a compound is the energy needed to break one mole of its
molecules into separate atoms.
Bond strength refers to how strongly one atom attracts and is held to another
atom.
Bond energy is the energy needed to break one mole of its molecules into separate atoms. The more
energy released in forming a bond means that there is a stronger bond. Removing or separating
atoms from molecules with stronger bonds is more difficult than separating atoms from molecules
with weak bonds.
Bond strength depends on the length of the bond, the size of the atoms, and the number of bonds
between the two atoms.
It is possible to work out the amount of energy released to produce a certain molecule. Look at the
list of bond energies given below. Remember, the line between the atoms represents a bond. If
there is one line, it is a single bond, two lines are a double bond, and three lines would be a triple
bond.
Table 1: Bond energies of single bonds.
Bond Bond Energy (kJ.mol-1)
H-H 436
C-H 413
C-F 565
N-H 389
O-H 463
H-S 338
C-C 348
F-F 155
Cl - Cl 243
This is not a complete list and we will add double and triple bonds soon. Let’s try to determine the
energy released when a carbon tetrafluoride bond is formed.
A chemical bond is a mutual attraction between two atoms resulting from the simultaneous
attraction between their nuclei and the outer electrons
Types of chemical bonds
3.2.1 Covalent bonds
A covalent bond forms when two atoms come close together and their half-filled orbitals overlap,
and the electrons are found in orbitals around both atoms. Simply put, this is when atoms share
electrons to achieve a noble gas state. We use Lewis structures to show how the atoms share
electrons. A covalent bond is the sharing of electrons between non-metals.
Covalent bonds can be single, double, or triple, depending on the number of half-filled orbitals that
overlap. Atoms in a single bond share two electrons, atoms in a double bond share four electrons,
and atoms in a triple bond share six electrons.
Ionic bonds
An ionic bond is formed when there is a transfer of electrons between two atoms. This happens
when the difference in electronegativity between the two atoms is greater than 1,7, which is often
the case between a metal and a nonmetal. Sodium chloride (NaCl) is a common ionic compound,
also known as table salt. Let’s look at how sodium and chlorine form an ionic bond to produce
sodium chloride. Sodium has 1 valence electron that it can donate to chlorine, which has 7 valence
electrons. After donating this electron, both atoms have a full valence shell, but let’s look at the
charge on each ion after the electron transfer. The nature of the charge on the sodium ion is now
positive, and on the chlorine ion, it is negative. We know that unlike charges attract each other, so
the positive sodium ion now attracts the negative chlorine ion. It is this attractive force that creates
the ionic bond between the two atoms. This attraction between cations and anions forms a crystal
lattice where ionic bonds hold the atoms together. We do not find individual sodium atoms
attracted to individual chlorine atoms but, rather, a crystal lattice of ions packed together in a 1: 1
ratio. It is mostly referred to as a compound.
Metallic bonds
Metallic bonds occur between metal atoms. Metallic bonds are very different from the covalent and
ionic bonds that we have been looking at so far. These bonds occur when metal nuclei are packed
together with the valence electrons that are free to move among all the nuclei. These free valence
electrons hold the metallic structure together.
Properties of metallic compounds
• Metals can conduct electricity well due to their sea of delocalised electrons that are free to
move.
• Metals conduct heat well because the nuclei are packed together closely, allowing energy to
be transferred easily.
• Metals are dense because of the tightly-packed nuclei.
• Metals mostly have high melting points because the nuclei that are tightly packed together
indicate strong forces between the nuclei, making us need more energy to overcome the
forces between the particles.
,3.3 Valence electrons
Valence electrons are the electrons in the outermost electron shell of an atom. The number of
electrons in an atom's outermost valence shell governs its bonding behaviour. Elements in Groups 1
- 3 have the same number that corresponds to the group number. Elements in Group 13 have 3
valence electrons. Group 14 has 4 valence electrons, Group 15 has 5 valence electrons. See the
pattern? So, Groups 6 and 17 will have 6 and 7, respectively.
What are Lewis diagrams?
Gilbert Newton Lewis published his important work in 1916, in which he proposed that a chemical
bond is made up of two atoms sharing a pair of electrons. Knowing an atom's Lewis structure allows
you to predict how, and how many bonds it will form. This understanding will eventually allow us to
understand molecule shapes and chemical characteristics.
We know that covalent bonds are the sharing of electrons between nonmetals, but how do we know
how many bonds are formed? This is the beauty of the Lewis notation. Lewis notation is a way to
represent molecules with atoms, using their valence electrons. We use the chemical symbol to
represent the nucleus and core electrons, and we use dots or crosses to represent the valence
electrons.
Rules for drawing a Lewis diagram for an atom
How do we draw a Lewis diagram? We are going to start by looking at an element and, in the
following pages, get more complicated diagrams of molecules.
Let’s start!
Step 1: Write down the symbol for the element (remember that the first letter is a capital and the
second letter, if there is one, is lowercase).
Step 2: Find the number of valence electrons by looking at the periodic table. (Download the
periodic table here.)
Step 3: Represent each electron with a dot or cross. Place one dot or cross on each of the four sides
of the element symbol.
Step 4: Add the rest of the dots or crosses to make pairs around the atom, until you have used all
the valence electrons.
We will work through some worked examples on the next page.
Drawing Lewis diagrams for molecules with single bonds
One important characteristic is that most molecules have an even number of electrons. GN Lewis
proposed that electrons from different atoms form pairs and that covalent bonding is the sharing of
a pair of electrons. We can use Lewis diagrams to represent molecules.
Let’s consider some terminology, first. The pair of electrons between the elements is called a shared
pair, or a bonding pair of electrons. The other pairs of electrons are called lone pairs.
,A bonding pair is a pair of electrons that is shared between two atoms in a covalent bond.
A lone pair is a pair of electrons in the valence shell of an atom that is not shared with another atom.
A single bond occurs when there is only one bonding pair of electrons.
A stable arrangement is attended when the atom is surrounded by 8 electrons. This octet can be
made up of its electrons and some electrons which are shared. Thus, an atom continues to form
bonds until an octet of electrons is made.
A double bond is formed when atoms share 2 pairs of electrons.
A dative covalent bond is the description of covalent bonding that occurs between two atoms, in
which both electrons shared in the bond come from the same atom.
To summarise what we have learnt so far:
• Any electron will attempt to pair up with another electron. As a result, atoms with at least 1
unpaired electron can establish bonds with any other atom with an unpaired electron. This is
not only limited to 2 atoms.
• An atom with an electron pair will not ordinarily share that pair, to create a bond. A lone pair
is a name given to this electron pair.
• An atom can create numerous bonds to another atom if it has more than 1 unpaired
electron. Double and triple bonds are created in this fashion.
• An atom with no electrons and an atom with a lone pair can create a dative covalent
connection.
Lone pair refers to paired valence electrons that do not become involved in the chemical bonding
between two atoms.
Bonding pair is a pair of electrons that are formed when a chemical bond is formed.
Bond length is the distance between the nuclei of two atoms when they bond.
Bond energy is the amount of energy that must be added to a system to break the bond that has
formed or, the energy that is released when a bond is formed.
Bond strength refers to how strongly one atom attracts and is held to another atom
Bond Energy
We saw, on the last page, that a bond is an electrostatic force of attraction that holds a molecule
together. Remember that as atoms approach each other, the energy decreases (energy is released
from the system). A bond is formed at position B when atoms have released a certain amount of
potential energy.
Figure 7: Graph showing the change in energy as bonding takes place.
, Key terms
Bond energy of a compound is the energy needed to break one mole of its
molecules into separate atoms.
Bond strength refers to how strongly one atom attracts and is held to another
atom.
Bond energy is the energy needed to break one mole of its molecules into separate atoms. The more
energy released in forming a bond means that there is a stronger bond. Removing or separating
atoms from molecules with stronger bonds is more difficult than separating atoms from molecules
with weak bonds.
Bond strength depends on the length of the bond, the size of the atoms, and the number of bonds
between the two atoms.
It is possible to work out the amount of energy released to produce a certain molecule. Look at the
list of bond energies given below. Remember, the line between the atoms represents a bond. If
there is one line, it is a single bond, two lines are a double bond, and three lines would be a triple
bond.
Table 1: Bond energies of single bonds.
Bond Bond Energy (kJ.mol-1)
H-H 436
C-H 413
C-F 565
N-H 389
O-H 463
H-S 338
C-C 348
F-F 155
Cl - Cl 243
This is not a complete list and we will add double and triple bonds soon. Let’s try to determine the
energy released when a carbon tetrafluoride bond is formed.
