Unit 4. Covalent Bonding

Question 1

When does covalent bonding occur?

Answer 1

occurs between elements with a difference in electronegativity of fewer than 1.8 units. It occurs between non metal elements

Question 2

What is a covalent bond

Answer 2

A covalent bond is the electrostatic attraction between positively charged nuclei and shared pairs of bonding electrons.

Question 3

How are diatomic molecules bonded

Answer 3

Single Covalent bonds for Halogens
Oxygen and nitrogen are also diatomic but the atoms are via double and triple bond.

A double covalent bond consists of four shared electrons, while a triple covalent bond involves the sharing of six bonding electrons.

Question 4

How does bond length change across single, double and ripple bonds

Answer 4

Consider the group 17 elements, which form diatomic molecules. Atomic radius increases down a group, so the length of the bond between the atoms also increases.

multiple bonds are shorter and stronger than single bonds as they share more electrons between the atoms, so the electrostatic attraction between the bonded nuclei is greater. This greater attraction brings the nuclei closer together

Question 5

How do covalent bonds weaken?

Answer 5

Longer bonds are weaker than shorter bonds due to the increased distance between the nuclei and the shared pairs of bonding electrons, which results in a weaker electrostatic attraction between the atoms

Question 6

How do we know of differences in bond lengths in strength

Answer 6

by comparing multiple bonds between atoms of the same element - for example, by comparing the lengths of the carbon-to-carbon bonds in different compounds.

Question 7

How does bond strengths affect the reactivity of a compound

Answer 7

Bond strength can affect the reactivity of a compound. Nitrogen is unreactive because it has a triple bond, N≡N. It takes a lot of energy to break this bond: 945 kJ mol−1. Oxygen is much more reactive as it only takes 498 kJ of energy to break a mole of O=O bonds.

Question 8

What is a coordinate covalent bond and give an example

Answer 8

A coordinate covalent bond differs from a ‘regular’ covalent bond because both the bonding electrons come from one atom.
Ex. CO

Question 9

How do you represent a coordinate covalent bond

Answer 9

With an arrow on the bonds in a Lewis structure. The arrow comes from to atom from which the electrons originate from.

Question 10

Give an example of a coordinate covalent bond in a polyatomic ionic compound

Answer 10

the hydronium ion (H3O+) and the ammonium ion (NH4+). In the hydronium ion, the coordinate covalent bond is formed between a hydrogen ion and a non-bonding pair of electrons on the oxygen atom. In the ammonium ion, it is formed between a hydrogen ion and the non-bonding pair of electrons on the nitrogen atom.

Question 11

When do coordinate covalent bonds form? Give an example

Answer 11

Atoms in molecules that are electron-deficient (lacking in electrons) are able to form coordinate covalent bonds. An example of such a molecule is aluminium chloride. Despite being a metal and non-metal atom bonded together, the bonding in aluminium chloride is actually polar covalent, not ionic.

Question 12

What is a dimer

Answer 12

A dimer is a larger molecule composed of two identical smaller molecules and can be linked by coordinate covalent bonds or by hydrogen bonds.

Question 13

What is bond order

Answer 13

The bond order is the number of bonds between a pair of atoms. Single bonds have a bond order of 1, double bonds have a bond order of 2, etc.

The higher the bond order, the greater the strength of the bond. Fractional bond orders are possible

Question 14

How do you calculate bond order

Answer 14

A simple calculation of bond order is to divide the sum of the individual bond orders by the number of bonding groups.

Ex: bond order of CO3 2–.

The carbonate ion is composed of two single bonds and one double bond. The sum of the individual bond orders is 1+1+2 = 4. There are 3 bonding groups in the ion (two single bonds and one double bond), therefore, the bond order is:

4/3 =1.33

Question 15

What are the two types of covalent bonds

Answer 15

Non-polar and polar

Question 16

Electronegativity difference for non-polar and polar bonds

Answer 16

Polar covalent= 0.5-1.7
Non polar= 0.1-0.4

Question 17

What are pure covalent bonds?

Answer 17

Pure covalent bonds occur between atoms that have no difference in electronegativity, such as those found between the diatomic molecules.

Question 18

What are polar bonds

Answer 18

Polar covalent bonds occur between atoms that have a difference in electronegativity of between 0.5 and 1.7 units.
the bonding electrons are not equally shared between the atoms, creating a bond dipole.
A bond dipole will result in one atom in the bond having a partial positive charge and the other having a partial negative charge.
These partial positive and negative charges are shown by the δ+ and δ- signs respectively.

Question 19

What is a bond dipole caused by

Answer 19

the unequal sharing of electrons in a covalent bond.

Question 20

What is the octet rule

Answer 20

The octet rule states that the most stable arrangement for an atom is to have eight electrons in its outermost energy level with the electron configuration of a noble gas.

Question 21

What are the exceptions to the octet rule

Answer 21

Hydrogen is stable with only two electrons in its outer shell.
Atoms such as boron, beryllium and aluminium (in compounds) are stable with fewer than eight electrons in their outer shell.
Atoms in period three and higher, such as sulfur, can form expanded octets with up to twelve electrons in their valence shell.

Question 22

What do you call atoms with less than eight electrons in their outer shells

Answer 22

Incomplete octet or electron deficient

Question 23

What are Lewis structures

Answer 23

Lewis structures (also known as electron dot diagrams) represent the bonding in a molecule; they show both the bonding electrons and the non-bonding electrons (or lone pairs).

Question 24

What are resonance structures?

Answer 24

When molecules contain multiple bonds (double or triple bonds), there is more than one possible Lewis structure that can be drawn. The different Lewis structures that can be drawn for the same molecule, or polyatomic ion, are called resonance structures.

Question 25

What is a resonance hybrid structure

Answer 25

The actual structure of compounds with resonance structures. In the resonance hybrid structure all bonds are of equal lengths and strengths which is not the case in resonance structures.

It is important to realise that the molecule for which we depict several different resonance structures is not flipping between these forms. The actual structure is an intermediate between the forms known as the resonance hybrid structure.

Question 26

What is VSEPR

Answer 26

The valence shell electron pair repulsion theory

Question 27

What is the order of repulsion between non-bonding and bonding domains?

Answer 27

Non-bonding domain > bonding domain

Question 28

What is the electron domain geometry

Answer 28

the total number of electron domains (both bonding and non-bonding) around the central atom.

Question 29

Electron domain geometry of molecules with two electron domains

Answer 29

• linear
  -180 degrees
  Ex. CO2

Question 30

Electron domain geometry with three electron domains

Answer 30

• trigonal planer
• molecular geometry can be either bent or trigonal planar, depending on lone pairs on central atom. No lone pair= trigonal planar, lone pair= bent
• if both geometries trigonal planar then 120 degrees, or else if bent then a little less than 120.

Question 31

Electron domain geometry with four electron domains

Answer 31

• tetrahedral
• molecular geometry: tetrahedral, trigonal pyramidal or bent
• no lone pairs = tetrahedral, 109.5 degrees
• one lone pair= trigonal pyramidal, 107.8 degrees
• two lone pair= bent, 104.5

Question 32

What are electron domains

Answer 32

Both bonding electrons and non-bonding electrons are collectively known as electron domains.

Question 33

What is the net dipole moment of a molecule

Answer 33

The net (or overall) dipole moment of a molecule is a measure of its overall polarity. It is the sum of all the bond dipoles in a molecule.

Question 34

Two things to consider when evaluating the polarity of a molecule

Answer 34

• the presence of polar bonds
• the molecular geometry

Question 35

What molecular geometry do non-polar molecules have

Answer 35

Symmetrical so the net dipole moment cancels out, even if the bonds in the molecules themselves are polar.

Question 36

What molecular geometry do polar molecules have

Answer 36

Usually not symmetrical molecules are polar

Question 37

What structure to covalent substances form other than discrete molecules, like H2O or CO2

Answer 37

Giant covalent structures, ex diamond, silicon and silicon dioxide

Question 38

Properties of silicon and silicon dioxide

Answer 38

the strong covalent bonds between the atoms are responsible for the properties of silicon dioxide: SiO2 is a very hard substance, a poor conductor of electricity at low temperature and has a high melting and boiling point. Sand is an impure form of silicon dioxide with a yellow colour due to the presence of iron(III) oxide.

Question 39

What are allotropes

Answer 39

different forms of the same element in the same physical state.

Question 40

Name all three allotropes of carbon

Answer 40

Diamond, graphite and fullerenes

Question 41

Properties of diamond

Answer 41

The strong covalent bonds between the carbon atoms mean that diamond is a very hard substance with a high melting and boiling point. It is a poor electrical conductor as it has no delocalised electrons within its structure.

Question 42

Properties of graphite and structure

Answer 42

In graphite, each carbon atom is covalently bonded to three other carbon atoms. Graphite has a layered structure consisting of carbon atoms arranged in fused hexagonal rings. A graphite crystal is composed of many such planar sheets of hexagonally arranged carbon atoms, stacked on top of one another. The layers are held together by relatively weak London dispersion forces. Each carbon atom has an electron which becomes delocalised across the plane.

• soft, can conduct electricity

Question 43

Properties and structure of fullerene C60

Answer 43

Simple covalent structure

The structure is made up of carbon atoms bonded together in 20 hexagons (six-carbon rings) and 12 pentagons (five-carbon rings), known as a truncated icosahedron. Like graphite, each carbon atom is covalently bonded to 3 other carbon atoms. C60 does have delocalised electrons but the electrons cannot jump between individual fullerenes, therefore, it is a poorer electrical conductor than graphite.

Question 44

What is graphene

Answer 44

A single layer of graphite which

very high tensile strength, and high electrical and thermal conductivity
Very reactive due to unoccupied bonds in the edges of the structure

It is composed of single layers of graphite with a bond angle of 120o between the carbon atoms in a trigonal planar arrangement.