Chemical bonding

Question 1

What is ionic bonding?

Answer 1

The bond formed from the electrostatic attraction between two oppositely charged ions; usually between a metal and a non-metal but can be between ionic compounds (non-metals) and non-metals.

Question 2

How do cations form?

Answer 2

When a metal atom loses all of its valent electrons to have a full outer shell, leaving the net charge of the atom positive.

Question 3

How do anions form?

Answer 3

When a non-metal gains electrons in its valent shell to form a full outer shell, leaving the net charge negative.

Question 4

How can you work out ionic charge from the periodic table?

Answer 4

• Group 1-3 elements usually form cations with a charge in accordance with their group number.
• Group 5-7 elements usually form anions with a charge of 8-n (n being the group number).
• Group 0 elements do not usually react at all as they are very stable.
• Group 4 ions don’t usually form ions as too much energy is required to transfer electrons between them.
• The transition metals can usually form more than one ions. Their oxidation state will determine their ionic charge.

Question 5

How do ionic compounds form solids?

Answer 5

They form ‘Giant ionic lattices’. These lattices consist of billions of ions packed in a regular repeating pattern. They are packed in a way so that each ion is surrounded by the maximum number of oppositely charged ions but is as far away from ions of the same charge as possible.

Question 6

What does ‘giant’ imply in this context?

Answer 6

Giant describes a structure that does not have a definite end/ has the potential to reach an infinite size provided with an infinite number of components. There isn’t a formula to describe the exact size of the structure, only simple ratios. For example, water is a molecule with a definite size, but Sodium Chloride crystals can be of any size.

Question 7

What are the typical physical properties of ionic compounds?

Answer 7

1. High melting and boiling points.
2. Don’t conduct electricity when solid but do when liquid or in solution.
3. Ofter soluble in water.
4. Brittle.

Question 8

Why do giant ionic lattices have high melting and boiling points?

Answer 8

The electrostatic force of attraction between the anions and cations are strong and many in an ionic lattice, so a lot of heat energy is required to break them; so ionic lattices have high melting and boiling points.

Question 9

Why do giant ionic lattices conduct as liquids and in solution but not when solid?

Answer 9

As a solid, all ions are held tightly in place and there are no free electrons or charge carriers, so an electric current cannot pass through the solid. However, as a liquid or in solution, the ions are free to move around. During electrolysis, cations move towards the cathode and gain electrons to form elements; the anions lose electrons at the anode. This process maintains a potential difference so a current can flow.

Question 10

Why do ionic compounds dissolve in water?

Answer 10

The ions are attracted to the polar water molecules. If that attraction is strong enough to break the ionic bonds between the ions, then the compound dissolves in water.

Question 11

Why are giant ionic lattices brittle?

Answer 11

When stress is applied to an ionic crystal, the rows of ions within the crystals shift and try to slide across each other. However, this brings ions of the same charge side by side and they repel each other, so cracks form along the contours of the rows of ions. This makes ionic crystals brittle.

Question 12

What is covalent bonding?

Answer 12

A bond formed when pairs of electrons are shared between atoms to achieve full outer shells.

Question 13

How do you determine the number of bonds an element can form from the periodic table?

Answer 13

• Hydrogen usually forms 1 covalent bond.

- Group 4-7 elements usually form 8-n (n being the group number) covalent bonds but can vary depending on context.

Question 14

Are there exceptions to the rule?

Answer 14

Sometimes a compound is more stable when an element in that compound does not achieve a full outer shell. This is the case with boron in boron trichloride when the boron only has 6 electrons in the outer shell and sulfur in sulfur hexafluoride when the sulfur has 10 electrons in its outer shell.
The boron is described as electron deficient and the sulfur is described as having an expanded octet.

Question 15

Rules for exceptions to the octet rule.

Answer 15

1. The maximum number of covalent bonds that can be formed by an element is equal to the number of electrons in the outer shell.
2. Expanded octets can only be formed with elements of the 3rd period or above as they have d shells.

Question 16

What is a dative covalent bond?

Answer 16

When the bonding pair of electrons in a covalent bond is provided by only one of the atoms in the bond.

Question 17

What is a simple covalent molecule?

Answer 17

Elements bonded by covalent bonds in molecules.

Question 18

What are the properties of simple covalent molecules?

Answer 18

1. Low melting and boiling points. Intermolecular forces are weak and do not require a lot of heat energy to break.
2. Do not conduct electricity. Electrons are not free to move and carry a current.
3. Soluble in organic solvents. Intermolecular forces are weak and easily broken by attraction to solvent molecules.

Question 19

What is a giant covalent lattice?

Answer 19

Three dimensional lattice structure consisting of atoms held together by strong covalent bonds.

Question 20

What are the properties of giant covalent lattices?

Answer 20

1. High melting and boiling points. Covalent bonds are strong and require lots of energy to break.
2. Do not conduct electricity (except graphite). No free moving charge carriers to carry an electric current except for graphite who has one free electron per atom to carry electric current.
3. Insoluble in organic solvents. Covalent bonds are too strong to be broken by the attraction to solvent molecules.

Question 21

What is the structure and physical properties of diamond?

Answer 21

1. Diamond has a tetrahedral structure. Each atom of carbon makes 4 bonds to 4 other carbon atoms.
2. Does not conduct electricity. No free moving charge carriers within the structure.
3. Hard. The tetrahedral structure of diamond distributes force applied to the structure evenly across the whole structure. There are no weak points, making diamond very hard.

Question 22

What is the structure and physical properties of graphite?

Answer 22

1. Graphite has a layered hexagonal structure. Each carbon makes 3 bonds to 3 other atoms to form a two dimensional hexagonal structure. The layers are then stacked on top of each other to form the final three dimensional structure.
2. Does conduct electricity. Each carbon atom only makes 3 bonds so only 3 electrons are used. The 4th electron is free to jump between the carbon atoms in the same plane and layer. This means that they are able to carry an electric current through the structure and so graphite does conduct electricity.
3. Soft. The forces holding each layer together within the graphite crystal are relatively weakly bonded together with Van Der Waal’s interactions. They slide past each other relatively easily. This allows for graphite to be used as quite an effective lubricant.

Question 23

What is metallic bonding?

Answer 23

The electrostatic force of attraction between a positive metallic cation and the surrounding sea of delocalised electrons.

Question 24

What are the properties of a metallic crystal?

Answer 24

1. Metallic crystals have high melting and boiling points. The metallic cations are fixed in place by the strong electrostatic force between them and the delocalised electrons. It takes a lot of heat energy to break that force of attraction and break apart the metallic lattice, so metals usually have fairly high melting and boiling points.
2. Good conductors of electricity. The electrons in the sea of delocalised electrons are non-specific to a metallic cation and are free to jump between any cation. This ability means that when a potential difference is applied, the electrons in the sea of electrons are free to move in any direction to carry the current.
3. Insoluble. Metallic bonds are too strong to be broken by attraction of cations in the lattice to solvent molecules.
4. Malleable and ductile. Metallic bonds are non-directional, so will hold no matter what position metallic cations are in. This allows the layers in the metallic lattice to slide past each other easily. When the positive cations are side by side, the sea of electrons also act as shielding to prevent them from repelling each other. This results in metal having the ability to change shape easily and being ductile and malleable.

Question 25

What does VSEPR theory state?

Answer 25

1. Electron pairs in the outer shell are all negatively charged and thus will repel each other as far away from each other as possible.
2. Lone pairs of electrons have a greater repulsion than bonded pairs.

Question 26

How do we determine the shape of a simple molecule?

Answer 26

1. Draw a dot-to-cross diagram of the molecule.
2. Count the number of lone pairs around the central atom to determine the shape.
3. Account for any lone pairs in the process.
4. Draw the three dimensional structure of the molecule.

Question 27

What are the corresponding shapes to the number of valent shell electron pairs?

Answer 27

• 2 pairs correspond to a linear shape. Bonding angle: 180 degrees.
• 3 pairs correspond to a trigonal plainer shape. Bonding angle: 120 degrees.
• 4 pairs correspond to a tetrahedral shape. Bond angle: 109.5 degrees.
• 6 pairs correspond to a octahedral shape. Bond angle: 90 degrees.

Question 28

What happens when lone pairs are involved?

Answer 28

Each lone pair reduces the bond angle by 2.5 degrees.

Question 29

What are Van der Waal’s interactions?

Answer 29

The attraction between the induced dipoles of neighbouring molecules.

Question 30

What causes Van der Waal’s interactions?

Answer 30

Because electrons in a molecule are constantly moving and fluctuating, there will be imbalances of electron and charge distribution throughout a molecule which causes an instantaneous dipole. The instantaneous dipole then causes an induced dipole on neighbouring molecules by either pulling electrons towards it or repelling them away. The molecules are then attracted to each other due to the induced dipole. This weak force of attraction are Van der Waal’s interactions.

Question 31

What determines the strength of Van der Waal’s interactions?

Answer 31

1. The number of electrons in a molecule is directly related to the strength of Van der Waal’s interactions between them. The greater the number of electrons, the greater the fluctuations in the electron cloud and thus the stronger the instantaneous dipoles. The stronger the instantaneous dipoles, the stronger the induced dipoles. The stronger the induced dipoles, the stronger the Van der Waal’s interactions.
2. The shape of the molecules also affect the strength of Van de Waal’s interactions between them. For example, molecules containing the same number of electrons will have different melting/ boiling points depending on how branched they are. The more branched the molecule, the smaller the contact area between the molecules and the weaker the Van der Waal’s forces between them.

Question 32

What is a permanent dipole?

Answer 32

A small charge difference across a covalent bond as a result of differing electronegativities of the bonding atoms.

Question 33

What are polar covalent bonds and polar molecules?

Answer 33

Polar covalent bonds are bonds with permanent dipoles across them.
Polar molecules are molecules that have overall charge differences across the whole molecule.

Question 34

What are dipole-dipole interactions?

Answer 34

The force of attraction between oppositely charged poles between polar molecules in addition to the existing Van der Waal’s attractions.

Question 35

What affects the strength of dipole-dipole interactions?

Answer 35

The difference in electronegativity between the atoms in a polar covalent bond. The greater the electronegativity, the greater the polarity of the molecule and the greater the dipole-dipole interactions.

Question 36

What are hydrogen bonds?

Answer 36

The force of attraction between an electron deficient hydrogen in a polar molecule and a lone pair of electrons of an electronegative atom on a different molecule.