2. Covalent Substances

Question 1

What is a covalent bond (t)

Answer 1

A covalent bond occurs between non-metallic atoms and involves sharing electron pairs to attain a noble gas configuration.

Question 2

What is electronegativity? (t)

Answer 2

Electronegativity is the relative attraction that an atom has for shared electrons in a covalent bond
Electronegativity table - https://docs.google.com/presentation/d/1e1tTpWLPDFEMlLKXyaSbhsTwngQ8d4AjEFt7mv1XPpk/edit#slide=id.g20c319d64e9_0_0

Question 3

What is a molecule? (t) When are they formed?

Answer 3

A molecule is formed when two or more non-metallic atoms join using covalent bonds. The atoms are combined in a fixed ratio and are electrically neutral.
They are formed when a fixed number of atoms combine and share electrons in covalent bonds.

Question 4

Which atoms form diatomic molecules? (s)

Answer 4

Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, iodine.

Question 5

How does a covalent bond work? Why does electronegativity affect this? (s)

Answer 5

The positive nuclei of an atom will be attracted to the negative space created by the electrons. The higher the level of electronegativity (the relative attraction that an atoms has for shared electrons in a covalent bond, increasing across [right] and up the periodic table), the stronger the covalent bond.

Question 6

Explain: single bonds > double bonds > triple bonds, in terms of the length of the bonds? (s)

Answer 6

the larger bonds will always be shorter, as the double bond means that 2 electrons are shared between atoms, and triple means 3. There are more electrons in the space between the nuclei, which means the pull between the electrons in that space and the nucleus is stronger (the bond is stronger). This means the nuclei are pulled further towards the electrons which makes the bond shorter

Question 7

What is a covalent molecular element? (t)

Answer 7

A covalent molecular element is made up of identical atoms held together in discrete groups by covalent bonds. They are described as discrete because each molecule is separate and distinct from the others.

Question 8

What is a covalent molecular compound? (t)

Answer 8

A covalent molecular compound is a compound where atoms of different non-metallic elements share electrons with each other.

Question 9

What is the theory for the structure of covalent molecular substances?

Answer 9

The major features of the current theory of the structure of covalent molecular substances are:
- the basic units of covalent molecular substances are groups of atoms called molecules. All molecules within a pure substance are identical.
- adjacent atoms within a molecule share electrons in order to achieve a stable outer shell.
- Electrostatic attraction between the nuclei of adjacent atoms and the shared electrons causes the atoms in a molecule to the held together. This force of attraction is called covalent bonding.
- The overall charge on each molecule is zero and so adjacent molecules are only held together by weak intermolecular forces.

Question 10

What is the octet rule? (s)

Answer 10

A generalisation that works for many (but not all) atoms, stating that atoms will donate or share electrons in order to achieve eight electrons in their outer shells.

Question 11

What does a lewis dot structure tell us? (s)

Answer 11

A lewis dot structure tells us how many bonds the atoms needs to form (indicated by the single electrons). Pairs of electrons don’t take part in covalent bonding, they are called lone pairs.
NOTE: Atoms in the same groups have the same number of valance electrons and so have the same electron dot structure.

Question 12

What is the difference between Valence and lewis structures?

Answer 12

Valence structures substitute a dash (-) for each pair of electrons. They can represent bonding pairs or lone pairs depending on location.
Lewis structures use dashes for bonding pairs and dots for lone pairs.

Question 13

What is VSEPR

Answer 13

Valence shell electron pair repulsion theory
Electron pairs in the molecule repel each other and take up positions as far from one another as possible.

Question 14

What are the most common structures of covalent molecules?

Answer 14

Tetrahedral: 0 lone pairs, 4 bonding electrons.
Pyramidal: 1 lone pair, 3 bonding electrons
Bent, or V-shaped: 2 lone pairs, 2 bonding electrons
Linear: Eg. Ethyne
- 0 lone pairs for each carbon atom
- 2 bonding pairs for each carbon atom
a triple bond between the carbons
Planar: Eg. Ethene
- 0 lone pairs for each carbon atom
- 3 bonding electrons for each carbon atom
- double bond between the carbons

In general:
- Molecules with no lone pairs around the central atom are tetrahedral, unless a double or triple bond creates a linear or planar structure (depending on the distribution of atoms around the central atom)
- Molecules with a single lone pair around the central atom are pyramidal
- Molecules with two long pairs around the central atom are bent.

Question 15

Tips to recognise the shapes of a molecule

Answer 15

• Determine the arrangement of the electron pairs around the central atom
• Decide on the basic structure (eg. 4 pairs of electrons will be tetrahedral)
• To decide on the final shape, look at the arrangement of the bonding pairs only (eg. if only 2 bonding pairs are in the tetrahedral arrangement, the shape is described as bent._)

Question 16

What is polarity?

Answer 16

Polarity is a localised imbalance in electric charges within a molecule resulting in a negatively charged end and a positively charged end.
Even though molecules are electrically neutral, the possibility exists of regions within a molecule where small, localised imbalances may occur.

Question 17

What is a polar covalent bond?

Answer 17

A polar covalent bond is formed when two atoms that have different electronegativities share electrons unevenly. The shared pair of electrons will move closer to the more electronegative atoms. If one atom has a lower electronegativity than the other, there will be a slight deficiency or electrons in that region (and therefore a slight positive charge).

Question 18

Summary of the Pauling scale of electronegativities

Answer 18

• electronegativites increase from left to right within a period
• electronegativites decrease from top to bottom within a group
• metals generally have lower electronegativities than non-metals.

Question 19

What is a bond dipole?

Answer 19

a bond dipole is a separation of charge in a polar covalent bond. When an arrow diagram is used, the arrow points to the negative end.

Question 20

What is a non polar covalent bond?

Answer 20

A non polar covalent bond is a covalent bond in which the bonding electron pair is shared equally and uniformly distributed between the nuclei of two bonded atoms. Such bonds can result only when two atoms of the same element or two atoms of equal electronegativity simultaneously attract a shared pair of electrons.

Question 21

What is a non polar molecule?

Answer 21

A non polar molecule is a molecule that does not have permanent dipoles or is symmetrical.

Question 22

What is a polar molecule?

Answer 22

A polar molecule is a molecule which, due to its polar bonds and asymmetric shape, has an overall imbalance in the distribution of its electrons. All polar molecules contain polar bonds, however some molecules with polar bonds are non polar.

Question 23

Tips for identifying polar and non polar molecules

Answer 23

• linear, planar and tetrahedral molecules with equal polar bonds that cancel each other out are non polar
• if the bond dipoles are not equal or do not cancel each other out, the molecule is polar. For example, methane is non polar whereas Bromomethane is polar because bromine is more electronegative and the resulting C-Br bond dipole is not cancelled by the C-H bond dipoles.
• Bent or pyramidal molecules are polar because their polar bonds do not cancel each other out.

Question 24

What is intramolecular bonding?

Answer 24

Intramolecular bonding is internal bonds within a molecule. For example, the covalent bonds between atoms in a molecule. They are relatively strong bonds compared to the wea interactions that hold atoms and molecules together in the liquid and solid states, which are described as intermolecular forces

Question 25

What are intermolecular forces? What are their effect?

Answer 25

Intermolecular forces are forces that exist between molecules. They effect melting and boiling points.

Question 26

What are the different types of intermolecular forces, from weakest to strongest?

Answer 26

1. Dispersion forces - the bond between adjacent molecules formed by instantaneous dipoles; this is and weak non directional
2. Dipole-dipole interactions (slightly stronger than disp) - weak bonding caused by the positive end of one dipole attracting the negative end of another dipole.
3. Hydrogen bonding (much stronger) - the bond between a hydrogen atom covalently bonded to an atom of F, O or N and another molecule that also contains an atom of H, F, O or N

Question 27

What factors influence dispersion forces?

Answer 27

1. Number of electrons in molecules creates stronger dispersion forces
2. The shapes of the molecules, since they affect how closely the molecules may approach each other in solid and liquid states. The closer the molecule can get, the stronger the attraction.

DISPERSION FORCES EXIST BETWEEN ALL ATOMS AND MOLECULES as they all contain electrons that can be attracted to nuclei of other atoms or molecules.

Question 28

How do dipole dipole interactions work?

Answer 28

If the molecules in a sample are polar, the presence of molecular dipoles causes simultaneous intermolecular attraction. The positive side of one molecule attracts the negative side of another, which attracts the next and so one. They operate between polar molecules.

Question 29

How does Hydrogen Bonding Work?

Answer 29

Special case of dipole dipole attraction - when a hydrogen atom bonds to a more electronegative atom (N, O or F), its electrons move towards that atom, and the positively charged hydrogen nucleus is exposed or unshielded. Therefore, the formed molecule has a strong dipole.
Hydrogen bonds only form between molecules and only when hydrogen has been bonded to F O or N. They are relatively weak but the strongest of the weak.

Question 30

What is bond dissociation energy?

Answer 30

Bond dissociation energy measures how difficult it is to break the covalent bonds within a molecule in the gaseous state and separate it into its component atoms. The higher the value, the stronger the bonds.

Question 31

What are the physical properties of Covalent Compounds?

Answer 31

• they have strong intramolecular bonds and weak intermolecular bonds
• It is hard to pull them apart into constituent elements
• It is relatively easy to pull molecules apart from each other
• They do not conduct electricity
• They don’t conduct electricity when dissolved in water (they are electrically neutral)
• Some covalent molecules will ionise in water (become ions) - then they will conduct electricity
• Solubility - like dissolves like - polar substances will dissolve in polar solvents, non polar substances will dissolve in non polar solvents
• Covalent compounds have low melting and boiling points because the intermolecular forces are weak.
• Covalent compounds are soft and easily scratched because they have weak intermolecular forces.

Question 32

What is an allotrope?

Answer 32

An allotrope is a different form of an element

Question 33

Allotropes of carbon

Answer 33

Diamonds:
- hard and refract light
- used for cutting and drilling
- each atom is covalently bound to four other carbon atoms (strong bonds which need to be broken in order to diamond to be cut or scratched)
- they break or fracture along straight lines
- no free electrons so they don’t conduct electricity
- bonds are rigid, so when heat or kinetic energy is put into one end of the diamond it is passed along the lattice efficiently, so diamond is a good conductor of heat.

Graphite:
- carbon atoms are covalently bonded to 3 other carbon atoms to form planes of hexagonal rings, layers are held together by dispersion forces
- the unbound electrons (one per atom) can become delocalised. They can move between layers, making them good conductors of electricity and heat
- Weak disp forces so layers can be shed off easily, like a pencil, and makes powdered graphite slippery (used as a lubricant)
- hexagonal shape makes it chemically inert, lots of industrial uses

Amorphous carbon: (eg. charcoal or soot)
- no crystalline structure
- conducts electricity

Graphene:
- single layer of carbon atoms, covalently bonded in hexagonal shapes - essentially a layer of graphite
- good at conducting electricity
- used for smart windows or electronic displays

Fullerenes (buckyballs)
- each atom bonded to 3 other carbon atoms forming a ball shape
- sea of electrons inside means they conduct electricity
- weak intermolecular forces, low melting point.