Covalent structures

Question 1

What is the octet rule?

Answer 1

• The tendency of atoms to gain the most stable arrangement which has 8 electrons in its outermost energy level (noble gas configuration)

Question 2

What are examples of exceptions of the octet rule?

Answer 2

• Hydrogen is stable with only 2 valence electrons
• Boron, Beryllium and aluminum (in compounds) are stable with fewer than 8 valence electrons
• Atoms is period 3 and higher e.g. sulfur, can form expanded octets with up to 12 valence electrons. They have 5 or 6 electron pairs around the central atom

Question 3

Why do Boron and Beryllium form incomplete octets and what are incomplete octets?

Answer 3

• Incomplete octets are molecules that are electron-deficient (less than 8 valence electrons)
• e.g. BF3 Boron only has 3 valence electrons (cannot reach noble structure)
• The Boron has formed the maximum number of bonds that it can under the circumstances, therefore is stable
• In BeCl2, the beryllium atom is stable with only 4 electrons in its valence shell

Question 4

What are Lewis structures?

Answer 4

• Dot-cross diagrams to represent the bonding between atoms in molecular covalent substances
• It represents the bonding in a molecule, they show the bonding electrons and the non-bonding electrons
• Pairs of electrons can be represented by lines and dots

Question 5

How is the Lewis structure different to ions?

Answer 5

• For cations, ions must be subtracted. For anions, ions must be added. The amount added/subtracted must be equal to the charge on the ion

Question 6

How do you draw a Lewis structure?

Answer 6

• Determine the total number of valence electrons in all of the atoms or ions
• Always determine the number of electrons needed to complete its outer shell (most need 8)
• Draw it with the least electronegative atom at the center and connect all atoms using only single bonds (some molecule’s central atom does not have the lowest electronegativity value
• Complete by adding all non-bonding electrons
• If the outer shell of all atoms cannot be filled (may use multiple bonds)
• Up to 4 pairs of electrons on each atom
  (Check book for example)

Question 7

What are resonance structures?

Answer 7

• When molecules contain multiple bonds, there is more than one possible Lewis structure that can be drawn
• Resonance structures: when there is more than one possible position for a double bond in a molecule
  (position of double/triple bond changes)
• The number of possible resonance structure is equal to the number of different positions for the multiple bond

Question 8

What is a resonance hybrid structure?

Answer 8

• The actual structure
• They exist for all molecules in which there is more than one position for a multiple bond in a molecule
• All bonds are identical and intermediate in strength and length between a single and double bond
• The double bond is drawn in a dotted line and split between all other bonds
• The number of possible resonance structure is equal to the number of different positions for the multiple bond

Question 9

Give an example of a resonance hybrid structure.

Answer 9

• Ozone (O3) has one double bond and one single bond.
• The bonds are however equal in length and strength (intermediate between that of a single covalent bond and a double covalent bond)
• Dashed lines between the oxygen atom represents the intermediate bonds
• There is a dashed and normal bond between each oxygen
• Benzene C6H6 (check book) is usually represented in a ring structure
• The nitrate ion (NO3−) has resonance structures too. The compound requires on extra ion to full fill the octet rule, hence the charge of -1 (anion)

Question 10

What are delocalised electrons and how are they linked to resonance structures?

Answer 10

• Covalent molecules have a defined shape, as the electrons in the covalent bonds are located in specific positions
• In some molecules, the bonding electrons, especially those in a multiple bond, they are shared between more than two nuclei in a molecule
• These are delocalised electrons
• Delocalized electrons come from the p-orbital
• They give greater stability to a molecule and they exist in resonance structures (more than one position for a multiple bond)

Question 11

What is the VSEPR theory and what is the electron domain?

Answer 11

• Valence shell electron pair repulsion theory (VSEPR)
• It predicts the shapes of molecules.
• Electron pairs in molecules repel each other and orient themselves as far away from each other as possible
• A molecules will shape in a way that minimizes the repulsion between the electron pairs
• Electron pairs can be bonding electrons or non-bonding electrons
• Bonding electrons and non-bonding electrons are called electron domains
• Single, double and triple covalent bonds count as one electron domain a.k.a bonding domain (contains bonding pairs of electrons)
• Non-bonding domain: non-bonding electrons together

Question 12

Lone pair = non-bonding pair of electrons

Question 13

How do you determine the molecular geometry and electron domain geometry? State the order of most repulsive to least repulsive.

Answer 13

• Need to count the number of electron domains around the central atom (determined from the Lewis structure of the molecule or ion)
• Non-bonding electrons cause slightly more repulsion than bond pairs of electrons
• Order from most repulsion to least repulsion:
  Non-bonding domain–non-bonding domain > non-bonding domain–bonding domain > bonding domain–bonding domain
  (greatest repulsion between non-bonding domains)

Question 14

What is the electron domain geometry and molecular geometry?

Answer 14

• The total number of electron domains (bonding and non-bonding) around the central atom
• The molecular geometry takes into account the extra repulsion between bonding and non-bonding domains
• This is why they are sometimes different

Question 15

What is the molecular geometry and electron domain geometry of two electron domains?

Answer 15

• They are both linear and the bond angle is 180°
• The bonding pairs are as far apart as possible, which minimizes the repulsion within the molecule
  E.g. CO2, and ethyne, C2H2
  Check book

Question 16

What is the molecular geometry and electron domain geometry of three electron domains?

Answer 16

• Have a trigonal planar (triangular planar) electron domain geometry
• The molecular geometry is either trigonal planer or bent (V-shaped), this depends on the presence of non-bonding electrons
• If there are no non-bonding pairs, the molecular geometry will be trigonal planar, if they are present it will be bent
• The bent geometry is caused by the stronger repulsion between the non-bonding electrons and bonding electrons
• It is trigonal when its only the bonding electrons as the repulsion is weaker e.g. BH3 and CO3 2- (bond angle is 120°)
• A molecules with lone pairs of electrons has bond angles slightly less than 120°
  Check book

Question 17

What is the molecular geometry and electron domain geometry of four electron domains?

Answer 17

• Have a tetrahedral electron domain geometry
• The molecular geometry can be either tetrahedral, trigonal pyramidal or bent depending on the number of non-bonding pairs
• Tetrahedral (no non-bonding pairs) e.g. CH4 (109.5°)
• Trigonal pyramidal (one non-bonding pair) e.g. NH3 (107.8°)
• Bent (two non-bonding pairs) e.g. H2O (104.5°) has the smallest bond angle

Check book

Question 18

Why do the bond angles become smaller?

Answer 18

• The angles are the bond angles (space between the atoms in the molecule)
• The more lone pairs there are, the smaller the angles become between the bottom atoms e.g. Hydrogen atoms become. The angle between the atom with lone electrons and the bonded electrons becomes larger because of the stronger repulsion
• Happening on the central atom

Check book

Question 19

Table of the molecular geometry and electron domain geometry.

Answer 19

In Book

Question 20

What is molecular polarity? What is a net dipole moment?

Answer 20

• Polar molecules have a net dipole moment, non-polar molecules do not
• The net (overall) dipole (opposite electric charges) moment is a measure of its overall polarity. It is the sum of all the bond dipoles in a molecule
• The presence of polar bonds and the molecular geometry are linked to the molecular polarity

Question 21

When is a molecule polar and when is it non-polar?

Answer 21

• If the molecule is symmetrical (even with polar bonds) it will be non-polar
• The bond dipoles cancel each other out (they have no net dipole moment)
• Bond dipoles are represented by a plus connected to an arrow
• E.g. BF3

Question 22

Rules if a molecule is polar or non-polar:

Answer 22

• Calculate the difference in electronegativity and compare it with the table
• If it is symmetrical e.g. CH4, BH3: non-polar
  this depends if there are lone pairs of electrons
• When an element is different e.g. CClH3: polar
• Lone pairs on the central atom: polar
• Same elements attached e.g. H2: non-polar
• Nobel gases: non-polar
• In linear molecules it depends on the difference in electronegativity
• Exception: HCl, NH3 (lone pairs) polar
• If the molecular geometry is tetrahedral or trigonal planar: non-polar (contains no lone pairs)

Question 23

What are two other descriptions of giant covalent structures?

Answer 23

• Network covalent structure

- Macro molecular structure

Question 24

What are the simple properties of Diamond?

Answer 24

• Structure is made up purely of carbon atoms bonded by strong covalent bonds
• Have a very high melting and boiling point
• Poor conductor of electricity or heat as it does not have any delocalised electrons within its structure
• Many atoms bonded together in a regular arrangement
• Carbon to carbon is tetrahedral arrangement with bond angle 109.5°
• Strong covalent bonds

Question 25

What are the properties of silicon and silicon dioxide?

Answer 25

• Each silicon atom is bonded to four oxygen atoms in a tetrahedral arrangement, bond angle 109.5°
• Electrons in a fixed position and also a poor conductor of electricity at low temperatures
• To improve the electrical conductivity, doping can be done
• Doping: involved the addition of small amounts of elements such as phosphorous or boron to pure silicon
• SiO2 (empirical formula, it is not a molecule) also has a tetrahedral structure around the silicon atoms with bond angle of 109.5°
• The silicon to oxygen to silicon bonds have a bent arrangement due to the lone pairs on the oxygen atoms
• Sand is an impure form of silicon dioxide (contains iron(III) oxide)

Question 26

What are allotropes and what are the allotropes of carbon?

Answer 26

• Different forms of the same element in the same physical state
• Diamond, graphite and the fullerenes
• They have different properties due to the different bonding within the structures

Question 27

What are the properties of Fullerene C60?

Answer 27

• Simple molecular substance
• Structure is made up of carbon atoms bonded together on 20 hexagons (6 carbon rings) and 12 pentagons (5 carbon rings) known as truncated isosahedron
• Each carbon atom is bonded to 3 other carbon atoms
• The delocalized electrons cannot jump between fullerenes, therefore is a poorer electrical conductor than graphite

Question 28

What are the 6 properties of Graphite? Where is it used?

Answer 28

• Each carbon is covatently bonded to three other carbon atoms
• It is a layered structure, carbon atoms arranged in fused hexagonal rings
• A graphite crystal is composed of many planar sheets of hexagonally arranged carbon atoms stacked on top of each other
• Layers are held together by weak London dispersion forces
• Each carbon atom has one delocalized electron therefore can conduct electricity
• It is soft and the layers can slide over one another due to the intermolecular forces between the layers
• Used in pencils and electrodes and lubricant

Question 29

Where is graphene used and what are its properties?

Answer 29

• It is composed of single layers of graphite with a bond angle of 120° between the carbon atoms in a trigonal planer arrangement
• Found in pencils
• Comes from graphite
• Very high tensile strength, high electrical and thermal conductor
• Behaves as a semi-metal (metalloid)
• When 1% of graphene is added to plastics, they could conduct electricity
• Most chemically reactive from all allotropes of carbon