S2.2- The covalent model

What does the octet rule refer to?

It refers to the tendency of atoms to gain a valence shell with a total of 8 electrons

What are the exceptions to the octet rule?

Hydrogen and helium→both in period 1; outer shell can only contain 2 valence electrons)

Beryllium and boron→incomplete octet (4 and 6 valence electrons respectively)

Period 3 elements (such as S, P)→expanded octets

What are the steps to finding Lewis structure

1. Draw the skeletal formula (central atom usually the least electronegative)

2. Calculate total valence electrons

3. Subtract paired electrons from bonds

4. Fill side atoms with the electrons left first

5. Any remaining electrons goes on the central atom

6. If the central atom does not satisfy an octet, form double or triple bonds where necessary

What is a covalent bond?

It’s formed by an electrostatic attraction between a shared pair of electrons and the +vely charged nuclei

Ionic bonds only form between different elements, while covalent bonds can form between atoms of the same element. Why is that so?

Main idea: electronegativity difference

Ionic bonds require a complete transfer of electrons to form oppositely charged ions, which only happens between elements with a large electronegativity difference.

Covalent bonds share electrons and can form between atoms with similar electronegativity.

What is bond energy and its unit?

The amount of energy required to break one mole of a covalent bond in the gaseous state

kJmol^1

What happens to bond strength when bond energy increases?

It increases

What happens to bond length when more electrons are shared?

Double or triple bonds form→electron density between atoms increase→electrostatic attraction increases→bond strength increases

True or false: a triple bond is weaker than a single bond

False.

A triple bond is shorter and stronger

What is a coordinate bond?

A covalent bond in which both electrons of the shared pair originates from the same atom. It is represented by an arrow →

True or false: coordinate bonds and regular covalent bonds formed from a shared electron pair are not the same

False. A coordinate bond is identical to a regular covalent bond once formed.

What are some examples of coordinate covalent bonds in molecules/ions?

H3O+

NH4+

Al2Cl6

HL: What is an example of coordinate bonding in transition metal complexes?

Reactivity 3.4: why do Lewis acid-base reactions lead to the formation of covalent bonds?

Both electrons in the new bond come from the Lewis base (electron donor but proton acceptors) while the Lewis base accepts the electron

What is the valence shell electron pair repulsion (VSEPR) theory used for?

To predict the geometry (shape) of molecules

What is the descending order of repulsion involved in lone pairs (lp) and bond pairs (bp)?

lp-lp>lp-bp>bp-bp

What does electron domain mean?

It refers to bonds or lone pairs of electrons around an atom in a molecule

True or false: single bonds, double bonds, triple bonds and lone pairs each count as one electron domain

True

What does a molecule with 2lp and 2bp have a smaller angle (104.5°) than one with 1lp and 2bp (<120°) despite having the same molecular geometry (bent shape)?

2 lps repel more than 1 lp, pushing the atom closer together and have a smaller angle

What is the bond angle of a molecule with tetrahedral molecular geometry?

109.5°

What is the bond angle of a molecule with trigonal pyramidal molecular geometry?

107°

What is the bond angle of a molecule with trigonal planar molecular geometry?

120°

What is the bond angle of a molecule with linear molecular geometry?

180°

What is the bond angle of a molecule (2lp, 2bp) with bent molecular geometry?

104.5°

What is the bond angle of a molecule (1lp, 2bp) with bent molecular geometry?

118°

What is the difference between electron domain geometry (EDG) and molecular geometry (MG)?

EDG takes into account lone pairs, MG ignores lone pairs and shows the actual shape of the molecule

What does bond polarity result from?

Difference in electronegativities of bonded atoms

At which electronegativity range is a bond non-polar?

0 to 0.4

Pure at 0 (eg: Cl-Cl)

Weakly polar at 0.1 to 0.4

When is a bond polar?

Electronegativity from 0.5 to 1.7

How does a polar covalent bond work in terms of electrons?

Unequal sharing of electrons due to partial +ve and -ve charge from large electronegativity difference

Where does the arrow point in a dipole symbol?

Towards the partially negative atom

Which 2 factors affect the polarity of a molecule?

1. Bond polarity

2. Molecular geometry

Which basic molecular geometries allow molecules with polar molecules to be non-polar and why?

Linear, trigonal planar and tetrahedral

Symmetrical geometry so dipoles cancel out

True or false: Tetrahedral molecules with different atoms bonded to the central atom are non-polar

False. They are polar

True or false: giant covalent structures except graphite have only covalent bonds between atoms but no intermolecular forces

True

What are examples of giant covalent structures? (6)

Silicon

Silicon dioxide

Diamond

Graphite

Graphene

Buckminsterfullerine (not usually considered giant structure as it has fixed formula C60)

Describe the structure of silicon

Each silicon atom is covalently bonded to 4 others, forming tetrahedral with bond angle 109.5°

Describe the structure of silicon (IV) oxide/silicon dioxide

Each silicon atom is covalently bonded to 4 oxygen atoms

Each oxygen atom is bonded to 2 silicon atoms

Tetrahedral geometry

Describe the structure of diamond

Each carbon atom is covalently bonded to 4 others

Tetrahedral 109.5°

Why does diamond not conduct electricity?

All 4 outer electrons on each carbon atom are used in bonding

Describe the structure of graphite

Each carbon covalently bonded to 3 others in hexagonal rings arranged in flat layers, held by weak intermolecular forces

Bond angle 120°, trigonal planar

Does graphite conduct electricity and why?

The fourth outer electrons of each carbon is delocalised and moves freely between layers

Describe buckminsterfullerene’s (C60) structure

Football-like, consisting of hexagons and pentagons

Each carbon covalently bonds with 3 others

Does buckminsterfullerene conduct electricity?

Semiconductor

Remaining electron on each carbon is delocalised, allowing limited electron movement through structure

What is graphene’s structure like?

One layer of graphite (1 atom thick)

What is the melting and boiling points of giant covalent lattices like?

Very high due to large amt of strong covalent bonds

Describe the hardness of diamond, silicon dioxide, graphite and graphene.

Diamond and silicon dioxide are very hard due to their 3D covalent networks

Graphite it soft as the layers are held by weak forces that can slide

Graphene is very strong, thin and flexible

Describe the electrical conductivity of giant covalent structures

Most don’t conduct electricity as all electrons r used in bonding

Graphite and Graphene conduct electricity due to delocalised electrons

Buckminsterfullerene is a semiconductor

Are giant covalent structures except graphite soluble?

No, most are insoluble in water

True or false: covalent bonds are intermolecular forces

False, they are intramolecular forces

True or false: polar molecules dissolve in non-polar solvents

False. Like dissolves like so they only dissolve in polar solvents

What are the 4 types of intermolecular force?

London dispersion forces (van der waals)

Permanent dipole-dipole attractions (vdw)

Dipole-induced dipole attractions (vdw)

Hydrogen bonding

What are London dispersion forces?

Electrons in any molecules can be unevenly spread at any moment due to constant movement→produces temporary/instantaneous dipoles, which can induce another dipole in a neighbouring molecule

It’s present in all molecules and is the weakest IMF

Only IMF present in non-polar molecules

What increases the strength of London dispersion forces?

Higher amt of electrons

Larger molecular mass

Larger surface area available for contact

Are London dispersion forces (LDF) stronger in molecules with branched or unbranched shapes?

Branched due to higher surface area

What is a permanent dipole-dipole attraction?

Present in polar molecules

It’s a permanent separation of charge within bonds due to electronegativity difference (>0.4)

Stronger than LDF

Which type of molecules does a dipole-induced dipole attraction form in?

Polar and non-polar molecules

How does a dipole-induced dipole attraction form?

The permanent dipole of the polar molecule distorts the electron cloud of the non-polar molecule, forming a temporary dipole in the non-polar molecule (weak IMF)

What is hydrogen bonding?

When hydrogen bonds directly to F, O or N in molecules

Strongest IMF

What is the difference between inter and intramolecular forces

Inter-occurs between molecules (eg: hydrogen bonds)

Intra-occurs within molecules (eg: covalent)

What happens when an ionic compound dissolves in water?

Ion-dipoles form between ions and oppositely-charged dipoles of water molecules

The water molecules surround the ion to form a hydration shell

What does a substance need to conduct electricity?

delocalised electrons or free moving (mobile) ions

Why are benzene and buckminsterfullerene poor conductors of electricity?

The delocalised electrons can only move within the molecule, not between molecules

What happens to volatility when intermolecular forces of a molecule get stronger

It becomes less volatile as the bonds require more energy to break, making them less likely to evaporate

What happens to LDF strength when molar mass increases?

It increases

What is the formula for Rf value in chromatography?

Distance travelled by solute/distance travelled by solvent front