The Covalent Model (DP IB Chemistry)

Lewis formulas show all of the valence electrons, including bonding and non-bonding pairs in a covalently bonded structure.

True.

Lewis formulas can also be known as electron dot and Lewis structures.

Lewis formulas can be represented using dots, crosses or dashes.

False.

The Lewis formula of chlorine can be shown as:

The Lewis formula for PF₅ using dashes is:

The Lewis formula for nitrous oxide, HNO₂ is:

The first step in drawing a Lewis formula is to count the total number of valence electrons.

An incomplete octet in Lewis structures refers to elements that form stable compounds with less than 8 electrons in their valence shell.

The Lewis formula for CCl₄ using dots is:

The Lewis formula for BeCl₂ is:

One shared pair of electrons is involved in a single covalent bond.

Three shared pairs of electrons are involved in a triple covalent bond.

Four electrons are shared in a double covalent covalent bond.

False.

Triple bonds are shorter than single bonds.

As bond length decreases, bond strength increases.

False.

It is not possible to form quadruple bonds as the repulsion from having 8 electrons in the same region between two nuclei is too great.

Triple bonds are stronger than single bonds due to greater electron density between the nuclei.

Single covalent bonds are the longest and weakest.

True.

The greater the forces of attraction between the electrons in the covalent bond and the nuclei, the stronger the covalent bond.

Triple covalent bonds are the shortest and strongest.

A coordination bond is a covalent bond in which both the electrons of the shared pair originate from the same atom.

Coordination bonds are also known as dative covalent bonds and coordinate bonds.

False.

Both metals and non-metals can form coordinate bonds.

The displayed formula of NH₃BF₃ is:

The type of bond formed between an ammonia molecule and hydrogen ion to form an ammonium ion is a coordination bond.

False.

Coordination bonds are just as strong as regular covalent bonds once formed.

VSEPR theory (Valence Shell Electron Pair Repulsion theory) is a model used to predict the shapes of molecules based on the repulsion between electron pairs.

The rules of VSEPR theory are:

• All electron pairs arrange themselves as far apart in space as is possible

• Lone pairs repel more strongly than bonding pairs

• Repulsion between multiple and single bonds is treated the same as for repulsion between single bonds

The bond angle in a tetrahedral molecule is approximately 109.5°.

False.

Molecules with a linear shape have bond angles of 180^o.

A molecule with four electron domains, including one lone pair, adopts a trigonal pyramidal shape.

The molecular shape of a molecule with three electron domains in which there are no lone pairs is trigonal planar / triangular planar.

True.

Molecules with four electron domains can be tetrahedral, trigonal pyrimidal or bent linear depending on how many lone pairs they have.

The bond angle in a molecule with trigonal planar domain geometry and one lone pair is 118^o.

A CCl₄ molecule has a tetrahedral shape.

The shape and bond angle for a BF₃ molecule is:

• Shape: Trigonal planar

• Bond angle: 120^o

Electron domain geometry includes all electron domains (bonding and non-bonding), while molecular geometry only considers the arrangement of atoms.

The domain shape of a molecule of ammonia is tetrahedral.

The molecular shape of a molecule of ammonia is trigonal pyramid.

Electronegativity is the ability of an atom to draw an electron pair towards itself in a covalent bond.

A polar bond is a covalent bond in which electrons are unequally shared between atoms, resulting in a partial positive charge on one atom and a partial negative charge on the other.

δ⁺ symbolises a partial positive charge in a polar bond and is assigned to the less electronegative atom.

δ^- symbolises a partial negative charge in a polar bond and is assigned to the more electronegative atom.

False.

Not all covalent bonds are polar; bonds between atoms with equal electronegativity are non-polar.

The extent of polarity in a covalent bond is determined by the difference in electronegativity between the bonded atoms.

True.

The greater the difference in electronegativity, the more polar the bond.

A non-polar bond is a covalent bond in which electrons are shared equally between atoms with similar electronegativities.

The bond between two chlorine atoms is non-polar because the two atoms have the same electronegativities so the electrons are shared equally between them.

A dipole moment is a measure of the polarity of a bond, represented by an arrow pointing towards the partially negative end of the dipole.

Fluorine has the highest electronegativity on the Pauling scale.

Bond polarity affects properties such as solubility, boiling point, and reactivity of a molecule.

The polarity of a molecule is determined by both the polarity of individual bonds and the overall geometry of the molecule.

False.

Not all molecules with polar bonds are polar; the overall geometry can result in a non-polar molecule.

False.

CCl₄ is a non-polar molecule because although there are four polar covalent bonds, the individual dipole moments cancel each other out.

BF₃ is a non-polar molecule because it is symmetrical.

Molecules with polar bonds may not be polar overall because the arrangement of the polar bonds within the molecule allow the individual dipole moments to cancel each other out.

Giant covalent structures are solids with high melting points, consisting of a huge number of non-metal atoms bonded to other non-metal atoms via strong covalent bonds.

An allotrope is a different structural form of the same element.

True.

Giant covalent structures are also called giant lattices.

This giant covalent structure is graphite.

Giant covalent structures have high melting points because there are many strong covalent bonds between atoms which require lots of energy to overcome.

This giant covalent structure is diamond.

Diamond and graphite are made from carbon atoms.

Silicon dioxide (silica) is made from silicon and oxygen.

False.

Giant covalent structures are solids at room temperature.

False.

Giant covalent structures have a fixed ratio of atoms in the overall structure.

This giant covalent structure is silicon dioxide.

In graphite:

• Each carbon atom is bonded to three others

• There are hexagonal layers

• There is one free / delocalised electron per carbon atom

Graphite conducts electricity because it has delocalised electrons that can move and carry a charge throughout the structure.

Graphite is soft because there are weak intermolecular forces betwen the layers, which allows the layers to slide over each other.

In graphite each carbon atom forms three bonds.

Diamond is hard because each carbon atom is bonded to four other carbon atoms via strong covalent bonds.

Each carbon atom forms four bonds.

The bond angle between carbon atoms in diamond is 109.5^o.

The formula of Buckminsterfullerene is C₆₀.

False.

Graphene is essentially a 2D molecule since it is only one atom thick.

True.

Silicon dioxide has a tetrahedral shape.

Diamond and silicon dioxide cannot conduct electricity because all four electrons are involved in a covalent bond so there are no free electrons to carry a charge.

Silicon has each atom bonded covalently to four other silicon atoms in a giant lattice structure.

Properties of silicon include:

• Good thermal conductivity

• Good mechanical strength

• Poor electrical conductivity

• High melting and boiling point

Intermolecular forces are forces of attraction between molecules.

Three types of intermolecular forces are:

• London (dispersion) forces

• Dipole-induced dipole

• Dipole-dipole attraction

• Hydrogen bonding

London (dispersion) forces are weak intermolecular forces arising from temporary fluctuations in electron distribution, present between all atoms and molecules.

The strongest type of intermolecular force in hydrogen chloride is permanent dipole- permanent dipole.

False.

Hydrogen bonding is the strongest type of intermolecular force.

For hydrogen bonding to take place:

• A species which has a very electronegative atom, O, N or F, with an available lone pair of electrons

• A hydrogen attached to the O, N or F

The strongest type of intermolecular force in ammonia, NH₃ is hydrogen bonding.

True.

London (dispersion) forces are present between all atoms and molecules.

As the size of a molecule increases, the strength of the London (dispersion) forces increases.

False.

Krypton has stronger London (dispersion) forces than argon because it has a greater number of electrons.

False.

Van der Waals forces include:

• London (dispersion) forces

• Dipole-induced dipole attractions

• Dipole-dipole attractions.

Permanent dipole-permanent dipole forces are forces of attraction between the δ+ of one polar molecule and the δ- of another polar molecule.

London (dispersion) forces are the weakest type of intermolecular force.

Hydrogen bonds are the strongest type of intermolecular force.

The more polar a molecule, the stronger the intermolecular forces.

False.

CH₃CH₂CH₂CH₃ has a lower boiling point than CH₃CH₂CH₂OH because it has only London (dispersion) forces whereas CH₃CH₂CH₂OH also has hydrogen bonding.

As the covalent molecule becomes larger, their solubility decreases.

True.

Polar covalent substances generally dissolve in polar solvents.

Many covalent substances are unable to conduct electricity because there are no freely moving charged particles.

False.

Substances with low melting and boiling points are said to be very volatile

Butan-1-ol has a higher boiling point than butanal because it contains hydrogen bonding whereas butanal contains dipole-dipole forces. Hydrogen bonds are stronger than dipole-dipole forces so need more energy to overcome them.

Ethane is insoluble in water because it does not have any polar bonds, so will not dissolve well in water which does contain polar bonds. Ethane cannot form hydrogen bonds with the water.

Two types of chromatography are:

• Paper chromatography

• Thin-layer chromatography (TLC)

Chromatography is a technique used to separate the components of a mixture based on their relative attractions to mobile and stationary phases.

A single spot on a chromatogram indicates that the substance is pure.

The baseline in paper chromatography should be drawn in pencil to prevent it running along with / contaminating the samples.

False.

The solvent level should not start above the pencil line, as this would ruin the chromatogram.

There are 3 substances in D.

False.

Multiple spots on a chromatogram indicate that the substance is a mixture.

True.

If two substances are the same, they will produce identical chromatograms.

The black food colouring contains A, E and an unknown.

Substance B remains on the baseline because it is insoluble in the solvent used.

The equation for the retardation factor (R_f value) is:

An R_f value close to 1 indicates that the component is very / highly soluble in the solvent used.

False.

The R_f value of a compound depends on the solvent used.

If the solvent is changed, the R_f value will change.

The purpose of calculating R_f values is to allow chemists to identify unknown substances.

An R_f value does not have units because it is a ratio.

The R_f values ranges between 0 and 1.

The mobile phase is the water, and the stationary phase is the chromatography paper.

The mobile and stationary phase in thin-layer chromatography are:

• Mobile phase - solvent

• Stationary phase - inert substance e.g. silica or alumina supported on a flat, unreactive surface such as glass

False.

During TLC, the components of the sample form hydrogen bonds with the stationary phase.