HL Chemistry - The Ionic, Covalent, and Metallic Model

Ionic bonds involve

the transfer of electrons from a metallic element to a non-metallic element

Metals ___ electrons from their valence shell forming ______ charged ______

lose, positively, cations

Non-metal atoms ____ electrons forming ______ charged ______

gain, negatively, anions

Once the atoms become ions, their electronic configurations

are the same as a noble gas.

definition of ionic bonding

'the force of attraction between oppositely charged species / ions'

force of attraction in ionic bonding

very strong and requires a lot of energy to overcome

This causes high melting points in ionic compounds

What is the charge of an ionic compound?

• Ionic compounds are electrically neutral; the positive charges equal the negative charges

• This means that the overall charge of an ionic compound is 0

non-metal positive ions

• ammonium, NH₄⁺, and hydrogen, H⁺

ionic bonds structure

lattice structure, known as an ionic lattice

This is an evenly distributed crystalline structure

regular repeating pattern so that positive charges cancel out negative charges

What forces hold together an ionic lattice?

there are strong electrostatic forces of attraction between the oppositely charged ions

Properties of Ionic Compounds

strong, brittle (ionic crystals can split), high melting and boiling points, not volatile

soluble in water as they can form ion-dipole bonds

• When molten or in solution, the ions can freely move around and conduct electricity

• As a solid, the ions are in a fixed position and unable to move around

covalent bonds are between

nonmetal and nonmetal

covalent bond involves the

electrostatic attraction between nuclei of two atoms and the electrons of their outer shells

No electrons are transferred but only shared in this type of bonding

electrons in a covalent bond are

in a state of constant motion and are best regarded as charge clouds

bond energy

the energy required to break one mole of a particular covalent bond in the gaseous states

bond length

internuclear distance of two covalently bonded atoms

• It is the distance from the nucleus of one atom to another atom which forms the covalent bond

dative covalent bond/coordinate

• Some molecules have a lone pair of electrons that can be donated to form a bond with an electron-deficient atom

• So both electrons are from the same atom

VSEPR theory predicts the molecular geometry and angles through three basic rules:

1. All electron pairs and all lone pairs arrange themselves as far apart in space as is possible.

2. Lone pairs repel more strongly than bonding pairs.

3. Multiple bonds behave like single bonds

electronegativity

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

• The higher the value, the more electronegative the element is 

polar bond

When two atoms in a covalent bond have different electronegativities

the covalent bond is polar and the electrons will be drawn towards the more electronegative atom

• The bigger the difference in electronegativity, the higher the polarity of the covalent bond

polar bonds result in

• The negative charge centre and positive charge centre do not coincide with each other

• This means that the electron distribution is asymmetric

• The less electronegative atom gets a partial charge of δ+ (delta positive)

• The more electronegative atom gets a partial charge of δ- (delta negative)

dipole moment

a measure of how polar a bond is

The direction of the dipole moment is shown by the following sign in which the arrow points to the partially negatively charged end of the dipole

Diamond

• giant lattice of carbon atoms

• c is covalently bonded to four others in a tetrahedral arrangement

• The result is a giant lattice with strong bonds in all directions

• Diamond is the hardest substance known

• v high mp and bp

• transparant crystal

• non conductor

Graphite

carbon atom is bonded to three others in a layered structure

The layers are made of hexagons bond angle 120

The spare electron is delocalised and occupies the space in between the layers

different layers are held together by weak intermolecular forces

high bp and mp

grey solid

good conductor

soft and slippery

Graphene

infinite lattice of covalently bonded atoms in two dimensions only to form layers.

single layer of carbon atoms that are bonded together in a repeating pattern of hexagons

high mp and bp

transparant

good conductor

thin and strong

Fullerene

60 carbon atoms, each of which is bonded to three others by single covalent bonds

The fourth electron is delocalised so the electrons can migrate throughout the structure making the buckyball a semi-conductor

low mp and bp

black powder

bad conductor

light n strong

Silicon

tetrahedral arrangement, just like that of the carbon atoms in diamond

• Each silicon atom is covalently bonded to four other silicon atoms 

high mp and bp

greywhite solid

poor conductor

good mechanical strength

Silicon dioxide

tetrahedral units all bonded by strong covalent bonds

Each silicon is shared by four oxygens and each oxygen is shared by two silicon atoms

high mp and bp

transparant crystal

non conductor

Properties of Giant Covalent Structures

very high melting and boiling points

• These compounds have a large number of covalent bonds linking the whole structure

• A lot of energy is required to break the lattice

hard or soft

• Most compounds are insoluble with water

• Most compounds do not conduct electricity

  • Graphite delocalised electrons

  • Graphene is an excellent conductors of electricity due to the delocalised electrons

  • Buckminsterfullerene is a semi-conductor

  • Diamond and silicon(IV) oxide do not conduct electricity as all four outer electrons on every carbon atom is involved in a covalent bond so there are no free electrons available

London (dispersion) forces

• The electrons in atoms are not static; they are in a state of constant motion

  • the distribution of electrons will not be exactly symmetrical - slight surplus of electrons on one side of the atoms

present between all atoms and molecules, very weak

• depends on two factors:

• the number of electrons in the molecule

• Surface area of the molecules

a temporary attractive force due to the formation of temporary dipoles in a nonpolar molecule

Dipole-dipole attractions

attraction between a permanent dipole on one molecule and a permanent dipole on another.

only in polar

Dipole-induced dipole attraction

mixtures might contain both polar and nonpolar molecules.

The permanent dipole of a polar molecule an cause a temporary separation of charge on a non-polar molecule 

hydrogen bonding

Hydrogen bonding is the strongest type of intermolecular force

When hydrogen is covalently bonded to an electronegative atom, the bond becomes very highly polarised

Van der Waals' forces

• The term Van der Waal's forces is used to include:

  • London dispersion forces

  • Dipole-induced dipole attractions

  • Dipole-dipole attractions

• These forces occur between molecules (intermolecularly), as well within a molecule (intramolecularly)

The strength of the intermolecular forces increases with

• the size of the molecule

• the increase in the polarity of the molecule

• Drawing the structure of the molecule helps identify and rank molecules according to boiling point as the following example shows

solubility principle

like dissolves like.

covalent conductivity

• do not contain any freely moving charged particles they are unable to conduct electricity in either the solid or liquid state

• However, under certain conditions some polar covalent molecules can ionise and will conduct electricity

• Some giant covalent structures are capable of conducting electricity due to delocalised electrons but they are exceptions to the general rule

non polar covalent

low bpmp, high volatility, insoluable in polar, can in nonpolar, no conductivity

polar covalent

low bpmp, high volatility, Some solubility
depending on molecular size for both, no conductivity

giant covalent

high mpbp, low volatility, insoluable, no conductivity except graphite and graphene

ionic

high mpbp, low volatility, insoluable in non polar, no conductivity except when molten

Delocalised electrons

electrons in a molecule, ion or solid metal that are not associated with a single atom or one covalent bond

resonance structures

two or more possible electron structures

all the bonds are equal in length and the electron density is spread evenly between the three oxygen atoms

• The bond length is intermediate between a single and a double bond

• The actual structure is something in between the resonance structures and is known as a resonance hybrid

resonance examples

Species	Lewis Resonance Formulas	Resonance Hybrid
Carbonate ion, CO32-
Benzene, C6H6
Ozone, O3
Carboxylate ion, RCOO-

Structure of Benzene

6 carbon atoms in a hexagonal ring, with alternating single and double carbon-carbon bonds

Evidence for delocalisation in benzene

• Enthalpy changes of hydrogenation

  • one c-c bond 120, expected 360 bc triple

  enthalpy change obtained is far less exothermic, ΔH^ꝋ = -208 kJ mol^-1 

  • less energy produced than expected, more stabillity

Carbon-carbon bond lengths

• All of the carbon-carbon bond lengths are 140 pm suggesting that they are all the same and also intermediate of the single C-C and double C=C bonds

Saturation tests

• Benzene does not decolourise bromine water suggesting that there are no double C=C bonds

Infrared spectroscopy

Molecular geometry

the shape of the molecules based on the relative orientation of the atoms

Electron domain geometry

the relative orientation of all the bonding and lone pairs of electrons

Formal Charge formula

FC= (number of valence electrons) - ½(number of bonding electrons) - (number of non-bonding electrons)

or

FC= V - ½B - N

preferred when: the difference in FC of the atoms is closest to zero

• has negative charges located on the most electronegative atoms

Sigma (σ) bonds

• the head-on / end-to-end combination or overlap of atomic orbitals

• The electron density is concentrated along the bond axis (an imaginary line between the two nuclei)

• s orbitals overlap this way as well as p to p, and s with p orbitals

Pi (π) bonds

the lateral (sideways) combination or overlap of adjacent p orbitals

• This maximises the overlap of the p orbitals

• The electron density is concentrated on opposite sides of the bond axis

sp³

4 electron domains

sp²

3 electron domains

sp

2 electron domains