Molecular shape and bonding

name the 3 types of chemical bonds

ionic, covalent, metallic

describe an ionic bond

metal and non-metal, electrons transferred, electrostatic attraction holds ions together

describe a metallic bond

metal and metal, electrons pooled

describe thermodynamic properties of ionic compound formation

ionisation energy of the metal to form the cation is endothermic

acceptance of an electron (electron affinity) by the non-metal is exothermic

explain why the heat of formation of an ionic compound is a large exothermic value

due to formation of a highly ordered structure known as the crystal lattice

(despite the IE of metal generally being larger than EA of non-metal)

describe the properties of the electrostatic attraction within ionic compounds

non-directional, there is no direct anion-cation pair

therefore there is no ionic molecule

what does the chemical formula of an ionic compound actually tell us?

empirical formula - gives the ratio of ions based on charge balance

why do chemical bonds form?

they lower the potential energy between the charged particles that constitute atoms

describe a polar covalent bond

electrons shared unequally due to increased electronegativity dfference

give the principle of Lewis theory of bonding

uses the valence electrons which are held most loosely and hence used in bonding

(elements in a particular column of the periodic table have the same properties because they have the same number of valence electrons)

describe the Lewis theory for ionic compounds

electrons are transferred from one atom to another atom, forming ions

describe Lewis theory for covalent compounds

atoms achieve their octet by sharing electrons - a bond consists of a shared pair of electrons

what is a lone pair of electrons?

electrons that are not shared between atoms

how does Lewis theory explain the shape of covalent molecules?

electrostatic repulsion between electron-pair bonds and lone pairs

equation for formal charge

number of valence electrons - (number of lone pair electrons + ½ number bonding electrons)

what is a free radical?

a species where one atom has a single unpaired electron

describe a pure covalent bond

non-metal and non-metal, electrons shared equally

describe some properties that influence strength of lattice energies

ion size - larger ion = weaker attraction so weaker

ion charge - larger charge = stronger attraction so higher

all ionic compounds have high mp and bp, and are solids at room temperature

state the 2 types of covalent bond

pure and polar

define electronegativity

the ability of an atom to pull bonding electrons towards itself

what is a dipole?

a covalent bond with one positive and one negative end (polar)

define atomic number

the number of protons in nucleus (= electrons if neutral)

define mass number

the number of protons + number of neutrons

define an element

all atoms with the same atomic number

define an isotope

atoms of an element that have different atomic number (different number of neutrons)

define molecular weight (Mr)

sum of all atomic weights of all atoms in a molecule

define an orbital

a mathematically defined region of space where there is a high probability of ‘finding’ an electron

electronic configuration

outline the aufbau principle

electrons are assigned to the available orbital of lowest energy

outline the Pauli exclusion principle

1. no more than two electrons can occupy each atomic orbital

2. the electrons must have opposite spins (denoted by an arrow)

outline Hund’s rule of maximum multiplicity

in the case of degenerate orbitals, an electron will occupy an empty orbital before it will pair up with another electron (minimises electron repulsion)

describe the octet rule

the tendency of atoms to prefer to have 8 electrons in their valence shell

(when they have fewer, they tend to react to form stable compounds)

what is periodicity?

the idea that elements in the same column of the periodic table have the same number of valence electrons (and this denotes chemical and physical properties)

define a valence shell

outermost electron shell on an atom

what is closed-shell configuration?

when an atom’s valence shell is completely filled with electrons, resulting in high stability and chemical inertness

what is the ground state of an atom?

its lowest energy, most stable state, where all electrons occupy the lowest possible energy levels or orbitals (adhering to aufbau principle)

define a node

a region where probability of finding an electrons falls to zero

result of wave-like properties

what shape is an s atomic orbital indicated to be?

sphere

what shape are p orbitals indicated to be?

dumbbell

have nodal plane that ‘cuts’ the dumbbell into two lobes

describe 2p orbitals

3 degenerate (same energy) p atomic orbitals

have symmetry about the x, y and z axes

each orbital is perpendicular to the other two

higher in energy than 2s orbitals

what two main theories are employed to describe a covalent bond?

molecular orbital (MO) and valence bond

(MO - atomic orbitals are combined to make MOs)

(valence - electrons within the molecule remain in the atomic orbital, but we don’t know in which atomic orbital)

outline MO theory

molecules are viewed as made up from nuclei and electrons - nuclei are ‘held together’ by molecular orbitals (rather than overlap of atomic orbitals)

electrons are delocalised throughout the entire molecule

define a molecular orbital (MO)

an electron distribution with electron density over the whole molecule

explain the difference between bonding and antibonding molecular orbital formation?

bonding: AOs of same phase sign overlap (eg, + and +)

antibonding: AOs of opposite phase sign overlap (+ and -)

antibonding does not mean its not allowed, merely that it is higher in energy

where are electrons likely to be found in bonding and anti-bonding MOs?

bonding - between the nuclei

anti-bonding - has a node, so anywhere but between the nuclei

which is stronger, pi or sigma bonds?

sigma

what is hybridisation?

using various combinations of orbitals to make new hybrid orbitals in order to simplify the maths (not a physical proces)

describe how we would employ hybridisation for CH4

promote an electron into empty 2p orbital

mix the 2s and 2p orbitals together, to construct 4 new degenerate sp3 hybrid orbitals

these are ¼ s and ¾ p in character

each of the 4 sp3 orbitals overlaps with a hydrogen atom (1s orbital) to form CH4

what shape and bond angle would sp hybrid orbitals form?

linear, 180 (2 electron groups)

what shape and bond angle would sp2 hybrid orbitals form?

trigonal planar, 120 (3 electron groups)

what shape and bond angle would sp3 hybrid orbitals form?

tetrahedral, 109.5 (4 electron groups)

what type of bonds comprise single, double and triple bonds?

single - one sigma

double - one sigma, one pi

triple - one sigma, two pi