LECTURE 1

Ionic Bonding

IB Chemistry Power Points

Topic 4

Bonding

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STRUCTURE AND BONDING

Key Concept:The physical properties of a substance (such as hardness, density, melting point and conductivity) depend on the structure of the substance. Bonding determines arrangement of particles and therefore, the type of structure. Basic theory – things you should know

1. The noble gases (He, Ne, Ar, Kr, Xe and Rn) are in Group VIII, they are all relatively inert meaning they do not react and bond like other elements.

2. We can conclude that noble gas electron configuration appears to confer stability. All noble gases have just filled their ‘outer shell’ of electrons

3. Other elements have a tendency to obtain the same electron arrangement (a full outer shell of valence electrons). Obtaining this electron arrangement is accomplished by different methods of chemical bonding.

4. The type of bonding depends on an element’s position in the periodic table

TYPES OF BONDING

CHEMICAL BONDS....are strong intramolecular bonds that hold atoms together in molecules, ionic solids and metals. These bonds are broken and made in chemical reactions. ionic

covalent dative covalent (or co-

ordinate) metallic

PHYSICAL BONDS....are intermolecular forces that exist between two or more separate molecules. These are the attractions involved in physical changes.

van der Waals‘ forces - weakest dipole-dipole interactions

hydrogen bonds - strongest

PART 1: CHEMICAL BONDS

THE IONIC BOND

Ionic bonds are formed between elements whose atoms tend to “lose” electrons to gain the nearest noble gas electronic configuration and those which tend to “gain” electrons. The electrons are transferred from one atom to the other.

Sodium Chloride

Na ——> Na+ + e¯ and Cl + e¯ ——> Cl¯

THE IONIC BOND

Ionic bonds are formed between elements whose atoms tend to “lose” electrons to gain the nearest noble gas electronic configuration and those which tend to “gain” electrons. The electrons are transferred from one atom to the other.

Sodium Chloride

Na ——> Na+ + e¯ and Cl + e¯ ——> Cl¯

2,8,1 2,8 2,8,7 2,8,8

THE IONIC BOND

Ionic bonds are formed between elements whose atoms tend to “lose” electrons to gain the nearest noble gas electronic configuration and those which tend to “gain” electrons. The electrons are transferred from one atom to the other.

Sodium Chloride

Na ——> Na+ + e¯ and Cl + e¯ ——> Cl¯

2,8,1 2,8 2,8,7 2,8,8

or 1s2 2s2 2p6 3s1 1s2 2s2 2p6 1s2 2s2 2p6 3s2 3p5 1s2 2s2 2p6 3s2 3p6

HL only responsible for s,p,d,f electron configurations

THE IONIC BOND

Ionic bonds are formed between elements whose atoms tend to “lose” electrons to gain the nearest noble gas electronic configuration and those which tend to “gain” electrons. The electrons are transferred from one atom to the other.

Sodium Chloride

Na ——> Na+ + e¯ and Cl + e¯ ——> Cl¯

2,8,1 2,8 2,8,7 2,8,8

or 1s2 2s2 2p6 3s1 1s2 2s2 2p6 1s2 2s2 2p6 3s2 3p5 1s2 2s2 2p6 3s2 3p6both species end up with the electronic configuration of the nearest

noble gas

the resulting ions are held together in a crystal lattice by electrostatic attraction.

SODIUM CHLORIDE

Cl

SODIUM ATOM2,8,1

Na

CHLORINE ATOM2,8,7

SODIUM CHLORIDE

SODIUM ION2,8

CHLORIDE ION2,8,8

both species now have ‘full’ outer shells; ie they have the electronic configuration of a noble gas

Na+

Cl

ELECTRON

TRANSFER

Mg ——> Mg2+ + 2e¯ and 2Cl + 2e¯ ——> 2 Cl¯

Mg

Cl

Cl

e¯

e¯

THE IONIC BOND

FORMATION OF MAGNESIUM CHLORIDE

MAGNESIUM CHLORIDE

Cl

MAGNESIUM ATOM2,8,2

MgCHLORINE ATOMS

2,8,7

Cl

MAGNESIUM CHLORIDE

Cl

MAGNESIUM ION2,8

MgCHLORIDE IONS

2,8,8

Cl

2+

• also known as cations; they are smaller than the original atom.

• formed when electrons are removed from atoms.• the energy associated with the process is known as the

ionisation energy

Recall: 1st IONISATION ENERGY (1st I.E.)The energy required to remove one mole of electrons (to infinity) from the one mole of gaseous atoms to form one mole of gaseous positive ions.

e.g. Na(g) ——> Na+(g) + e¯ or Mg(g) ——> Mg+(g) + e¯

Other points (HL topic 12)

Successive IE’s (2nd , 3rd , etc) get larger as the proton:electron ratio increases.

Large jumps in value occur when electrons are removed from shells nearer the nucleus because there is less shielding and more energy is required to overcome the attraction.

If the I.E. values are very high, covalent bonding will be favoured (e.g. beryllium).

THE FORMATION OF POSITIVE IONS

• known as anions• are slightly larger than the neutral atom - electron repulsion

in outer shell• formed when electrons are added to atoms• energy is released as the nucleus pulls in an electron• this energy is the electron affinity.

ELECTRON AFFINITYThe energy change when one mole of gaseous atoms acquires one mole of electrons (from infinity) to form one mole of gaseous negative ion

e.g. Cl(g) + e¯ ——> Cl¯(g) and O(g) + e¯ ——> O¯(g)

The greater the effective nuclear charge (E.N.C.)of a species, the easier an electron is attracted.

THE FORMATION OF NEGATIVE IONS

Ions form from a tendency toward a lower energy, stable, noble gas electron configuration

Transition metals have more complex electron arrangements (d – subshell HL material) thus can form more than one ion.

IONIC COMPOUNDS - CRYSTAL LATTICE STRUCTURE

Cl-

Chloride ion

Na+

Sodium ion

Oppositely charged ions held in a regular3-dimensional lattice by electrostatic attraction

The arrangement of ions in a crystal lattice depends on the relative sizes of the ions

The Na+ ion is small enough relative to a Cl¯ ion to fit in the spaces so that both ions occur in every plane.

Each Na+ is surrounded by 6 Cl¯and each Cl¯ is surrounded by 6 Na+

Oppositely charged ions held in a regular 3-dimensional lattice by electrostatic attraction:

The arrangement of ions in a crystal lattice depends on the relative sizes of the ions

Physical properties of ionic compounds

Melting pointvery high A large amount of energy must be put in to overcome the

strong electrostatic attractions and separate the ions.

StrengthVery brittle Any dislocation leads to the layers moving and similar

ions being adjacent. The repulsion splits the crystal.

Electrical don’t conduct when solid - ions held strongly in the lattice

conduct when molten or in aqueous solution - the ions

become mobile and conduction takes place.

Solubility Insoluble in non-polar solvents but soluble in water

Water is a polar solvent and stabilises the separated ions.

IONIC COMPOUNDS - ELECTRICAL PROPERTIES

SOLID IONIC COMPOUNDS DO NOT CONDUCT ELECTRICITY

Na+

Cl- Na+

Cl-

Na+

Cl-Na+

Cl-

Na+

Cl- Na+

Cl-

IONS ARE HELD STRONGLY TOGETHER

+ IONS CAN’T MOVE

- IONS CAN’T MOVE

MOLTEN IONIC COMPOUNDS DO

CONDUCT ELECTRICITY

Na+

Cl-

Na+

Cl-

Na+

Cl-

Na+Cl-

IONS HAVE MORE FREEDOM IN A LIQUID SO CAN MOVE TO THE ELECTRODES

SOLUTIONS OF IONIC COMPOUNDS

IN WATER DO CONDUCT

ELECTRICITY

DISSOLVING AN IONIC COMPOUND IN WATER BREAKS UP THE STRUCTURE SO IONS ARE FREE TO MOVE TO THE ELECTRODES

Many ionic compounds are soluble in water - dissolving