Structure and Bonding

Page 1

Formation of Ions

Ions are charged particles - can be single atoms or groups of atoms

Atoms lose or gain electrons to form ions - get a full outer shell

Atoms with full outer shells are very stable

Metals form ions - lose electrons from outer shell to form positive ions

Non-metals form ions - gain electrons to form negative ions

Number of electrons lost or gained is same as the charge

Group 1 and 2 elements are metals - lose electrons

Group 6 and 7 elements are non-metals - gain electrons

Elements in same group all have same number of outer electrons

Form ions with the same charges

Page 2

Ionic Bonding

Metal and a non-metal

Metal atom loses electrons to form a positively charged ion

Non-metal gains these electrons to form a negatively charged ion

Oppositely charged ions - strongly attracted to one another by electrostatic forces

Called ionic bond

Dot and cross diagrams show arrangement of electrons in an atom or ion

Each electron is represented by a dot or a cross

These diagrams can show which atom electrons originally came from

Dot and cross diagrams are useful for showing how ionic compounds are formed

But they don't show structure of compound, size of ions or arrangement

Page 3

Ionic Compounds

Ionic compounds have a giant ionic lattice structure

Ions form closely packed lattice structure

Very strongly electrostatic forces of attraction between oppositely charged ions

Single crystal of sodium chloride is one giant ionic lattice

The Na+ and Cl- ions are held together in a regular lattice

All have high melting and boiling points due to many strong bons between ions - takes a lot of energy to overcome this attraction

When they're solid - ions are held in place so compound can't conduct electricity

When ionic compounds melt, the ions are free to move and they'll carry electric current

Ionic compounds dissolve easily in water - ions separate and free to move so they'll carry electric current

Page 4

Covalent Bonding

Non-metal atoms bond together - share pairs of electrons - make covalent bonds

Positively charged nuclei of bonded atoms are attracted to shared pair of electrons by electrostatic forces - making covalent bonds very strong

Atoms only share electrons in outer shells

Each single covalent bond provides one extra shared electron for each atom

Each atom involved makes enough covalent bonds to fill outer shell

Gives them electronic structure of a noble gas - very stable

Covalent bonding happens in compounds of non-metals and non-metal elements

Page 5

Simple Molecular Substances

Made up of molecules, containing few atoms joined together by covalent bonds

Hydrogen, H2

Have one electron

Only need one electron to complete outer shell

Often form covalent bonds - either with other hydrogen atoms or other elements

Chlorine, Cl2

Needs one electron to complete outer shell

Two chlorine atoms - can share one pair of electrons and form simple covalent bond

Substances containing covalent bonds usually have simple molecular structures

Atoms within molecules are held together by strong covalent bonds - forces of attraction between molecules are very weak

To melt/boil you only need to break these weak intermolecular forces and not covalent bonds

Melting and boiling points are very low - molecules are easily parted from each other

Most molecular substances are gases/liquids at room temp

As molecules get bigger, strength of intermolecular forces increases

More energy is needed to break them and melting/boiling points increase

Molecular compounds don't conduct electricity - aren't charged

No free electrons or ions

Page 6

Polymers and Giant Covalent Structures

In a polymer, lots of small units are linked together - form long molecule with repeating sections

All atoms are joined by strong covalent bonds

Intermolecular forces between polymer molecules are larger than simple covalent molecules

More energy is needed to break them - means most polymers are solid at room temperature

Intermolecular forces are still weaker than ionic/covalent bonds

Generally have lower boiling points than ionic/giant molecular compounds

Giant Covalent structures are macromolecules

Giant covalent structures - all atoms are bonded to each other by strong covalent bonds

High melting and boiling points - lots of energy needed to break bonds

Don't contain charged particles - don't conduct electricity even when molten

Diamond

Each carbon atom forms four covalent bonds in a very rigid giant covalent structure

Graphite

Each carbon atom forms three covalent bonds to create layers of hexagons

Each carbon atom also has one delocalised electron

Can conduct electricity - one delocalised electron

Silicon Dioxide

Silica

What sand is made of

Each grain of sand is one giant structure of silicon and oxygen

Page 7

Allotropes of Carbon

Diamond

Giant Covalent Structure - made of carbon atoms that each form 4 covalent bonds - makes diamond very hard

Strong covalent bonds take a lot of energy to break - very high melting point

Doesn't conduct electricity - no free electrons or ions

Graphite

Each carbon atom only forms three covalent bonds

Creating sheets of carbon atoms arranged in hexagons

Aren't covalent bonds between layers

Held together weakly

Free to move over each other - makes graphite soft and slippery

High melting point

Covalent bonds in layers need lots of energy to break

Only 3 out of each carbon's 4 outer electrons are used in bonds

Each carbon atom has one electron that's delocalised and can move - can conduct electricity and thermal energy

Graphene is one layer of graphite

Sheet of carbon atoms joined together in hexagons

One atom thick, making it two-dimensional compound

Network of covalent bonds makes it very strong

Very light - can be added to composite materials to improve strength without adding much weight

Contains delocalised electrons

Conduct electricity through whole structure - used in electronics

Fullerenes form spheres and tubes

Molecules of carbon, shaped like hollow balls/closed tubes

Mainly made of carbon atoms arranged in hexagons - can contain pentagons or heptagons

Can be used to "cage" other molecules

Structure forms around another atom or molecule - is then trapped inside - can be used for drug delivery

Have a huge surface area - could help make industrial catalysts/lubricants

Can form nanotubes - tiny carbon cylinders

Ratio between length and diameter of nanotubes is very high

Nanotubes - conduct electricity and thermal energy

Don't break when stretched

Can be used in electronics or to strengthen materials without adding much weight - like tennis racket frames

Page 8

Metallic Bonding

Metals also consist of a giant structure

Electrons in outer shell are delocalised

Strong forces of electrostatic attraction between positive metal ions and shared negative electrons

Forces of attraction hold atoms together in regular structure

Known as metallic bonding - very strong

Substances held together by metallic bonding include metallic elements and alloys

Delocalised electrons produce all properties of metals

Most metals are solid at room temp

Electrostatic forces are very strong

Lots of energy needed to be broken

Means most compounds with metallic bonds have very high melting and boiling points

Generally solid at room temperature

Good conductors of electricity and heat

Delocalised electrons carry electrical current and thermal energy through whole structure

Good conductors of electricity and heat

Most are malleable

Layers of atoms in a metal can slide over each other

Means they can be bent/hammered/rolled into flat sheets

Alloys are harder than pure metals

Pure metals often aren't right for certain jobs

Often too soft so are mixed with other metals to make them harder

Most metals used everyday are alloys - mixture of or more metals/a metal and another element

Alloys are harder and more useful

Different elements have different sized atoms

When another element is mixed with a pure metal, new metal atoms will distort layers of metal atoms

More difficult for layers to slide over one another - makes alloys harder

Page 9

States of Matter

Materials come in three different forms - solid, liquid and gas

State of something depends on how strong forces of attraction are

Forces of attraction depends on three things:

Material

Temperature

Pressure

Solids

Strong forces of attraction between particles

Holds them together in fixed positions to form regular lattice

Particles don't move

Solids keep a definite shape and volume

Particles vibrate

Hotter the solid becomes, the more they vibrate (causing solids to expand slightly when heated)

Liquids

Weak forces of attraction

Randomly arranged and free to move past each other

Tend to stick closely together

Definite volume

Don't have a definite shape

Will flow to fill container

Particles are constantly moving with random motion

Hotter the liquid gets, faster they move

Causes liquids to expand slightly when heated

Gases

Force of attraction is very weak

Free to move and are far apart

Particles in gases travel in straight lines

Don't keep definite shape or volume

Will always fill container

Particles move constantly with random motion

Hotter the gas gets, faster they move

Gases either expand when heated, or pressure increases

Page 10

Changing State

Physical changes don't change particles - just their arrangement or their energy

When a solid is heated, particles gain more energy

Makes particles vibrate more, which weakens forces that hold solid together

At melting point, particles have enough energy to break free from their position - this is melting

When liquid is heated, again particles gain more energy

Energy makes particles move faster - weakens and breaks bonds holding liquid together

At boiling point, particles have enough energy to break bonds - this is boiling

As a gas cools, particles no longer have enough energy to overcome forces of attraction between them

Bonds form between particles

At boiling point, so many bonds have formed between gas particles that gas becomes liquid - condensation

When a liquid cools, particles have less energy - so move around less

There's not enough energy to overcome attraction between particles, so more bonds form between them

At melting point, so many bonds have formed between particles that they're held in place - freezing

Amount of energy needed for a substance to change state depends on how strong forces between particles are

Stronger the forces, the more energy is needed to break them and so the higher the melting point and boiling points of the substance