Bonding, Bonding, Structure & Structure & Properties of Properties of SubstancesSubstances

UNIT 2 Aim A

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Why do atoms form bonds?

Bonds involve the electrons in the outer shells of atoms.

Filled electron shells are very stable.

1st shell holds a maximum of 2 electrons

2nd shell holds a maximum of 8 electrons

3rd shell holds a maximum of 8 electrons

Each shell has a maximum number of electrons that it can hold. Electrons fill the shells nearest the nucleus first.

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Why do atoms form bonds?

The atoms of noble gases have completely full outer shells and so are stable.

The atoms of other elements have incomplete outer electron shells and so are unstable.

This makes the noble gases very unreactive and so they do not usually form bonds.

By forming bonds, the atoms of these elements are able to have filled outer shells and become stable.

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What are the types of bonding?

Different types of bonds are formed depending on the types of atoms involved:

All bonds involve electrons and all bonding involve changes to the number of electrons in the outer shells of atoms.

ionic bonding – occurs between metal and non-metal atoms.

covalent bonding – occurs between non-metals atoms only.

metallic bonding – occurs between metal atoms only.

How do you think electrons are involved in ionic bonding?

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Positive and negative ions?

The electron configuration of an atom shows how many electrons it must lose or gain to have a filled outer shell.

An atom that loses electrons has more protons than electrons and so has a positive overall charge.

This is called a positive ion.

An atom that gains electrons has more electrons than protons and so has a negative overall charge.

This is called a negative ion.

Atoms with a nearly empty outer shell, will lose electrons to obtain a full outer shell.

Atoms with a nearly full outer shell, will gain electrons to obtain a full outer shell.

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How are ionic bonds formed?

The positive sodium ions and the negative chloride ions are strongly attracted to each other.

-

Cl

It is this electrostatic attraction that forms ionic bonds in sodium chloride and other ionic compounds.

Na

+

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Formation of an ionic bond

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Do ionic compounds conduct electricity?

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How are covalent bonds formed?

How do non-metal atoms form covalent bonds?

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Two common ways to represent a covalent bond are:

Only outer shells of electrons are involved in bonding, so the inner shells do not always have to be included in diagrams.

solid line

Cl Cl–

–Cl Cl

A covalent bond consists of a shared pair of electrons.

covalent bond

Cl

simplified dot and cross

diagramCl

How is a covalent bond drawn?

Cl Cl

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Covalent bonding in carbon dioxide

How do carbon and oxygen atoms form covalent bonds in a molecule of carbon dioxide?

C O

(2.4) (2.6)

4electrons needed

electron configuration

ratio of atoms

element

2

1 2

CO2 or O C O

O OC

double bonds

A double bond is when two pairs of electrons are shared.

In carbon dioxide there are two double bonds – one between each oxygen atom and the carbon atom.

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Covalent bonding in hydrogen

Many non-metal elements, such as hydrogen, exist as simple diatomic molecules that contain covalent bonds.

How is a covalent bond formed in hydrogen?

Some molecules contain double or triple covalent bonds. How are these are formed?

H H H

Each hydrogen atom needs one more electron in its outer shell and so each atom shares its single unpaired electron.

H

This shared pair of electrons forms a covalent bond and so creates a diatomic molecule of hydrogen.

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H

Covalent bonding in ammonia

How do nitrogen and hydrogen atoms form covalent bonds in a molecule of ammonia?

NH3 or H N H

H

N H

(2.5) (1)

3electrons needed

electron configuration

ratio of atoms

element

1

1 3

H HN

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H

Covalent bonding in methane

How do carbon and hydrogen atoms form covalent bonds in a molecule of methane?

CH4 or H C H

H

H

C H

(2.4) (1)

4electrons needed

electron configuration

ratio of atoms

element

1

1 4

H

H HC

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What is the structure of diamond?

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What are the properties of diamond?

In diamond, all the electrons in the outer shell of each carbon atom (2.4) are involved in forming covalent bonds.

This affects the properties of this allotrope of carbon:

Diamond is very hard – the hardest natural substance.

Diamond has a very high melting and boiling point – a lot of energy is needed to break the covalent bonds.

Diamond cannot conduct electricity – there are no free electrons or ions to carry a charge.

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What is the structure of graphite?

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What are the properties of graphite?

In graphite, only three of the four electrons in the outer shell of each carbon atom (2.4) are involved in covalent bonds.

Graphite is soft and slippery – layers can easily slide over each other because the weak forces of attraction are easily broken. This is why graphite is used as a lubricant.

Graphite conducts electricity – the only non-metal to do so. The free electron from each carbon means that each layer has delocalized electrons, which can carry charge.

This affects the properties of this allotrope of carbon:

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What is the structure of metals?

sea of electrons

metal ions

The atoms in a pure metal are in tightly-packed layers, which form a regular lattice structure.

The outer electrons of the metal atoms separate from the atoms and create a ‘sea of electrons’.

These electrons are delocalized and so are free to move through the whole structure.

The metal atoms become positively charged ions and are attracted to the sea of electrons. This attraction is called metallic bonding.

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Electrons and metallic bonding

How does the sea of electrons affect the properties of metals?

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State at room temperature

Particles in structureBonding

Bonding and structures

The type of bonding in a substance affects the properties of that substance. Can you fill in the gaps in the table below?

Type of structure

giant ionic lattice

millions of metal and non-metal ions

solidionic

covalent

metallic

simple molecular

few non-metal atoms

usually liquid or solid

giant covalent lattice

millions of non-metal atoms

solid

giant metallic lattice

millions of metal ions

solid (except mercury – liquid)

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Substances with giant structures generally have high melting and boiling points because all the atoms are strongly bonded together to form a continuous 3D lattice. A large amount of energy is needed to break these bonds.

strong ionic bonds holds ions together

strong covalent bonds holds atoms together

strong metallic bonds holds ions together

Melting and boiling point: giant structures

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Effect of structure on properties

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The effect of bonding on properties