why do atoms form bonds?

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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.


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.


What is a covalent bond?

Non-metal elements usually just need one or two electrons to fill their outer shells. So how do they form a bond?

The shared electrons join the atoms together. This is called a covalent bond.

The two non-metal atoms cannot form a bond by transferring electrons from one to another. Instead, they share electrons.

Each atom now has a full, stable outer shell.

incomplete outer shellsCl

Cl

Cl

Cl


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


Comparing covalent and ionic bonding


How are covalent bonds formed?

How do non-metal atoms form covalent bonds?


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.


What are the types of covalent bonds?


Can compounds contain covalent bonds?

Covalent bonding can also occur between atoms of different non-metals to create molecules of covalent compounds. These covalent bonds can be single, double or triple.

How is a covalent bond formed in hydrogen chloride (HCl, also represented as H–Cl)?

H ClH Cl

Hydrogen and chlorine both need one more electron to fill outer shells. By sharing one electron each, they both have a stable outer shell and a covalent bond is formed.


Covalent bonding in water

Compounds can contain more than one covalent bond.

The oxygen atom shares 1 electron with 1 hydrogen atom, and a second electron with another hydrogen atom.

H HO

Oxygen (2.6) needs 2 more electrons, but hydrogen [1] only needs 1 more. How can these three elements be joined by covalent bonding?

What is the name of the molecule that is formed?

H2O (or H–O–H) is water.


How is the ratio of atoms calculated?

To calculate the ratio of atoms in a stable covalent compound:

For example, how many nitrogen and hydrogen atoms bond together in an ammonia molecule?

1. Work out how many electrons are needed by each non-metal element to complete its outer electron shell.

2. Work out the ratio of atoms that will provide enough shared electrons to fill all the outer shells.

N H

(2.5) (1)

3electrons needed

electron configuration

ratio of atoms

element

1

1 3


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


ratio of atoms

element

1

1 3

H HN


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


ratio of atoms

element

1

1 4

H

H HC


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


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.


What are simple covalent structures?

Covalent molecules that contain only a few atoms are called simple covalent structures.

weak bonds between molecules

strong bonds within molecules

Most substances that contain simple covalent molecules have low melting and boiling points and are therefore liquids or gases at room temperature, e.g. water, oxygen, carbon dioxide, chlorine and hydrogen. Why?

The covalent bonds within these molecules are strong but the bonds between molecules are weak and easy to break.


What is the structure of a molecular solid?

A few substances that contain simple covalent molecules are solid at room temperature. These are molecular solids.

The solid is formed because millions of iodine molecules are held together by weak forces of attraction to create a 3D molecular lattice.

Two iodine atoms form a single covalent bond to become an iodine molecule.

weak forces of attraction

What properties would you expect molecular solids to have with this type of structure?

Iodine is a molecular solid at room temperature.


What are the properties of molecular solids?

low melting and boiling points;

usually soft and brittle – they shatter when hit.

The properties of a molecular solid, such as iodine, are:

Why do molecular solids have these properties?

The weak forces of attraction between the molecules can be broken by a small amount of energy. This means that the molecular solids are soft and brittle and melt and boil at low temperatures.

Molecular solids are also unable to conduct electricity because there are no free electrons or ions to carry a charge.

cannot conduct electricity.


Covalent bonds – true or false?


What are giant covalent structures?

In some substances, such as sand, diamond and graphite, millions of atoms are joined together by covalent bonds.

All the bonds are covalent, so giant covalent structures have very high melting and boiling points, and are usually hard.

The covalent bonds in these substances do not form molecules but vast networks of atoms called giant covalent structures.


What is the structure of sand?

Sand is mostly made of the mineral quartz, which is silicon dioxide. It has a giant covalent structure made up of silicon and oxygen atoms.

Each silicon atom (2.8.4) is bonded to four oxygen atoms, and each oxygen atom (2.6) is bonded to two silicon atoms.

Si

O

O

O

O


What are the allotropes of carbon?

Diamond and graphite appear to be very different substances but what do they have in common?

Both diamond and graphite are made up of carbon atoms.

These allotropes of carbon have different properties because the atoms are bonded in different arrangements which create different giant structures.

Different forms of the same element are called allotropes.


How does structure affect properties?

How do the different structures of diamond and graphite influence their properties?


What is the structure of diamond?


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.


What is the structure of graphite?


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:


Are there other allotropes of carbon?

A third class of carbon compounds have been discovered in recent years. These are called fullerenes. Buckminsterfullerene is one type of fullerene. It contains 60 carbon atoms, each of which is bonded to three others by two single bonds and one double bond.

The atoms in this allotrope of carbon form a sphere, like the shape of a football. The molecules can be called ‘bucky balls’.They are large but are not classified as giant structures.

C

C

C

C


Complete the sentences


Bonding and structure


Simple or giant covalent structure?


How does bonding affect properties?

Does the type of bonding in a substance affect its properties?


State at room temperature

Particles in structure

Bonding

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)


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


Effect of structure on properties


The effect of bonding on properties


Glossary (1/2)

allotrope – A structurally different form of an element with different physical properties.

covalent bond – A strong bond between two atoms in which each atom shares one or more electrons with the other.

covalent compound – A compound containing atoms joined by covalent bonds.

double bond – A covalent bond in which each atomshares two of its electrons.

giant structure – A structure containing millions of atoms or ions bonded together. The structure extends in three

dimensions until all available atoms are used up.


Glossary (2/2)

molecule – A small group of atoms which are held together

by covalent bonds.

molecular solid – A solid substance made up of molecules held together by weak forces of attraction, forming

a lattice.

single bond – A covalent bond in which each atom shares one of its electrons.

triple bond – A covalent bond in which each atom shares three of its electrons.


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Multiple-choice quiz

why do atoms form bonds?

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