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What are bonds?

Compounds are formed when different elements chemically react and form bonds with each other.

Some compounds, like water (H20), have small, simple structures with just a few atoms bonded together.

Others compounds, like DNA, have large, complex structures containing thousands or even millions of bonded atoms.

Elements are the simplest substances that exist on Earth.

Each element is made up of just one type of atom, usually joined to other atoms of the same element by bonds.

This forms molecules such as chlorine (Cl2).

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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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From atoms to ions

How can reactive metal atoms become stable positive ions?

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How do atoms form ions?

An ion is an atom or group of atoms that has an electrical charge, either positive or negative.

Atoms have an equal number of protons and electrons and so do not have an overall charge.

Atoms with incomplete outer electron shells are unstable. By either gaining or losing electrons, atoms can obtain full outer electron shells and become stable.

When this happens, atoms have an unequal number of protons and electrons and so have an overall charge. This is how atoms become ions.

How does an atom become a positive or negative ion?

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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 do atoms form positive ions?

An atom that loses one or more electrons forms a positive ion.

Positive ions have a small ‘+’ symbol and a number by this to indicate how many electrons have been lost.

This number is usually the same as the number of electronsin the atom’s outer shell. For example:

Metal atoms, such as sodium, magnesium and iron, form positive ions.

lithium ion [ 2 ]lithium atom 2.1

aluminium atom 2.8.3 aluminium ion [ 2.8 ]

magnesium atom 2.8.2 magnesium ion [ 2.8 ] = Mg2+

= Li+

= Al3+

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How is a sodium ion formed?

2.8.1(partially full outer shell)

11 protons = +11

11 electrons = -11

Total charge = 0

Sodium atom:11 protons = +11

10 electrons = -10

Total charge = +1

Sodium ion:

loses 1 electron

+

[2.8](full outer shell)

Na Na

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How is a magnesium ion formed?

2.8.2 (partially full outer shell)

12 protons = +12

12 electrons = -12

Total charge = 0

Magnesium atom:12 protons = +12

10 electrons = -10

Total charge = +2

Magnesium ion:

[2.8]2+

(full outer shell)

2+

Mg Mg

loses 2 electrons

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How do atoms form negative ions?

Negative ions have a small ‘-’ symbol and a number by this to indicate how many electrons have been gained to fill their outer shell. For example:

An atom that gains one or more electrons forms a negative ion.

The name of the ion is slightly different to the atom’s name.

Non-metal atoms, such as chlorine, oxygen and nitrogen, form negative ions.

chlorine atom chloride ion [ 2.8.8 ] = Cl-

oxygen atom oxide ion [ 2 ]

nitrogen atom nitride ion [ 2 ]

2.8.7

2.6

2.5 = N3-

= O2-

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How is a fluoride ion formed?

2.7 (partially full outer shell)

9 protons = +9

9 electrons = -9

Total charge = 0

Fluorine atom:9 protons = +9

10 electrons = -10

Total charge = -1

Fluoride ion:

[2.8]-

(full outer shell)

-

F F

gains 1 electron

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[2.8.8]2-

(full outer shell)2.8.6

(partially full outer shell)

How is a sulfide ion formed?

16 protons = +1616 electrons = -16

Total charge = 0

Sulfur atom:16 protons = +1618 electrons = -18

Total charge = -2

Sulfide ion:

2-

S S

gains 2 electrons

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Building an ion

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Calculating ion charges

What is the charge on the ion of each element?

+2 +2+1 -3 -1

2.8.8.2 1 2.8.5 2.7 2.2

calcium hydrogen phosphorus fluorine berylliumElement

Electron shells

Charge on ion

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Electron configuration of ions

The number of protons and neutrons however, remain different for each element. This means that each ion has different properties.

For example, oxygen and fluorine both gain electrons to become negative ions. The resulting ions have the same electron configuration:

F

-2-

O

When different elements gain or lose electrons to complete their outer electron shell, they can create ions that have the same electron configuration.

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Comparing electron configurations

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What is a compound ion?

Ions can be made up of a single atom or a group of atoms. An ion made up of a group of atoms is called a compound ion.

hydroxide

sulfate

nitrate

carbonate

ammonium

-1

-2

-1

-2

+1

Ion Formula Charge Atoms present

NH4+

CO32-

NO3-

SO42-

OH- O

O OO

O

O O O

OOO

H

H H HH

C

N

N

S

What atoms are present in the following compound ions?

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Comparing positive and negative ions

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Ions and attraction

Why do some ions attract while others repel?

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What is ionic bonding?

Compounds that contain ions are called ionic compounds. These compounds are usually formed by a reaction between a metal and a non-metal.

Why do these substances react together and form bonds?

The metal and non-metal atoms have incomplete outer electron shells and so are unstable.

Electrons are transferred from each metal atom to each non-metal atom. The metal and the non-metal atoms form ions with completely full outer shells and become stable.

The positive and negative ions are strongly attracted to each other. This electrostatic attraction is called ionic bonding.