Elements – different types of atomElements are the simplest substances. There are about 100 different elements.

Each element is made up of very tiny particles called atoms, and each element is made up of just one particular type of atom, which is different to the atoms in any other element.

Gold is an element made up of only gold atoms.

Carbon is an element made up of only carbon atoms.

Atoms – the building blocksJohn Dalton had the first ideas about the existence of atoms over 200 years ago.

However, it is only relatively recently that special microscopes (called electron microscopes) been invented that can actually ‘see’ atoms.

The yellow blobs are individual gold atoms, as seen through an electron microscope.

What are atoms made of?

What particles are atoms made of?For some time, people thought that atoms were the smallest particles and could not be broken into anything smaller.

proton neutron electron

Scientists now know that atoms are actually made from even smaller particles. There are three types:

How are these particles arranged inside the atom?

What is the structure of an atom?Protons, neutrons and electrons are not evenly distributed in an atom.

The electrons are spread out around the edge of the atom. They orbit the nucleus in layers called shells.

The protons and neutrons exist in a dense core at the centre of the atom. This is called the nucleus.

Most of the atom is empty space

If you imagine an atom being the size of Wembley stadium, the nucleus would be about the size of a on the centre spot.

The electrons would be two peas flying around the whole stadium. The rest of it: emptiness.

Mass and electrical chargeThere are two properties of protons, neutrons and electrons that are especially important: mass

electrical charge.

The atoms of an element contain equal numbers of protons and electrons and so have no overall charge.

-1almost 0electron

01neutron

+11proton

ChargeMassParticle

How many protons?The atoms of any particular element always contain the same number of protons. For example:

The number of protons in an atom is known as the atomic number (Z)or proton number.

It is the smaller of the two numbers shown in most periodic tables.

hydrogen atoms always contain 1 proton

carbon atoms always contain 6 protons

magnesium atoms always contain 12 protons.

What is the atomic number?What are the atomic numbers of these elements?

11

26

50

9

tin

iron

sodium

fluorine

Each element has a definite and fixed number of protons.

Changes in the number of particles in the nucleus (protons or neutrons) are very rare. They only take place in nuclear processes such as:

radioactive decay

nuclear bombs

nuclear reactors.

If the number of protons changes, then the atom becomes a different element.

What is mass number?Electrons have a mass of almost zero, which means that the mass of each atom results almost entirely from the number of protons and neutrons in the nucleus.

Mass Number = Number of + Number of(of an atom) protons neutrons

1413aluminium

43lithium

01hydrogen

NeutronsProtonsAtoms Mass number

27

7

1

number of neutrons = mass number - number of protons

= mass number - atomic number

How many neutrons?

How many neutrons are there in these atoms?

3888

919

24

50

10

2

strontium

fluorine

helium

Atomic number

Mass numberAtoms Neutrons

zirconium

uranium

91

238

40

92 146

51

How many electrons?Atoms have no overall electrical charge and are neutral.

The number of electrons is therefore the same as the atomic number.

Atomic number is the number of protons rather than the number of electrons, because atoms can lose or gain electrons but do not normally lose or gain protons.

This means atoms must have an equal number of positive protons and negative electrons.

29

2

53

ElectronsProtons Neutrons

helium

copper

iodine

Atoms

2 2

2935

74 53

How are electrons arranged?Electrons are not evenly spread but exist in layers called shells. (The shells can also be called energy levels).

The arrangement of electrons in these shells is often called the electron configuration..

Note that this diagram is not drawn to scale – the atom is mostly empty space. If the electrons are the size shown, the nucleus would be too small to see.

1st shell

2nd shell

3rd shell

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

1st shell holdsa maximum of2 electrons

2nd shell holdsa maximum of8 electrons

3rd shell holdsa maximum of8 electrons

This electron arrangement is written as 2,8,8.

Summary: the atom so farThe nucleus is:

made up of protons and neutrons

positively charged because of the protons

dense – it contains nearly all the mass of the atom in a tiny space.

Electrons are:

very small and light, and negatively charged

able to be lost or gained in chemical reactions

found thinly spread around the outside of the nucleus, orbiting in layers called shells.

Isotopes

Why is the mass number of chlorine not a whole number?

What is an isotope?Elements are made up of one type of atom, but there can be slightly different forms of the atoms in an element.

mass number is different

atomic number is the same

Atoms that differ in this way are called isotopes.

Although atoms of the same element always have the same number of protons, they may have different numbers of neutrons.

For example, two isotopes of carbon:

What are the isotopes of carbon?Most naturally-occurring carbon exists as carbon-12, about 1% is carbon-13 and a much smaller amount is carbon-14.

6 protons

6 electrons

6 neutrons

6 protons

6 electrons

7 neutrons

6 protons

6 electrons

8 neutrons

Properties of isotopesThe isotopes of an element are virtually identical in their chemical reactions.

The uncharged neutrons make little difference to chemical properties but do affect physical properties such as melting point and density.

Natural samples of elements are often a mixture of isotopes.

This is because they have the same number of protons and the same number of electrons.

What are the isotopes of hydrogen?Hydrogen-1 makes up the vast majority of the naturally-occurring element but two other isotopes exist.

1 proton

0 neutrons

1 electron

hydrogen

1 proton

1 neutrons

1 electron

deuterium

1 proton

2 neutrons

1 electron

tritium

What are the isotopes of chlorine?About 75% of naturally-occurring chlorine is chlorine-35 and 25% is chlorine-37.

17 protons

18 neutrons

17 electrons

17 protons

20 neutrons

17 electrons

What are the particle numbers in each isotope below?

What are the isotopes of oxygen?Almost all of naturally-occurring oxygen is oxygen-16, but about 0.2% is oxygen-18.

oxygen-16

8 protons

8 neutrons

8 electrons

oxygen-18

8 protons

10 neutrons

8 electrons

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.

From atoms to ionsHow can reactive metal atoms become stable positive ions?

How do atoms form ions?An ion is an atom or group of atoms that has an electrical charge, either positive and 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?

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.

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+

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

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

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 negtive 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-

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

[2.8.8]2-

(full outer shell)

2.8.6(partially full outer shell)

How is a sulphide ion formed?

16 protons = +16

16 electrons = -16

Total charge = 0

Sulfur atom:

16 protons = +16

18 electrons = -18

Total charge = -2

Sulfide ion:

2-

S S

gains 2 electrons

Calculating ion chargesWhat 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

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.

1. What is the composition of (numbers of electrons, protons and neutrons) of a neutral atom of the two main uranium atoms (Uranium-235 & Uranium 238)

2. Give the formula and charge of the ions formed by the elements Aluminium and chlorine

3. What is the composition of the ions of potassium-40 (K+) and chlorine-37 (Cl-)?

U-235 has 92 protons, 92 electrons and 143 neutrons

U-238 has 92 protons, 92 electrons and 146 neutrons

K+ has 19 protons, 18 electrons and 21 neutrons

Cl- has 17 protons, 18 electrons and 20 neutrons

Al3+ Cl-