Aims

Class work 6 September 2007

Mass NumberLO’s• What is the mass number?• What’s an isotope?

Write it down!! Page 50© C Meyer

Chemistry Unit 2 Sections

Structures Structures and bonding and properties How

much? 3 of 7

© C Meyer

All• Know the relative masses of sub

atomic particles• Know what an isotope isMost• Work out the mass number of an

atomSome• Explain how isotopes are different

Aims

T/F Quiz – Back• Atoms are charged particles• Electrons are found in energy levels• Electrons are found in shells• Electrons have a negative charge• Protons are in the nucleus of the atom• Neutrons are found in shells around the

nucleus• The atomic number is the number of

protons

What have I learnt?

• What does the structure of the atom look like?What have I learnt?

• There are two properties of sub-atomic particles that are especially important:– Electrical charge (looked at before)– Mass (Need to look at now)

• Mass of proton = mass of neutron (relative mass = 1)• Ignore mass of electron

m5

ParticleParticle ChargeCharge Relative MassRelative Mass

Protons +1 1

Neutrons 0 1

Electrons -1 0.0005

ChargeChargeParticleParticle

-1Electrons

0Neutrons

+1Protons

Overall neutral

Atomic/Proton Number• The atom of any particular element

alwaysalways contains the same number of protons. E.g.– Hydrogen atoms always contain 1 proton– Carbon atoms always contain 6 protons– Magnesium atoms always contain 12 protons

• The number of protons in an atom is known as its atomic/proton number.

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

12

C6

What have I learnt?

How Many Protons?

• Here is some data about elements taken from the Periodic Table.

• What is the atomic number of the elements?

23

Na11

56

Fe26

119

Sn50

19

F9

27

Al13

11 26 50 9 13

Activity

How Many Protons?

• NoteNote elements have a definite and elements have a definite and fixed number of protons.fixed number of protons.

• Changing number of protons in atom changes that atom into a different element.

What have I learnt?

Mass Number – What is it?

• The mass of each atom comes almost entirely from the number of protons and neutrons that are present. (Remember that electrons have a relatively tiny mass).

Atom Protons Neutrons Mass NumberHydrogen 1 0 1

Lithium 3 4 7

Aluminium 13 14 27

MASS NUMBER = Number of protons + number of neutrons

m2

What is the mass number?

• In back - Calculate the mass number of the atoms shown.

Atom Protons Neutrons Mass Number

Helium 2 2

Copper 29 35

Cobalt 27 32

Iodine 53 74

Germanium 32 41

4

64

59

127

73

MASS NUMBER = Number of protons + number of neutrons

Activity

What is the mass number?

• Atomic/proton number = protons.• It is the smaller of two numbers

shown in periodic table

12

C6

What is the mass number?

• Mass number = protons + neutrons• It is the bigger of two numbers shown

in periodic table

m1

• In back - Calculate the number of neutrons in these atoms.

Atom Mass Number

Atomic Number

Number of Neutrons

Helium 4 2

Fluorine 19 9

Strontium 88 38

Zirconium 91 40

Uranium 238 92

Number of Neutrons = mass number - atomic number

2

10

50

51

146

Activity

What is the mass number?

• In Front - Copy and fill in the blank columns

Atom Protons Neutrons Electrons Atomic number

Mass Number

Boron 5 6

Potassium 19 20

Chromium 24 28

Mercury 80 121

Argon 18 22

5 5 11

19 19 39

24 24 52

80 80 201

18 18 40

Note – atomic number is defined as the number of protons rather than electrons because atoms can lose (or gain)

electrons but do not normally lose protons

Activity What have I learnt?

• Natural samples of elements are often a mixture of isotopes. About 1% of natural carbon is carbon-13.

ProtonsElectronsNeutrons

C12

699% C

13

61%

6

6

6

6

6

7

What’s an isotope?

What’s the difference?

m1

ATOMS are just like the Ford Fiesta

ISOTOPEISOTOPE

What is relative mass?

m2

Version1

Version 2

There are different versions of the FIESTA

Version 3

SPEEDYSPEEDY

m3

We also have different versions of the same

element’s atoms

WE CALL THEM

ISOTOPESISOTOPESm1

The only difference between the atoms in an

isotope is that they contain

different numbers of

NEUTRONNEUTRONSS

• It is not strictly true to say that elements consist of one type of atom.

Remember: The number of protons defines

the element

What’s an isotope?

•Atoms in an element always have the same

number of protons but may have different

numbers of neutrons.

•Atoms like this are called isotopes.isotopes.

Isotopes: Hydrogen• Hydrogen exists as 3 isotopes although

Hydrogen-1 makes up the vast majority of the naturally occurring element.

H1

1 H2

1 H31

ProtonsElectronsNeutrons

Hydrogen

ProtonsElectronsNeutrons

(Deuterium)

ProtonsElectronsNeutrons

(Tritium)

All can make water

m4

Isotopes: Chlorine• About 75% of natural chlorine is 35Cl the rest is 37Cl.

Cl35

1775%

17ProtonsElectronsNeutrons

17

18ProtonsElectronsNeutrons

171720

Cl37

1725%

How manyneutrons?

Isotopes of Oxygen

• Almost all of natural oxygen is 16O but about 0.2% is 18O.

• Produce a Table showing the particles in each isotope.

Protons

Electrons

Neutrons

16

O8

Oxygen-18Oxygen-16

8

8

8

Protons

Electrons

Neutrons

8

8

10

Activity

• Isotopes are virtually identical in their chemical reactions as have same number of protons and electrons.

• The uncharged neutrons only affect physical properties such as melting point and density.

What’s an isotope?

• Draw the electronic structure of carbon 12 and carbon 14

• Say how many protons, neutron• Draw up a table of similar

properties and differences

What have I learnt?

Same Different

What have I learnt?

C126

99% C13

61%

ProtonsElectronsNeutrons

6

6

7

6

6

6

Same Different

Same number electrons Different number neutrons

Same number protons Different mass number

Same chemical properties Different physical props

Same proton number

• Section 2 test

Homework

What have I learnt?

The End