How Many Neutrons?

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

• Which sub-atomic particle must always be of the same number in atoms of an element?

Remember: The number of protons defines

the element

• Atoms of the same element always have the same number of protons, but they may

have different numbers of neutrons.

• Atoms that differ in this way are called isotopesisotopes..

Write this down!

Isotopes are atomic forms of the same element, with the

same number of protons but differing numbers of

neutrons.

• Isotopes are virtually identical in their chemical reactions. WHY?

• The uncharged neutrons make no difference to chemical properties but do affect physical properties such as melting point and density.

Isotopes: Carbon• 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

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)

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

Cl35

1775%

17ProtonsElectronsNeutrons

17

18ProtonsElectronsNeutrons

171720

Cl37

1725%

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 and Relative Atomic Mass

• Many natural elements are a mixture of isotopes, so when we react them we are using a mixture of atoms with different mass numbers.

• The relative atomic mass given in the periodic table takes account of this.

E.g.. For 100 atoms of chlorine:Mass of 75 atoms of Chlorine 35: 75 x 35 =2625Mass of 25 atoms of Chlorine 37: 25 x 37 =925Total = 3550Average (divide by 100) = 35.535.5

Relative Atomic Mass

• Bromine contains 50.5% bromine-79 and 49.5% bromine-81.

• Calculate the relative atomic mass of natural bromine showing your working.

Egg. For 100 atoms of bromine:Mass of 50.5 atoms of bromine 79: 50.5 x 79 =3989.5Mass of 49.5 atoms of bromine 81: 49.5 x 81 =4009.5Total = 7999Average (divide by 100) = 80 80

Activity

1. Which of the following is not a sub-atomic particle?

A. Proton.

B. Isotope.

C. Neutron.

D. Electron.

2. The element Cobalt has a relative atomic mass of 59 and an atomic number of 27. Which of these is a true statement about each neutral cobalt atom?

A. It contains 59 neutrons.

B. It contains 27 electrons.

C. It contains 32 protons.

D. It contains equal numbers of neutrons and electrons.

3. The Periodic Table displays iron as shown below. This indicates that Fe atoms:

A. contain 56 neutrons.

B. contain 30 electrons.

C. contain 26 protons.

D. contains more protons than neutrons.

56

Fe26

4. Bromine consists of a mixture of two isotopes: Bromine-79 and Bromine-81

• Which of the following is true:

A. Both isotopes contain 35 protons.

B. Bromine 79 contains 46 neutrons.

C. Bromine 81 contains 44 neutrons.

D. Bromine-81 is more reactive than bromine-79.

80Br35

5. Natural boron consists of approx. 20% boron-10 and 80% boron-11.

• What will the relative atomic mass of natural boron will be?

A. 10.0

B. 11.0

C. 10.5

D. 10.8

?B5