Understanding Understanding Chemical Chemical ReactionsReactions

Lesson 1Lesson 1

Lesson ObjectivesLesson Objectives• Ensure familiarity with concept of atomic

structure.• Understand what isotopes are.• Understand the properties of isotopes• Understand the concept of relative atomic

mass (RAM)• Be able to calculate the RAM from

information about the isotopes of an element.

Isotopes of hydrogenIsotopes of hydrogen• There are 3 isotopes of hydrogen

Hydrogen-1Hydrogen-1• Contains 1 proton

and 1 electron – as all isotopes of H must contain.

• No neutrons• The most common

type of H atom.

Hydrogen-2, deuteriumHydrogen-2, deuterium• Contains 1 proton

and 1 neutron in the nucleus.

Hydrogen-3, tritiumHydrogen-3, tritium• Contains 1 proton

and 2 neutrons in the nucleus.

IsotopesIsotopes• All isotopes have identical chemical

properties but different physical properties.

IsotopesIsotopes• Chlorine exists as two

isotopes, chlorine-35 and chlorine-37.

• Chlorine has the atomic number 17.

• Work out the number of neutrons in chlorine-35 and chlorine-37.

IsotopesIsotopes• Chlorine- 35• 17 protons

• No. of neutrons= 35 – 17

= 18

IsotopesIsotopes• Chlorine- 37• 17 protons

• No. of neutrons= 37 – 17

= 20

Relative atomic massRelative atomic mass• Atoms are too small to be weighed

individually, even with the best balance!

• We can, however, compare the masses of different atoms.

Relative atomic massRelative atomic mass• We give the lightest

atom (a hydrogen-1 atom) a mass of 1.

• All other atoms are compared so many times heavier than this.

• We call this relative atomic mass (RAM).

Relative atomic massRelative atomic mass• E.g. a helium atom

has a RAM of 4.• This means a

helium atom is four times heavier than a hydrogen atom.

Relative atomic massRelative atomic mass• A magnesium atom has a RAM of 24.

How many times heavier is a magnesium atom than a helium atom?

• SIX times heavier! (He has a RAM of 4, 24/4 = 6)

Relative atomic massRelative atomic mass• Some elements have RAM that are

not whole numbers.• This is because the elements exist as

a number of isotopes – each of which has atoms of different mass.

• Tin has 10 different isotopes!

The RAM of chlorineThe RAM of chlorine• The RAM of Cl is 35.5, how do we

explain this?• Cl exists as two isotopes, Cl-35 & Cl-

37.• In any sample of Cl, 75% of the

atoms are Cl- 35 and 25% are Cl-37.

The RAM of chlorineThe RAM of chlorine• The average mass, taking into

consideration the different number of atoms of the two isotopes, is:

• (75% x 35) + (25% x 37)

• = 35.5

• The relative atomic mass of hydrogen is 1.008 rather than exactly 1.

• Why?