Understanding Understanding Chemical Chemical Reactions Reactions Lesson 1 Lesson 1
Dec 16, 2015
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?