Isotopes & Relative Atomic Mass

Isotopes

• Isotopes are atoms of the same element that have different number of neutrons. (In order for them to be atoms of the same element, their number of protons would be the same)

Example:

is an isotope of chlorine that has 17 protons and 18 neutrons

is an isotope of chlorine that has 17 protons and 20 neutrons

• Generally, isotopes behave the same way during chemical reactions.
• The extra neutrons just change the mass of the atom and its density.

Standard atom

• Since the mass of an atom would be extremely small when measured in grams, it would be more convenient to measure the masses of atoms relative to a standard atom.
• The standard atom chosen is (carbon-12) isotope. An atom of carbon-12 is taken to have a mass of 12 atomic mass unit (amu).
• Since one carbon-12 atom has 6 proton and 6 neutron,

mass of one proton (neutron) = mass of one carbon-12 atom

= 1 amu (atomic mass unit)

Relative atomic mass

• The relative atomic mass (A_r) of an element is the average mass of the naturally occurring atoms of the element. This quantity takes into account the percentage abundance of all the isotopes of an element which exist.
• The formula for relative atomic mass is

Example:

Given that the percentage abundance of is 75% and that of is 25%, calculate the A_r of chlorine.

Solution:

Example:

Bromine has two isotopes, Br-79 and Br-81. Both exist in equal amounts. Calculate the relative atomic mass of bromine.

Solution:

Example:

The neon element has three isotopes. They are 90.92% of , 0.26% of and 8.82% of

Solution:

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