Download Isotopes and mass number

Isotopes and mass number
Atoms of elements always have equal numbers of
 protons (positive charges) and
 electrons (negative charges).
The number of neutrons (which are neutral) an atom can have can vary.
Generally in an atom (with the exception of hydrogen)
the number of neutrons exceeds or equals the number of protons.
Isotopes are atoms of the same element which have differing numbers of neutrons.
Hydrogen has 3 isotopes - hydrogen, deuterium and tritium.



Hydrogen
1 proton,
1 electron,
no neutrons
represented as



Deuterium
1 proton,
1 electron,
1 neutron
represented as



Tritium
1 proton,
1 electron,
2 neutrons
represented as
Mass number
This is defined as the
number of protons and neutrons inside the nucleus of an atom.
All the mass of the atom is in the nucleus. (we can ignore the mass of an electron)
Protons and neutrons have the same mass.
Electrons are around 2000 times lighter than either a proton or neutron.
Atomic number
This is defined as the
number of protons OR electrons in an atom.
Protons have a positive charge. Electrons have a negative charge.
Atoms are neutral. (number of positive charges = number of negative charges).
Hence IN AN ATOM
number of electrons = number of protons
Calculating the number of neutrons in an atom
Mass number = number of protons + number of neutrons
Atomic number = number of protons
Mass number - atomic number = number of protons + number of neutrons - number of protons
Mass number - atomic number = number of neutrons
So if we know the
 mass number
 atomic number
of an atom we can determine how many protons, neutrons and electrons are present in that atom.
A short hand way to write all this is shown below: In this example using deuterium:
EXERCISES on Mass number / atomic number