Isotopes are variations in the number of neutrons in the nucleus of an atom of a particular element. For elements it doesn't have a significant effect on the chemistry of reactions they take part in.

i) indicates 12 superscript meaning 6 neutrons and 6 protons , ii) indicates 2 extra neutrons for 8 neutrons + 6 protons = 14 . iii) the usual way of writing is  14C it is often written as carbon-14

There are subtle variations though in density of the samples of the elements depending on their isotope. 

For example hydrogen 2H (called deuterium or D) has 1 extra neutron making it 'heavy'. And when combined with oxygen creates so called 'heavy water' or D2O because when compared with water made with 1H it actually weighs a little more, that extra neutron adds a little extra mass to the water molecule. In addition the standard boiling temperature point of heavy water is slightly higher at 101°C.

An important feature of some isotopes is that they can be stable and unstable. If an element as an unstable isotope this means the nucleus of the atom can 'fall apart' and release energy in the form of ionising radiation such as alpha radiation

This leads to some isotopes being called radioactive. This is also called radioactive decay. The nucleus can often decay into another type of element.  The study of nuclear decay is more associated with nuclear physics.