Elements are the basic building block of matter. There are 93 naturally occurring elements and these can be identified on any reasonable Periodic Table. The nucleus of each element's atom has positively charged protons within it. Similarly charged particles repel each other, and the strength of the repulsion is inversely proportional to the distance-squared. That is: the closer they are, the stronger the repulsion and within the very small dimensions of the nucleus the repulsion is extremely strong. Consequently, once the element gets above hydrogen there must be some kind of "glue" which holds the protons together. Generally, this seems to be neutrons. This is why from helium on up the chart there are always neutrons in the nucleus of elements along with protons.
All atoms of a particular element have to have the same number of protons. However, they do not need to have the same number of neutrons. Elements of the same element with differing numbers of neutrons are called isotopes in chemistry and nuclides in nuclear chemistry. It turns out that some combinations of neutrons and protons are not sufficient to hold all the protons together inside the nucleus. Over time the repulsive force will win and the atom will eject some protons and neutrons in the form of alpha-particles. On top of that, neutrons themselves are basically a positive proton and a negative electron bound together. Over time, it is possible for a neutron to "fall apart" or decay into its basic particles. This causes the electron produced to leave the nucleus in the form of beta-particles. And further, some nuclei are not packed together very efficiently. The protons and neutrons, over time, can rearrange themselves and repack more efficiently. The potential energy released with this more efficient arrangement is released from the nucleus in the form of gamma-radiation.
Any of these three methods of changing the internal structure of the nucleus is what we call nuclear radiation. Isotopes, or nuclides, which release radiation are called radionuclides. The process of releasing radiation in these manners is generally spontaneous and it does not happen all at once. The rate and timing of radiation release obeys rules of statistics and follows an exponential decay function. That is, if you plot a graph of the number of radionuclides that exist and how many are left after they decay over time you get a function that matches the mathematical expression for an exponential decay (X^-y).
One of the properties of an exponential decay is that it follows a rule by which 1/2 of the amount present will decay over the same amount of time, every time. For example, if you start with 100 and it takes 5 seconds to decay to 50, then it will take another 5 seconds for 50 to reduce to 25, in another 5 seconds the 25 that are left will decay down to 12.5, etc....
The amount of time for half of the radionuclides to decay in a sample is called the half-life of that nuclide. Half-life is an intrinsic property of the elements: that is, each radionuclide has its own unique half-life. For example, carbon-14 has a half-life of 5,340 years. So if you have 100 C-14 atoms after about 5,340 year only about 50 will be left.
Half life (the name itself suggest) is the time requried for an radioactive element to decay 50% by its mass, that is half of the concentration. The half life of a substance is a fixed tiome it doesnot according to temperature and pressure.
for carbon-14 th half life is 5730±40 years.