Atomic mass and Isotopes

1.  A pure chemical substance consisting of one type of atom that every different element is distinguished by its atomic number, which is the number of protons in its nucleus.

2. As of May 2011, 118 elements have been identified, the latest being ununoctium in 2002.

Ununoctium – it is a temporary name with temporary element Uuo by IUPAC.

IUPAC -International Union of Pure Applied Chemistry is  an international standard for chemistry.But besides IUPAC we also have IUPAP International Union of Pure and Applied Physics,both,are the members of International Council for Science (ICSU). The international headquarters of IUPAC is located in Zürich, Switzerland.It was first addressed in 1860 by German scientist.

3. Of the 118 known elements, only the first  94 are believed to occur naturally on Earth. Of these naturally occurring elements, 80 are stable or essentially so, while the others are radioactive, decaying into other.See figure below :

4. The number of nucleons (protons and neutrons) in the nucleus, =  the mass number.It is not same between two isotopes. For example, carbon-12, carbon-13 and carbon-14 are three isotopes of the element carbon with mass numbers 12, 13 and 14 respectively.

5. The atomic number of carbon is 6 (every carbon atom has 6 protons); therefore the neutron numbers in these isotopes are 6, 7 and 8 respectively.