When differentiating between kinds of atoms, it becomes important to be able to tell them apart by their masses. Atomic mass is the mass of an atom as measured in atomic mass units. . Although many standards for calculating the mass of atoms were used in the past, currently scientists use the atomic mass unit, which is the unit of mass equal to 1/12 the mass of a carbon-12 atom. The carbon-12 atom is an atom that has six protons, six neutrons as we will as 12 electrons, making in total a mass of 1 9926 x 10-23 g. This means that 1/12 of that mass or one atomic mass unit, is equal to 1 6605 x 10-24 g. The atomic mass unit is often abbreviated as amu as we will as more recently as just the letter u. The atomic mass of each element is given on the periodic table as we will as is actually a weighted average of all the naturally occurring isotopes of an element. For example, there’s isotopes of carbon-13 as we will as carbon-14, but due to the fact that they occur in much smaller amounts than carbon-12, they do not change the atomic mass of carbon very much. However, the mass of carbon is listed as 12 0111 u as we will as not exactly 12 u The discovery of the nucleus is usually credited to Rutherford as a result of his scattering experiments. The experiments of Rutherford as we will as others showed that all nuclei are composed to two particles, protons as we will as neutrons. These two types of particles, called nucleons, are both much more massive than an electron. Every nucleus can be specified by the numbers of protons as we will as neutrons it contains. The number of protons in a nucleus is called the atomic number, Z. Due to the fact that each element has a particular as we will as unique number of protons in it is nucleus, each has a characteristic value of Z. For example, the atomic number of He’s Z = 2, while the atomic number of O is Z = 8. Since atoms are electrically neutral, Z is also equal to the number of electrons in an atom. The number of neutrons in a nucleus is usually denoted by the symbol N. The value of N for a particular element can vary. For example, He nuclei can have N = 1, N = 2 or N = 4 as we will as other values are also possible. The number of neutrons in atomic nuclei generally increases as the atomic number increases .