1.All atom of the same element are alike. They all have particles concentrated in the centre called the nucleus. The nucleus consists of two types of particles, namely proton and neutron. The protons are positively charge while the neutrons do not have charge. Both of them have the same mass. Surrounding the nucleus are electrons which are negatively charged. The relative mass of electron is 1/1840.

The electrons circulate the nucleus in some fixed pattern. The first two electrons nearest to the nucleus circulate in an elliptical path. These two electrons are constantly in motion in the fixed path. This path is known as the first electron shell and can hold at most two electrons. Any other electron will have to go on to the second shell. The second shell can hold at most 4 pairs of electrons which are constantly circulating in pairs. Once the second shell is full with 8 electrons, the electrons move to the third shell which can also hold at most 8 shells. The number of protons in the nucleus is the atomic number. For an atom, the number of protons is equal to the number of electrons. The number of neutrons plus protons in the nucleus is known is the nucleon number.