The model we use to describe the atom is often called the nuclear atom. In this model of there is a very small dense nucleus at the centre of the atom, inside which are the protons and neutrons. The nucleus is very dense and most of the mass of the atom is found here. The electrons in an atom orbit the nucleus in a series of shells or energy levels. Atoms are very very small and have a radius of around 1 x 10-10m (0.0000000001m) or 0.1nm ( 1 nanometre= 1 x 10-9m), about 1 million atoms stacked end to end would be about as thick as a human hair!
Elements are simple substance which consist of only 1 type of atom, all
known elements are listed in the
periodic table. The elements in the periodic table are arranged in order of
their atomic number (symbol Z), that is the number of protons
in the nucleus. Since
atoms are electrical neutral the number of protons
is the same as the number of electrons. To calculate the number of
neutrons you simply take the atomic number from the
of the element.
The masses of protons are neutrons are almost identical and are shown in the table below. It is the mass of these two sub-atomic particles that make up almost all the mass of an atom, since the mass of an electron is negligble when compared to the masses of the proton and neutron. If you divide the mass of the proton by the mass of the electron given in the table below it will tell you just how much more massive the proton is when compared to the electron. The particles inside the nucleus (the protons and neutrons) are often referred to as nucleons. The diagram below should be familar to you and shows the atomic structure of an atom of the element carbon along with its atomic number and mass number.
|particle||relative mass||actual mass in Kg||charge||Where in the atom it is found|
|proton||1||1.67 x 10-27||+1||nucleus|
|neutron||1||1.67 x 10-27||0||nucleus|
|electron||0||9.11 x 10-31||-1||electron shells or rings|