# Atomic structure

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 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!

## Symbols for elements

Elements are simple substances 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 mass number 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 negligible 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 familiar to you and shows the atomic structure of an atom of the element carbon along with its atomic number and mass number.

## Charges and masses of protons, neutrons and electrons

The table below gives some information on the mass and charge of the 3 sub-atomic particles found inside the atom. The protons and neutrons which are found in the nucleus are heavy particles and are responsible for almost all the mass of the atom. In comparison the mass of the electron, is very small when compared to the masses of protons and neutrons. It takes almost 2000 electrons to make up the mass of a single neutron or proton, so for most practical purposes you can assume the mass of the electron is zero. This means that all the mass of the atom is concentrated inside the tiny nucleus. If you divide the mass of the proton by the mass of the neutron you get an answer of 1, this is why masses of the neutrons and protons are taken as 1. When compared to or relative to each other they are the same.

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

## Key Points

• Protons and electrons have equal but opposite charges.
• The mass of the atom is due to the protons and neutrons only. The mass of an electron is very small compared to that of a proton or neutron.
• All the mass of an atom is concentrated in the nucleus. The nucleus is tiny when compared to the size of the atom. The nucleus is about 1/10 0000 th the size of the atom.
• Atoms are mostly empty space.