The Structure of an Atom Explained With a Labeled Diagram

Most of an atom is just empty space and consists of a positively charged nucleus of protons and neutrons surrounded by a cloud of negatively charged electrons. The center of an atom is the nucleus and one or more electrons surrounding the nucleus. When one says an atom is electrically neutral, it means that the number of electrons equal the number of positive charges on the nucleus. This means the positive charge of protons and the negative charge of electrons cancel each other resulting in an electrically neutral atom.

The atomic model in the diagram below shows protons and neutrons concentrated at the atomic nucleus and electrons in the orbits surrounding it. Protons are positively charged, electrons are negatively charged, while the neutrons carry no charge.

Initially, people did not understand the atom and many scientific investigations were carried out to understand the model of an atom. The first person who discovered electrons, Sir J.J. Thompson, put forth his 'Plum Pudding' Model of an atom. He believed that the atom was a uniform sphere with a positive charge and had electrons embedded in it. He could explain the electrical neutrality of an atom, but could not explain the arrangement of the fundamental particles in an atom.

Later, Ernest Rutherford was studying the properties of spontaneously emitted particles of radioactive substances. Through his experiment of bombardment of alpha particles through a foil of gold atoms, he observed that alpha particles passed straight through the gold foil without deflection. This showed that there is a lot of empty space in an atom.

Next, he observed that some alpha particles were deflected through small and large angles. This proved that there was a 'center of positive charge' in an atom. This center repelled the positively charged alpha particles and deflected them from their original path. Only few alpha particles rebounded after hitting the gold foil and this proved that the nucleus is very dense and hard.

Thus, Rutherford could prove that the nucleus is positively charged, dense, and hard. The nucleus is very small compared to the size of atom and the entire mass of an atom is centered in the nucleus. This conclusion helped him propose 'Rutherford's Atomic Model'.

According to his atom diagram, the atom has a small, positively charged nucleus in center. This nucleus carries the entire mass of the atom. Electrons revolve around the nucleus at a high-speed. The number of electrons were equal to the number of protons and therefore, an atom is electrically neutral. The volume of a nucleus was considered to be negligible compared to the volume of an atom. The atom had a lot of empty space. Rutherford compared his atom diagram to the solar system where the nucleus is like sun and the electron like the planets revolves around it.

Yet, his model had some major drawbacks. His theory proposed electrons revolved in high-speed around positively charged nucleus. But, according to the electromagnetic theory this would lead to continuous radiation of energy. The loss of energy would ultimately slow down the electron speed and finally the electron would fall into the nucleus. However, this does not happen and Rutherford's model could not explain why the collapse does not occur.

In 1913, Neil Bohr proposed a theory to explain the stability of an atom, where electrons revolved around the nucleus at certain energy levels. Each orbital had a different radius and electrons revolving in a particular orbit did not radiate energy. This happened even if the electron had accelerated motion around the nucleus. Bohr put forth some postulates that supported his atom diagram.

Bohr's Postulates

• The electrons are in orbit around the nucleus. The electrostatic forces hold the electrons in an orbit.
• An electron can be only in a particular orbit and when it is present in that particular orbital, it does not radiate energy.
• When an electron falls from a higher energy state to a lower state, only then it emits radiation.
• The radii of the orbitals are quantized and each energy state has a specific radius that is proportional to h/2n.
• The angular momentum of electron will be mvr = nh/2n, where n is the principle quantum number.

The Bohr model of an atom was able to explain the stability of the atom and also could explain the phenomenon of atomic spectra and ionization of gases.

There were a lot of problems related to the calculation of the mass of the nucleus, although Rutherford's model could explain the scattering of alpha problem. One could calculate the charge of the nucleus by scattering experiments, but could not find the mass of the nucleus as the protons were only half the nucleus mass. William Draper Harkins, in 1920, suggested that there may be some other particles similar to protons present in the nucleus. He called this particle a neutron that had no charge. In 1932, James Chadwick discovered the neutron with his experiment. Chadwick bombarded the element beryllium with alpha particles and observed that the radiation was highly penetrating and it remained unaffected in an electric or magnetic field. This showed that the radiation was neutral. These particles emitted had the same mass as protons and the relative mass of such a particle was 1 a.m.u. The absolute mass was about 1.6×10 ^-24 g. These particles were electrically neutral and called neutrons.

With the discovery of protons, neutrons, and electrons, physicists could put forth a diagram of an atom. They could explain that an atom is made up of electrons, neutrons, and protons. The center of an atom is the nucleus that contain protons and neutrons. This makes the nucleus positively charged. The electrons are present on different shells or orbits that revolve around the nucleus. This helps in determining the size of an atom.

Many scientists have worked hard to help dissect the tiniest of all matters and propose the correct atom diagram.