Objective: At the end of this topic, you should be able to describe:

  • The structure of an atom.
  • The postulates made by different scientists.
  • The atomic structure of isotopes.

In the last article describing matter, we discussed the fact that matter is made up of small indivisible particles known as atoms. This definition where we see atoms as indivisible was postulated by John Dalton. However, further experiments by other scientists showed that Dalton’s hypothesis was not totally correct!

It was discovered that an atom is actually divisible into three main subatomic particles which are; proton, neutron and electron.

All of these three subatomic particles are studied in respect to the centre of the atom which is called the nucleus. The electron revolves around the nucleus and is negatively charged, the proton is positively charged and embedded within the nucleus. The neutron just like the name implies “neu- ” is without charge and also embedded within the nucleus.

In some cases, we might be asked to determine the atomic number and mass number of an element given the number of protons, neutrons and electrons. In such cases, we must know that the atomic number is the same as the number of protons in an element. To get the mass number, we simply add the number of protons and neutrons together.

The Atomic Theories

Dalton’s Atomic Theory

He suggested that all matter is made up of atoms, which are indivisible and indestructible. He also stated that all the atoms of an element were exactly the same, but the atoms of different elements differ in size and mass.

His postulates were summarized into six points:

  • Every matter is made up of atoms.
  • Atoms are indivisible.
  • Specific elements have only one type of atom in them.
  • Each atom has its own constant mass that varies from element to element.
  • Atoms undergo rearrangement during a chemical reaction.
  • Atoms can neither be created nor be destroyed but can be transformed from one form to another.

When he brought out his theories, other scientists began to investigate using his theories as a base-line and it was discovered that;

  • The existence of isotopes was unexplainable.
  • Nothing about the structure of the atom was appropriately explained.

Later, the scientists discovered particles inside the atom that proved that the atoms were divisible.

John Thomson’s Atomic Theory

This scientist discovered the electron as a subatomic particle through experiments on the discharge tube.

The experiment makes use of a tube made of glass which has two openings, one for the vacuum pump and the other for the inlet through which a gas is pumped in where the role of the vacuum pump is to maintain “partial vacuum” inside the glass chamber. A high voltage power supply is connected using electrodes, i.e. cathode and anode is fitted inside the glass tube. It is then discovered that when a high voltage power supply is switched on, there are rays emerging from the cathode towards the anode. This was confirmed by the ‘Fluorescent spots’ on the ZnS screen used. These rays were called “Cathode Rays”. It was also seen that when an external electric field is applied, the cathode rays get deflected towards the positive electrode, but in the absence of electric field, they travel in a straight line.

Lastly, Thomson noticed that when rotor blades were placed in the path of the cathode rays, they seemed to rotate. This proved that the cathode rays were made up of particles of a certain mass and that they have some energy. He called these cathode rays “electrons”.

Ernest Rutherford’s Atomic Theory

In 1911, Rutherford performed an experiment which helped him in his discovery of the nucleus.

He named his experiment ” the gold foil experiment” where he bombarded a thin foil of gold with alpha particles. He noticed that most of the particles passed through the few while only about one out of about nine thousand was deflected. From his observation, he concluded that most of the space inside that atom was empty and that 1/9000th of rays got strongly deflected because of a very strong positive charge in the center of the atom. He called this strong positive charge as “nucleus”.

Rutherford therefore proposed a spherical atomic structure where the nucleus is at the center of an atom and most of the charge and mass are concentrated there. The electrons, on the other hand, revolve around the nucleus in a circular orbit, similar to the way planets orbit the sun.

Atomic Structures of Isotopes

Frederick Soddy

Isotopes are elements having the same atomic number but different mass numbers.

This implies that isotopes of elements have the same number of protons but different number of neutrons.

For example, there exists three known naturally occurring isotopes of hydrogen, namely: protium, deuterium, and tritium and they have 1, 2 and 3 mass numbers respectively. These isotopes of hydrogen also have 0, 1 and 2 neutrons respectively. These isotopes vary in stability and have different half-lives. However, they generally have similar chemical behavior owing to the fact that they hold the same electronic structures.


1.How do the atomic structures of isotopes vary?

2.Why was lead used to shield alpha radiation fall-outs in Rutherford’s experiment?

3.The ratio of proton to neutron in an atom that contains 17 protons, 18 neutrons and 18 electrons can be represented in what way?

Leave your answers in the comment section below.

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