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The Structure of the Atom

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The tiniest bit of anything is the atom. The tiniest bit of gold is one atom of gold. The tiniest bit of oxygen, of mercury, indeed of every solid, liquid and gas in this Universe, is one atom of that element. It is the basic unit of all matter.

And yet, even atoms are made up of smaller particles!


The basic subatomic particles are –

  1. Protons
  2. Neutrons
  3. Electrons

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Each atom has a definite number of these subatomic particles. The protons and neutrons are at the center of the atom, the nucleus, with empty space around it. The electrons whirl around the nucleus in the empty space, completing billions of trips in less than a millionth of a second. The paths of these electrons are completely random. Even the protons and neutrons constantly move at random inside the nucleus. The nucleus, comprising the protons and neutrons, contributes to almost all of the mass of the atom. (Mass is the quantity of matter in the atom.) This means that electrons have really very little mass.

Properties of Atoms

This tiniest building block of the Universe is the most studied by Chemists. There are many properties of atoms to understand and study.

Some of the main ones are –

  1. The atomic number, which tells us how many protons an atom has
  2. The mass number, which is the sum of protons and neutrons in an atom
  3. The atomic weight
  4. The electric charge of an atom
  5. Radioactivity and many more properties


All atoms of an element have the same number of protons. But all atoms of that element can have different numbers of neutrons. This changes the mass number of that atom, creating isotopes of that atom. Most elements have more than one isotope. Hydrogen, for example, has zero, one or two neutrons in addition to one proton. The study of isotopes is of immense importance to areas such as Biology, Medicine, Earth & Planetary Sciences, and many more.

Some isotopes form naturally through changes in the nucleus of the atom. The actual number of protons and neutrons in an atom may undergo a change. When this happens to the nucleus, the atom becomes an atom of a different element. Such atoms are called radioactive because they decay very quickly and give off radiation in the form of rays.


Many heavy elements such as uranium and radium are naturally radioactive, i.e. they change naturally into other atoms and decay to give off radioactive rays. Physicists can even artificially create radioactive isotopes in a lab. The radioactive property is useful in many ways such as for energy generation, food sterilization and scanning with X-rays, to name a few.

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Check Point

  1. Which of the following are subatomic particles?
    1. Protons
    2. Crotons
    3. Electrons
    4. a) and c) above
  2. What does the nucleus consist of?
  3. ______ spin around in the empty space around the nucleus.
  4. Isotopes of the same atom have a different number of ________.
  5. If the number of protons and neutrons in an atom change then –
    1. The atom becomes an atom of another element
    2. It creates a radioactive atom
    3. It decays and emits rays
    4. All of the above

Answer Key

  1. d) Protons and Electrons
  2. Protons and Neutrons
  3. Electrons
  4. Neutrons
  5. d) all of the above

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