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Matter is made up of discrete particles. The main ones are atoms, molecules, and ions. An atom is the smallest part of an element which can take part in a chemical reaction. A molecule is the smallest particle of a substance that can exist alone and still retains the chemical properties of that substance. Molecules are made up of atoms.

Atomicity of an element is the number of atoms in one molecule of the element. We have monatomic, diatomic and triatomic for those elements that contain one atom, two atoms and three atoms respectively in their molecules.


Element                                              Atomicity

Hydrogen                                             Diatomic

Oxygen                                                            Diatomic

Nitrogen                                               Diatomic

Neon                                                    Monoatomic

Helium                                                  Monoatomic

Argon                                                   Monoatomic



  1. Define an atom.
  2. Give two examples of diatomic molecules.



An ion is an atom or group of atoms which carries an electric charge. Such groups of atoms that carry either a positive or negative charge are called RADICALS.

An acid radical is thus a small group of atoms carrying a negative charge that keeps its identity. Examples include S042-, N03 e.t.c.

Generally ions are grouped as cations and anions. Cations are positively charged ions e.g Ca2+, Na+, NH4+ e.t.c.

Anions are negatively charged ions e.g.. C032-, S042-, Cl, OH, etc.


  1. What are ions?
  2. State the cation and anion present in (I) H2S04   (ii) NaCl (iii) FeS04



John Dalton, British Physicist and Chemist (1808) proposed the atomic theory thus:

  1. All elements are made up of small indivisible particles called atoms.
  2. Atoms can neither be created nor destroyed in any chemical reaction.
  3. Atoms of the same elements are exactly alike in aspect and are different from atoms of all other elements.
  4. Atoms of different elements can combine in simple whole number ratios to form compounds.
  5. All chemical changes result from the combination or separation of atoms



Due to new discoveries in the twentieth century, Dalton’s atomic theory cannot hold in its entirety. There is need for its modification.

  1. The first statement has been proved wrong by Rutherford’s discovery of protons, electron and neutrons as constituents of the atom. An atom is not an indivisible solid piece.
  2. The second statement still holds good for ordinary chemical reactions. During nuclear reactions, however, the nucleus can be broken into simpler atoms giving out large amount of heat (nuclear fission). This destroys the atoms involved.
  • The discovery of isotopes makes the third statement unacceptable. Chlorine for example has two atoms with different nucleus content and hence different relative atomic masses although the same proton numbers.
  1. The fourth statement is true only for inorganic compounds which contain a few atoms per molecule. Carbon forms very large organic molecules such as proteins, starch and fats which

contain thousands of atoms.



  1. State the modifications of the Dalton’s atomic theory.
  2. A mixture contains propanone, ethanol and water with boiling point of 560C, 780C and 1000C respectively.
  3. What method will be used to separate the liquids
  4. Name the first liquid that will distil over. Explain your answer
  5. Name an industrial process that uses fractional distillation



  1. Which of the following is not a constituent of the atom (a) proton (b) electron (c) neutron  (d) isotope
  1. Which of the following statement about an atom is not correct? (a) it is indivisible (b) it is destructible in some cases (c) it is the smallest part of a substance that takes part in a reaction (d) it is made up of protons, neutrons and electrons
  2. Which of the following is a liquid at room temperature? (a) copper (b) gold (c) mercury (d) silver
  1. How can you separate a mixture of iron filings and sulphur powder? (a) distillation (b) chromatography (c) magnetization  (d) evaporation
  1. What is the atomicity of neon? (a) monoatomic (b) diatomic (c) triatomic (d) polyatomic



  1. Give any two postulates of the Dalton’s atomic theory.
  2. (a) Differentiate an atom from a molecule. (b)How will an atom become an ion?


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