Atoms, Molecules, Valency and Radicals

Atoms

John Dalton was the first to enunciate matter to be composed of invisible particles called atoms. Atoms means which cannot be cut (uncut). So, an atom may be defined as the smallest particle of an element or chemical substance which generally participates in the chemical reaction but it may or may not have an independent existence. This means an atom is unstable. For example- Hydrogen gas is the smallest molecule which is composed of two atoms of hydrogen. Similarly, Helium, Neon, Argon, etc. are examples of an atom.

Characteristics of atoms:

1) An atom is the smallest unit of an element.

2) It takes part in the chemical reaction.

3) It is composed of three subatomic particles namely proton, electron, and neutron.

4) It generally doesn't exist in nature independently because they are reactive and the life span of an atom is very short.

5) It is electrically neutral particle because the total number of protons are equal to the total number of electrons.

Molecules

A molecule can be defined as the smallest particle of an element or a compound, which does not generally participate in a chemical reaction but has an independent existence. This means a molecule is stable. The term 'molecule' was given by Avogadro. The properties of one molecule are same as the whole of the compound i.e. the properties of a compound is determined by molecule, not by atoms.

For example- H₂, O₂, N₂are the diatomic molecules.

He, Ne, Ar are monoatomic molecules.

O₃ and S₈ are polyatomic molecules.

CH₄ and PCl₅ are heteroatomic molecules.

Symbols and formulae and their significances

Symbols can be defined as the short representation of an element.

Example: Iron= Fe, Hydrogen= H, Oxygen= O, Sodium= Na, Gold= Au etc.

Elements having symbols derived from latin names.

Elements having name based on scientist

Significance of a symbol

A symbol is not only for an abbreviation for the full name of the element but it has qualitative and quantitative significance. For example- The symbol Na stands for qualitatively.

1) The element sodium quantitatively signifies-

• One atom of sodium
• One atomic weight of sodium (i:e 23, parts by weight of sodium)
• One gram atomic weight of sodium (i:e 23 gram of sodium or 6.023× 10²³. sodium ions).

Molecular formula

It is the short representation of a molecule.

Example: Water = H₂O, Ammonia = NH₂

The name and formulae of some common compounds are given below:

Significances of molecular formula or formulae

A chemical molecular formula has both qualitative and quantitative significance. A chemical formula gives the following information:

Let us take NH₃ for example:

1) Qualitatively a chemical formula represents:

• The compound is ammonia.
• Ammonia is composed of nitrogen and hydrogen.

2) Quantitatively the chemical formula represents:

• One molecule of ammonia.
• One mole of ammonia contains 6.023× 10²³molecules of ammonia.
• One molecule of ammonia contains one atom of nitrogen and 3 atoms of hydrogen.
• The molecular weight of ammonia is 17 (14+3).
• 17 parts by weight of ammonia contain 14 parts by weight of nitrogen and 3 parts by weight of hydrogen.

Valency

Valency (according to the classical concept):Different elements combine together to give millions of compounds. Each element has its own combining capacity to other elements which is known as valency. The valency of an element is combining the capacity of an element which is measured in terms of the number of the hydrogen atom or its equivalent that combine to one atom of an element. Consider the following compound-

Hydrogen chloride = HCl

Water = H₂O

Ammonia = NH₃

Methane = CH₄

In these compounds, one atom of Cl, O, N, and C are combined with 1, 2, 3, and 4 atoms of hydrogen respectively. So, the valency of Cl, O, N, and C are 1, 2, 3, and 4 respectively. Certain elements do not combine with hydrogen. For such elements combining capacity is measured with reference to chlorine. For example- NaCl, AlCl₃. In these compounds, one atom of Na and Al combines with 1 and 3 atoms of chlorine respectively. So the valency of Na and Al is 1 and 3 respectively.

In H₂O, the combining capacity of oxygen is twice that of hydrogen. So, twice the number of oxygen that combines with 1 atom of elements is also called valency. For example- in MgO, 1 atom of Mg combines with 1 atom of oxygen which is equivalent to 2 atoms of hydrogen, so the valency of Mg is 2.

In summary,the number of hydrogen atoms or the number of chlorine atoms or twice that number of the oxygen atom that combine with one atom of elements give the valency of that element.

Variable valency

Certain elements can have more than one valency. For instance, iron combines with chlorine to give ferrous chloride ( FeCl₂) and ferric chloride (FeCl₃). So, iron has the valency of 2 and 3 respectively in these compounds. The compound with the metal having the lower valency is called -ous compound and metal having higher the valency is called -ic compound.

Radicals

The atoms or a group of atoms which behave as a single unit during a chemical reaction and carry positive or negative charge are called radicals.

Most of the inorganic components are made up if two oppositely charged units. For example- FeSO₄consists of two parts Fe²⁺and SO₄^2-which are called radicals. Similarly, MgCl₂ consists of Mg⁺⁺and Cl^- which is also called radicals. The radicals may be classified into-

a) Simple radicals: The radicals which contain only one atom are called simple radicals. Magnesium (Mg⁺⁺), calcium (Ca⁺⁺), oxide (O^2-) are examples of simple radicals.

b) Compound radicals: The radicals which contain more than one atom are called compound radicals. Ammonium (NH₄⁺), sulphate (SO₄^--), nitrate (NO₃^-) are examples of compound radicals.

Depending upon the charge present in it, radicals are of two types:

Radicals containing a positive (+) charge orElectropositive/basic radicals.Like: NH₄⁺, Ca⁺⁺etc.

Radicals containing a negative (-) the charge or Electronegative/acidic radicals.Like: SO₄^2-, NO₂^-etc.

References:

Adhikari, Rameshwar; Khanal, Santosh; Subba , Bimala; Adhikari, Santosh; Khatiwada, Shankar Pd. Universal Chemistry XI. First. Vol. 1st. Kathmandu: Oasis Publication, 2069.

Chaudhary, Ganga Ram; Karna, Shila Kant Lal; Sharma, Kanchan; Singh, Sanjay; Gupta, Dipak Kumar. A Textbook of Higher Secondary Chemistry XI. Ed. 2nd. Kathmandu: Vidyarthi Pustak Bhandar, 2069 (2012).