Chapter 1 - BASIC CONCEPTS (Contd.)

MOLECULE, ITS ATOMICITY AND CHARACTERISTICS

The smallest particle of a pure substance which can exist independently is called molecule.

ATOMICITY: The numbers of atoms present in a molecule is called its atomicity. Thus a molecule can be:

• MONO ATOMIC: It contains only one atom like He, Ne, Ar etc.
• DI ATOMIC: It contains two atoms like H₂, O₂, N_2, etc.
• TRI ATOMIC: It contains three atoms like H₂O, CO₂, etc.
• POLY ATOMIC: It contains many atoms. Such molecules are called Macromolecules.

MACROMOLECULES: These are very large molecules containing very large number of atoms. For example HAEMOGLOBIN contains 10,000 atoms in one molecule and it is 68,000 times heavier than H-atom.

A molecule may contain atoms of same element or atoms of different elements.

• HOMOATOMIC molecules: H₂, O₂, P₄, S₈
• HETEROATOMIC MOLECULES: HCL, NH₃, C₆H₁₂O₆, H₂SO₄

PROPERTIES OF MOLECULES

1. Molecules of same substance are similar in all respects.
2. Molecules have empty spaces between them. These are maximum in gases and minimum in solids.
3. The molecules are in the state of constant motion. These movements are maximum in gases, lesser in liquids while solids have no vibration motion.
4. Molecules have attractive forces for each other
5. The molecules have definite kinetic and potential energy.
6. For a chemical reaction, molecules must collide with each other. The molecules exchange atoms as a result of collision and new compounds are formed.

WHAT IS ION? ENERGY CHANGES DURING FORMATION OF A POSITIVE AND NEGATIVE ION

ION: the species which carry positive or negative charge are called ions. There are two types of ions:

1. Positive ion (cations)
2. Negative ion (anion)

POSITIVE ION: positive ion is formed when one or more electrons are removed from a neutral atom.

A – e^- → A¹⁺

If an atom loses one electron, unipositive ion is formed.

If an atom loses two electrons, dipositive ion is formed.

If an atom loses three electrons, tripositive ion is formed.

Energy is required to remove an electron from an atom to produce positive ion. It is called ionization energy. Thus formation of positive ion is an endothermic process. The most common positive ions are Na¹⁺, K¹⁺, Mg²⁺, Al³⁺, Fe³⁺, Sn⁴⁺, etc.

NEGATIVE ION (ANION): Negative ION is formed when an atom gains one, two or more electrons.

B + e^- → B^1-

The formation of negative ion is exothermic process. the most common negative ions are Cl^1-, Br^1-, F^1-, S^2-, etc. uninegative, dinegative and trinegative ions are formed due to addition of one, two or three electrons in an atom.

Some negative ion consists of groups of atoms. For example OH^1-, SO₄^2-, PO₄^3-, MnO₄^1-, CrO₄^2- etc. Positive ions having group of atoms are less common. For example NH₄¹⁺ and some carbonations (carbon having +ve charge). Properties of ions are different from corresponding atoms.

MOLECULAR ION AND ITS SIGNIFICANCE

When a molecule gains or losses electron, a molecular ion is formed. For example CH₄¹⁺, CO₃^2-, N₂¹⁺ etc. Cationic (+ve) molecular ions are more abundant than anionic (-ve) molecular ions.

These molecular ions are produced by passing high energy electron beam or alpha-particles through a gas.

SIGNIFICANCE: The molecular ions are quite unstable. The breaking of molecular ion gives useful information about structure of natural products.

RELATIVE ATOMIC MASS: The mass of an atom of an element as compared to mass of carbon atom taken as 12 is called relative atomic mass. For example on C-12 scale mass of H = 1/12*12=1.0078 amu and mass of carbon is 12.0000 amu. The masses of atoms are extremely small. We don't have a balance to weight atoms. Thus we use relative atomic mass unit scale. The elements have fractional relative atomic masses due to their different isotopic abundance.