Molecule, Atomicity, Macromolecule, Ions, Molecular Ions, Atomic mass unit, Isotopes, Relative abundance of isotopes, FCS, Punjab text book

Molecule:

The smallest particle of a substance that can exist independently is called a molecule. For example H₂, O₂, Cl₂,. A molecule consists of one or different kinds of atoms, for example, P_4, S₈, NH₃, H₂SO₄, C₆H₁₂O_6.

Monoatomic molecule: A molecule that contains one atom is called a monoatomic molecule. For example, He, Ne, Ar, Kr, Xe, and Rn.

Diatomic molecule: A molecule that contains two atoms is called a Diatomic molecule. For Example H₂, O₂, Etc.

Triatomic molecule: A molecule that contains three atoms is called a Triatomic molecule. For Example: CO₂, H₂O, SO₂ etc.

Atomicity:

The total number of atoms in a molecule is called atomicity. For example, the atomicity of He is one, the atomicity of H₂ is 2, and the atomicity of H₂O is 3.

Macromolecule: 

A big molecule that contains a very large number of atoms is called a macromolecule. For example Haemoglobin, Chlorophyll. Hemoglobin is found in the blood. It helps to carry oxygen from the lungs to all parts of the body. A hemoglobin molecule is made up of nearly 10,000 atoms and it is 68000 times heavier than a hydrogen atom.

Ions:

The species which carry positive or negative charge are called ions. For example: Na⁺, Ca⁺², Al⁺³, Cl^-, Br^-, SO₄^-2, PO₄^-3, MnO4^-1, (Permanganate)^-.

CO₃^-2 (Carbonate) and Cr₂O₇^-2 (dichromate) etc.

A positive ion or cation is formed by the loss of one or more electrons.

A   ---------------------                        A⁺   +   e^-

The energy is required for the removal of electrons. So the formation of positive ions is an endothermic process.

A negative ion (anion) is formed by the gain of one or more electrons.

B     +    e^-     --------------------                B^-

The energy is released when an electron is added to an atom. So the formation of uni negative ion is an Exothermic process.

The metals usually form positive ions and non-metals form negative ions.

Molecular Ions: 

The ions formed by the loss or gain of electrons from a molecule are called molecular ions. For example: CH₄⁺, N₂⁺, CO⁺, etc.

They are generated by passing high-energy electrons or α-particles through a gas. It is necessary to note that cationic molecular ions are more abundant than anionic ones.

Relative Atomic Mass: 

The mass of an atom of the element as compared to the mass of an atom of the element as compared to the mass of ¹²C is called relative atomic mass. For Example: H= 1.0078amu, oxygen = 16amu, Na= 23amu, S= 32amu.

Atomic Mass Unit: 

The ½ ^th of the mass of one Carbon atom is called the atomic mass unit. The atomic masses are very small. So, the atom can not be weighted by any balance. Therefore we use the relative atomic mass unit Scale.

Isotopes: 

The atoms of the same element which have the same atomic number but a different mass number are called Isotopes. For example, Hydrogen has three isotopes (₁¹H, ₁²H, ¹₃H).

They are called protium, deuterium, and tritium.

Carbon has isotopes (¹²₆C,  ¹³₆C, ¹⁴₆C ). Oxygen has three isotopes, Nickel has five, Calcium has six, Palladium has Six, Cadmium has nine, & Tin has eleven isotopes. The isotopes have the same number of protons and electrons but a different number of neutrons. For example, ¹²₆C,  ¹³₆C, and ¹⁴₆C each have six protons, and six electrons but they have 6, 7, and 8 neutrons, respectively. The isotopes of an element have the same chemical properties and the same position in the periodic table. It is called isotopy.  This process was first discovered by Soddy. The separation of isotopes can be done by gaseous diffusion, thermal diffusion, distillation, ultracentrifuge, laser separation, electromagnetic separation, etc.

Relative Abundance of Isotopes:

The %age of different isotopes in a given sample of an element is called the Relative Abundance of Isotopes. For example, a sample of Chlorine has 75.53% ₁₇³⁵Cl and 24.47% ₁₇³⁷Cl.

Mono-isotopic elements: 

Some elements have only a single isotope. They are called Mono-isotopic elements. For example, Arsenic, Fluorine, Gold, and Iodine, etc.

The elements of odd atomic numbers have generally two stable isotopes. The elements of even atomic numbers have a larger number of isotopes. The isotopes whose mass numbers are multiple of four are the most abundant. For example, ¹⁶O, ²⁴Mg, ²⁸Si, ⁴⁰Ca, & ⁵⁶Fe form about 50% of the earth’s crust.

At present, about 280 different isotopes occur in nature. Of these 280 isotopes, 40 are radioactive and 154 have even mass numbers and even atomic numbers. About 300 unstable radioisotopes have been produced by artificial disintegration (radioactivity).