Atoms, Molecules and Radicals
Atoms and molecules
An atom is the smallest particle of an element that takes part in a chemical reaction. It is extremely small in size and cannot be seen even under the most powerful optical microscope. An atom consists of three sub-atomic particles or fundamental particles.
A molecule is an electrically neutral group of two or more atoms chemically bonded together. It is the smallest particle of a substance, which has the properties of that substance and can exist in a free state. Molecules are of two types. The two types of molecules are molecule of an element and molecule of a compound.
Molecule of an element consists of the same type of atoms. Atomicity is the number of atoms present in one molecule of a substance. The molecule of an element can be called as monoatomic, diatomic, triatomic, tetra-atomic and polyatomic on the basis of the number of atoms present in a molecule.
Molecule of a compound contains two or more different types of atoms chemically combined together in a definite proportion by mass and can exist freely. For example, water molecule, ammonia molecule. Depending upon the number of atoms present in a molecule, a molecule of a compound can be diatomic, triatomic, tetra-atomic and polyatomic.
Radicals and Valency
A radical is an atom or a group of atoms of the same or different elements which behaves as a single unit and has a positive or negative charge. It has definite valency and definite chemical formula.
Radicals are of two types: Basic or Positive radicals or Cations and Acidic or Negative radicals or Anions.
Valency is the combining capacity of an element. The numbers of electrons present in the outermost shell of an atom are called valence electrons.
A positively or negatively charged atom or group of atoms is known as ion. Cations are positively charged ions. Anions are negatively charged ions. Some metals exhibit variable valencies i.e., more than one valency.
Iron has two valencies 2 and 3. So it forms two types of salts, Fe 2+ salts or Ferrous salts and Fe 3+ salts or Ferric salts.
There are 118 elements known to us. Periodic table is a tabular arrangement of these elements in vertical columns and horizontal rows indicating the regular trends in the properties of elements. The horizontal rows of the periodic table are called periods and the vertical columns are called groups. All the elements belonging to same group have same valency.