The smallest particle of an element taking part in a chemical combination is an atom. The concept of the atom is more than 2500 years old. We know that the smallest particles of most of the substances are molecules. The molecules of a few substances contain only one atom, which means that Molecules are formed by a chemical combination of atoms.
The Indian philosopher, Kanad (6th century B.C.) expresses that there is a limit to dividing matter into small particles. The indivisible particles that constitute matter were named by Kanad Muni as ‘Paramanu’ (meaning the smallest particles). He also stated that ‘Paramanu’ is indestructible
The Greek philosopher Democritus (5th century B.C.) definite that matter is made of small particles and these cannot be divided. The smallest particle of matter was named by Democritus as an ‘Atom’. (In the Greek language ‘Atomos’ means the one which cannot be divided.
Dalton’s atomic theory :
British scientist John Dalton proposed ‘Atomic Theory’. in 1803 A.D.
- Matter is made of atoms.
- Atoms are indivisible and indestructible.
- All-atom of an element are alike, while different elements have different atoms with different
mass. - Mass is distributed uniformly in an atom
Thomson’s plum pudding model of atom :
Thomson proposed a first-time model of atomic structure in the year 1904.
- The positive charge is distributed throughout the atom.
- The negatively charged electrons are embedded in it.
- The distributed positive charge is balanced by the negative charge on the electrons. Therefore the atom becomes electrically neutral.
Rutherford’s nuclear model of the atom (1911) :
Rutherford’s stated that atom
- There is a positively charged nucleus at the center of an atom.
- Almost the entire mass of the atom is concentrated in the nucleus.
- Negatively charged particles called electrons revolve around the nucleus.
- The total negative charge on all the electrons is equal to the positive charge on the nucleus. As the opposite charges are balanced the atom is electrically neutral.
- There is an empty space between the revolving electron and the atomic nucleus.
Bohr’s stable orbit atomic model (1913) :
In the year 1913 Danish scientist Niels Bohr explained the stability of atoms by putting forth a stable orbit atomic model.
- The electrons revolving around the atomic nucleus lie in concentric circular orbits at a certain distance from the nucleus.
- The energy of an electron is constant while it is in a particular orbit.
- When an electron jumps from an inner orbit to an outer orbit it absorbs energy equal to the difference in its energy level and when it jumps from an outer orbit to an inner orbit it emits energy equal to the difference in its energy level.
Atomic structure :
Atoms are formed from electrons, protons, and neutrons. Protons and neutrons are contained in the nucleus and electrons revolve around the nucleus.
Nucleus :
The atomic nucleus is positively charged. Almost the entire mass of the atom is concentrated in the nucleus. The nucleus contains two types of subatomic particles together called nucleons. Protons and neutrons are the two types of nucleons.
Proton (p) :
The positive charge on the nucleus is due to the proton in it. A proton denoted ‘p’. Each proton carries a positive charge of +1e. The number of protons in an atom is called as Atomic Number of that element and is denoted by Z. The Mass of one proton is approximately 1u (1 Dalton)
Neutron (n) :
A neutron is an electrically neutral particle and is denoted by the symbol ‘n’. The number of neutrons in the nucleus is denoted by the symbol ‘N’. The mass of a neutron is approximately 1 u, which is almost equal to that of a proton.
Electron (e-) :
Electron is a negatively charged particle and is denoted by the symbol ‘e-’. Each electron carries one unit of negative charge (-1e). Charge equal on electron and proton.
The mass of an electron can be treated as negligible because the Mass of an electron is 1800 times less than that of a hydrogen atom. Electrons revolve in the orbits around the nucleus. The orbits are three-dimensional in nature. The energy of an electron is determined by the shell in which it is present. Number of electrons is equal to number of protons.
Note:
- Atomic Number (Z) = number of proton = number of electron
- Atomic mass (A) = number of pronton + number of nuetron
