Inside the Atom Science Notes
→ Indian philosopher Kanad (6th century B.C.) stated that there is a limit to divide 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.
→ Greek philosopher Democritus (5th century B.C.) stated that matter is made of small particles and these cannot be divided. The smallest particle of matter was named by Democritus as ‘Atom’. (In Greek language ‘Atomos’ means the one which cannot be cut.)
Dalton’s atomic theory:
According to Dalton’s atomic theory matter is made up of atoms and atoms are indivisible and indestructible.
All atoms of an element are alike while different element have different atom with different mass.
Thomson’s plum pudding model of atom
- According to Thomson’s model the positive change is distributed throughout the atom and the negatively charged electron 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 atomic model
- An atom has tiny, dense positively charged nucleus at centre of an atom.
- Most of the mass of the atom is concentrated in the nucleus.
- Negatively charged particles called electrons revolve around the nucleus.
- The total negative charged on all the electron 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:
In the year 1913 Danish scientist Niels Bohr explained the stability of atom by putting forth stable orbit atomic model.
- The electrons revolving around the atomic nucleus lie in the concentric circular orbits at certain distance from the nucleus.
- 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 specific amount of energy, and when it jumps from an outer orbit to an inner orbit it emits specific amount of energy.
- The energy emitted or absorbed during these transitions is equal to energy differences between the initial state and the final state of the electron.
When table salt (Sodium chloride) is thrown on LPG gas stove flame, immediately yellow spark forms on that place. If sodium metal is put in water, it burns to give yellow flame. On road, sodium vapour lamp gives yellow colour light. From all the above examples, the electron of sodium absorb energy and goes to outermost shell and come back to inner shell by emitting energy. The difference of energy level of these two shells of sodium is fixed. This difference is similar to energy of yellow light. Therefore in above examples same specific yellow light is emitted.
→ Atomic structure: An atom is formed from the nucleus and the extranuclear part. These contain three types of subatomic particles.
→ Nucleus: The atomic nucleus is positively charged. Almost 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 proton is the positively charged particle denoted by the symbol p. It is located in the nucleus of an atom.
→ Electron (e1): The electron is negatively charged particle denoted by the symbol e1. The charge on the electron (e1) is treated as one elementary unit of negative charge. Electrons revolve around the nucleus of the atom in specific orbits.
→ Atomic number (Z): The number of electrons or protons in an atom is called the atomic number. It is denoted by Z.
→ Atomic mass number (A): The total number of protons and neutrons in the nucleus of the atom is called the atomic mass number. It is denoted by A.
→ Distribution of electron: Electrons revolve in stable shells. These shells have a definite energy. The shell nearest to the nucleus is called the first shell. The next shell is called the second shell. A symbol ‘n’ is used for the ordinal number of a shell.
The shells are referred to by the symbols K, L, M, N, … corresponding to the ordinal numbers n = 1, 2, 3, 4, … The maximum number of electron a shell can contain is obtained by the formula ‘2n2’. As the magnitude of ‘n’ increases, the energy of an electron in that shell increases.
Electronic configuration of elements
The electrons in an atom are distributed in the shells according to their maximum capacity. The shells distribution of the electron in an atom of an element is called the electronic configuration of that element.
Each electron has a definite energy as per the shell in which it is present. Energy of an electron in the first shell (K shell) is the lowest. Energy of electron in the subsequent shells goes on increasing with ordinal number of the shell. The electronic configuration of an element is such that the energy of all the electrons together is the maximum possible.
Electrons get a place in the shells in accordance with the maximum capacity of the electron shell in an atom and the increasing order of energy.
Valency and valence electrons:
Valency of an atom is determined by the configuration of its outermost shell. Therefore the outermost shell is called | valence shell and electrons in the outermost shell are called valence electrons. “Valency of an element is same as the number of its valence electrons if this number is four or less j than four. On the other hand, when an j element has four or more valence electrons, the number of electron by which the octet is short of completion is the valency of that element.
Atoms of the same element having the same atomic number, but different atomic mass numbers are called isotopes.
→ Uses of Isotopes:
- Uranium – 235 is used for nuclear fission and production of electricity.
- Radioactive isotopes like Cobalt – 60 are used in the treatment of cancer.
- Iodine – 131 is used in the treatment of goitre, disease of thyroid gland.
- The radioactive isotopes are used for detection of cracks (leakage) in the underground pipes, E.g. Sodium -24.
- Radioactive isotopes are used for food preservation from microbes.
- The radioactive isotope C-14 is used for determining the age of archaeological objects.
→ Nuclear Reactor: Nuclear reactor is a machine that generates electricity on large scale by using atomic energy.