Structure Of Atom
Matter is all around us in the universe, it has mass and takes up space. Atoms are basic building blocks of matter, and cannot be chemically subdivided by ordinary means.
The word atom is derived from the Greek word atom which means indivisible. The Greeks concluded that matter could be broken down into particles too small to be seen. These particles were called atoms.
Atoms are composed of three type of particles: protons, neutrons, and electron. Protons and neutrons are responsible for most of the atomic mass.
Both the protons and neutrons reside in the nucleus. Protons have a positive (+) charge, neutrons have no charge --they are neutral. Electrons reside in orbitals around the nucleus. They have a negative charge (-).
It is the number of protons that determines the atomic number, e.g., H = 1. The number of protons in an element is constant (e.g., H=1, Ur=92) but neutron number may vary, so mass number (protons + neutrons) may vary.
The same element may contain varying numbers of neutrons; these forms of an element are called isotopes. The chemical properties of isotopes are the same, although the physical properties of some isotopes may be different. Some isotopes are radioactive-meaning they "radiate" energy as they decay to a more stable form, perhaps another element half-life: time required for half of the atoms of an element to decay into a stable form. Another example is oxygen, with an atomic number of 8 can have 8, 9, or 10 neutrons.
Various Models of atom are
(a) Thomson Model or Water Melon Model or Plum pudding Model According to this model electrons are embedded in the positively charged mass distributed uniformly throughout the atomic sphere. This model was proposed by Joseph James Thomson in 1897. This model is also known as apple pie model.
(b) Rutherford’s Model or Planetary Model: This model is based on experiments conducted by Rutherford. This model was given by Ernest Rutherford in 1911. According to this model, all the positively charged particles are present in a small space in the centre of the atom. This small space is called nucleus. The electrons revolve around the nucleus just as the planets revolve around the sun. Due to this similarity, Rutherford’s model is called the planetary model or Solar model. Electrons (negatively charged) revolve around the nucleus in orbits with a high speed to overcome the electrostatic force. Drawbacks of Rutherford’s Model - It is possible to have an infinite number of orbits. In practice, it is not the case. The moving electron must continuously lose energy and fall into the nucleus. Actually, it is not the case.
(c) Bohr’s Atomic Model: This model was given by Neils Bohr in 1913. According to this model, an atom consists of a heavy positively charged nucleus. The whole mass of the atom is concentrated in the nucleus. The electrons in an atom revolve around the nucleus in definite circular paths called orbits or energy level. Each energy level is associated with a definite amount of energy. The change in energy takes place when an electron jumps from one energy level to another energy level.