Write the chemical formula of barium chloride.
Enter your correct answer as A, B or C.
Important Points to Remember in Chapter -1 - Atomic Structure from Tamil Nadu Board Standard Eight Science Solutions
1. Dalton’s Atomic Theory
(i) According to Dalton’s atomic theory, all matter is made of atoms, which are indivisible. Atoms of a given element are identical in mass and properties.
(ii) Dalton’s theory was based on the law of conservation of mass and the law of constant composition.
2. Discovery of Fundamental Particles
(i) Electron was discovered by J.J. Thomson whereas proton was discovered by Goldstein.
(ii) Rutherford’s alpha-ray scattering experiment led to the discovery of the atomic nucleus.
(iii) Rutherford’s model of the atom proposed that a very tiny positively nucleus is present inside the atom and electrons revolve around the nucleus. The stability of the atom could not be explained by this model.
(iv) Neils Bohr proposed that electrons are distributed in different shells with discrete energy around the nucleus. Electrons revolve in orbits (shells) without losing any energy. This atomic model explains the stability of the atom.
(v) J. Chadwick discovered presence of neutrons in the nucleus of an atom. So, the three sub-atomic particles of an atom are:
(a) electrons,
(b) protons and
(c) neutrons.
Electrons are negatively charged, protons are positively charged, whereas neutrons have no charge. The mass of an electron is about 11840 times the mass of a hydrogen atom. The mass of a proton and a neutron is taken as one unit each.
3. Thomson's Atom Model
First atomic structure was given by J.J. Thomson who proposed that electrons are embedded in a positive sphere. Neutron was not known at the time of Thomson.
4. Valency
(i) Shells of an atom are designated as K, L, M, N etc.
(ii) The maximum number of electrons in an orbit is .
(iii) The outermost shell can have at the maximum electrons.
(iv) Valency is defined as the combining capacity of an atom.
5. Variable Valency
Some elements differ in their capacity to combine with other elements depending on the nature of the reaction, this property is called variable valency.
6. Types of Ions
The atoms which carry positive or negative charge are called ions. They are of two types, cations and anions.
7. Chemical Formula
Chemical formula is the shorthand notation of a molecule of a substance (compound).
8. Chemical Equation
A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and formulae, where the reactant entities are given on the left-hand side and the product entities on the right-hand side.
9. Laws of Chemical combination
(i) A chemical equation is balanced to follow the law of conservation of mass.
(ii) Law of constant proportion states that a given chemical compound always contains its component elements in fixed ratio (by mass) and does not depend on its source and method of preparation.
