Atoms and Molecules Class 9 Notes Science - Chapter 3
What Are Atoms And Molecules ?
Laws Of Chemical Combination
Antoine L. Lavoisier give two important laws of chemical combination.
Law of Conservation of Mass: It states that mass can neither be created nor destroyed in a chemical reaction.
Law of Constant Proportions or The Law of Definite Proportions: This law was given by Proust as “In a chemical substance the elements are always present in definite proportions by mass”.
This theory was given by British chemist John Dalton. The postulates of this theory may be stated as follows:
• All matter is made of very tiny particles called atoms.
• Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
• Atoms of a given element are identical in mass and chemical properties.
• Atoms of different elements have different masses and chemical properties.
• Atoms combine in the ratio of small whole numbers to form compounds.
• The relative number and kinds of atoms are constant in a given compound.
Atoms are the smallest particles of an element which can take part in a chemical reaction.
Atomic mass unit: One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12. Example: Atomic mass of H is 1, C is 12, N is 14, O is 16 , Na is 23 etc.
Representation Of Atoms Of Different Elements
An element symbol is a one- or two-letter abbreviation for a chemical element name. When a symbol consists of two letters, the first letter is always capitalised, while the second letter is lowercase.
Symbols of Some Elements
It can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.
The number of atoms constituting a molecule is known as its atomicity. Example:
An ion is a charged particle.
Types of Ion
The chemical formula of a compound is a symbolic representation of its composition.
The combining power (or capacity) of an element is known as its valency.
• The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.
• It is expressed in atomic mass units (u).
Formula Unit Mass
The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound.