Explore Atoms and Molecules with Future Classes - CBSE Class 9 Science
Atoms and molecules form the basis of class 9 science as per the CBSE curriculum. Atoms are the basic units of matter that combine to form molecules. Molecules are composed of two or more atoms bonded together. The study of atoms and molecules will help students understand the physical and chemical properties of matter. This chapter will also provide students with the knowledge to explain different phenomena in the world around them.
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Chapter - 3 Atom and molecules
Laws of Chemical Combination
The chemical reaction between two or more substances giving rise to products are governed by certain laws. These laws are called ‘Laws of Chemical Combination’.
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Law of Conservation of Mass
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Law of Constant Proportion
Law of Conservation of Mass
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According to this law, “Mass can neither be created nor destroyed.”
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“During a chemical reaction total mass of reactants will be equal to total mass of products.”
Law of Constant Proportion or law of definite proportions
According to this law, “in a chemical substance or compound, the elements are always present in definite proportions or ratios by mass.
In a compound such as water, the ratio of mass of hydrogen to the mass of oxygen is always 1:8, whatever the source of water. Thus, if 9g of water is decomposed, 1g of hydrogen and 8g of oxygen are always obtained.
Similarly, carbon dioxide (CO2) always contains carbon and oxygen in the ratio of 3:8. If a sample of carbon dioxide (CO2) contains 36g of carbon then it is compulsory that the sample has 96 g oxygen.
Atom
Atom are the smallest particles of an element which may or may not have independent existence but take part in a chemical reaction. These are the building blocks of all matter.
e.g., Atoms of hydrogen, oxygen, nitrogen etc., are not capable of independent existence whereas atoms of helium, neon etc., are capable of existing independently. Atoms are very small and their radius is measured in nanometers.
Hydrogen atom is the smallest atom and its radius is 0.1 nm.
Symbols for some elements
Atomic Mass
The mass of an atom of an element is called its atomic mass.
Atomic Mass Unit
The atomic mass unit is defined as the quantity of mass equal to 1/12 of mass of an atom of carbon-12.
Molecule
A molecule is a group of two or more atoms that are chemically bonded together. Atoms of the same element or of different elements can join together to form molecules.
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Molecules are of two types
Molecules of Elements
They contain same type of atoms of an element. Molecules of many elements are made up of only one atom of that element.
Molecules of compounds
Atom of different element joins together in definite proportions to form molecules of compounds.
Ions
When atoms, groups of atoms or molecules loss or gain electron(s) they become charged. These charged species are known as ions. Atoms in solution generally exist in the form of ions. These can be negatively or positively charged, thus can be categorized into two groups.
Cations The positively charged ions are known as cations.
These are formed when elements lose electrons. Usually, metals form cations.
Anions The negatively charged ions are known as anions.
These are formed when elements gain electrons, Usually, non-metals form anions.
Polyatomic ion
A group of atoms carrying charge and act as a single entity is known as a polyatomic ion. It carries a fixed charge. e.g., nitrate ion, carbonate ion and sulphate ion etc.
Chemical formula
The chemical formula of a compound is a symbolic representation of its composition.
Rules for writing chemical formulae
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We first write symbols of elements which form compound.
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Below the symbol of each element, we should write their valency.
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Now cross over the valencies of combining atoms.
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With first atom, we write the valency of second atom (as a subscript).
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With second atom, we write the valency of first atom (subscript).
Molecular Mass
It is the sum of atomic masses of all the atoms in a molecule of that substance.
Formula Unit Mass
It is the sum of atomic mass of ions and atoms present in formula for a compound.
