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Grade 9 Atoms and Molecules


Mass can neither be created nor destroyed in a chemical reaction. This is known as
the law of conservation of mass. It means that the sum of the masses of the reactants
and the products remains the same during a reaction.
The law of constant proportion states that a chemical substance always contains the
same elements in a fixed proportion by mass, irrespective of the source of
compound.
Matter is made up of very tiny particles and these particles are called atoms. Atoms
cannot be divided further i.e., atoms are indivisible. An atom can be defined as the
smallest particle of matter that can neither be created nor destroyed by chemical
means.
The symbol of the element is made from one or two letters of the English or the
Latin name of the element.
The mass of an atom is known as the atomic mass. The atomic mass of an atom of an
element is also known as its relative atomic mass, since it is determined relative to
the mass of C-12 isotope.
A molecule is formed when two or more atoms of the same element or different
elements get combined chemically. The number of atoms that combine to form a
molecule is called the atomicity of the molecule.
An ion is a charged species in which an atom or a group of atoms possess a net
electric charge (positive or negative). Positively charged ions are called cations and
negatively charged ions are called anions. Compounds in which molecules are
formed by the combination of cations (positively charged ions) and anions
(negatively charged ions) are known as ionic compounds.
A chemical formula is the representation of the composition of a molecule in terms
of the symbols of elements present in that molecule.
The molecular mass of a substance is the sum of the atomic masses of all the atoms
present in a molecule of that substance. The formula unit mass of a substance is the
sum of the atomic masses of all the atoms present in a formula unit of that substance.
One mole of a substance is the quantity of the substance containing 6.022 1023
numbers of particles (atoms, molecules, or ions). The number i.e., 6.022 1023 is
an experimentally obtained value and is known as the Avogadro number or the
Avogadro constant. It means that one mole of any substance (element or compound)

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