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Science Class 9 Notes – Atoms and Molecules
1. Law of conservation of mass :This law was stated by Lavoisier in 1744. It states that “In
all physical and chemical changes, the total mass of reactants is equal to total mass of
products.”
2. Law of constant proportions (or constant com-position) : This law was first stated by
Proust in 1797. According to the law “a chemical compound is always found to be made up of
the same elements combined together in the same proportions by weight” e.g. the ratio of
hydrogen and oxygen in pure water is always 1: 8 by weight. This law is also called law of
definite proportions.
3. Dalton’s Atomic theory : Postulates of Dalton Atomic Theory
• Matter is made up of extremely small indivisible particles called atoms.
• Atoms of the same substance are identical in all respects i.e., they possess same size,
shape, mass, chemical properties etc. Atoms of different substances are different in all
respects i.e., they possess different size, shape, mass etc.
• Atom is the smallest particle that takes part in a chemical reaction.
• Atoms of different elements May combine with each other in a fixed simple, whole
number ratio to form compound atoms.
• Atoms can neither be created nor destroyed i.e., atoms are indestructible.
4. Atom : It is the smallest particle of an element which can take part in a chemical change. It
may or may not be capable of independent existence.
5. Symbol: The abbreviation used for lengthy names of elements are termed as their symbols.
The symbol of an element is the first letter or the first and another letter of English name or
Latin name of the element. While writing a symbol, the first letter is always capital and the
second is always small.
6. Molecule : It is the smallest particle of an element or compound that is capable of inde-
pendent existence and shows all the properties of that substance.
[The molecules of an element is made up of only one and same type of atoms, while the
molecule of a compound is made up of dissimilar atoms]
7. Atomicity : The number of atoms present in a molecule of an element or a compound is
known as its atomicity. e.g. the atomicity of oxygen is 2 while atomicity ozone is 3.
8. Ion : It is an electrically charged atom or group of atom. It is formed by the loss or gain of
electrons by an atom. Ions are of two types :
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Science Class 9 Notes – Atoms and Molecules
1. Law of conservation of mass :This law was stated by Lavoisier in 1744. It states that “In
all physical and chemical changes, the total mass of reactants is equal to total mass of
products.”
2. Law of constant proportions (or constant com-position) : This law was first stated by
Proust in 1797. According to the law “a chemical compound is always found to be made up of
the same elements combined together in the same proportions by weight” e.g. the ratio of
hydrogen and oxygen in pure water is always 1: 8 by weight. This law is also called law of
definite proportions.
3. Dalton’s Atomic theory : Postulates of Dalton Atomic Theory
• Matter is made up of extremely small indivisible particles called atoms.
• Atoms of the same substance are identical in all respects i.e., they possess same size,
shape, mass, chemical properties etc. Atoms of different substances are different in all
respects i.e., they possess different size, shape, mass etc.
• Atom is the smallest particle that takes part in a chemical reaction.
• Atoms of different elements May combine with each other in a fixed simple, whole
number ratio to form compound atoms.
• Atoms can neither be created nor destroyed i.e., atoms are indestructible.
4. Atom : It is the smallest particle of an element which can take part in a chemical change. It
may or may not be capable of independent existence.
5. Symbol: The abbreviation used for lengthy names of elements are termed as their symbols.
The symbol of an element is the first letter or the first and another letter of English name or
Latin name of the element. While writing a symbol, the first letter is always capital and the
second is always small.
6. Molecule : It is the smallest particle of an element or compound that is capable of inde-
pendent existence and shows all the properties of that substance.
[The molecules of an element is made up of only one and same type of atoms, while the
molecule of a compound is made up of dissimilar atoms]
7. Atomicity : The number of atoms present in a molecule of an element or a compound is
known as its atomicity. e.g. the atomicity of oxygen is 2 while atomicity ozone is 3.
8. Ion : It is an electrically charged atom or group of atom. It is formed by the loss or gain of
electrons by an atom. Ions are of two types :
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