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Law of Conservation of

MassLaw of conservation of Mass states that mass can neither be created or destroyed in a chemical reaction.

Law of Constant Proportion

In a chemical substance the elements are always present in definite proportion by mass

Dalton’s Theory

1.All matter is made up of tiny particles of tiny particles called atoms.

2.Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.

3.Atoms of a given element are identical in mass and chemical properties.

4. Atoms of different of elements have different masses and chemical properties.

5. Atoms combine in the property of small whole numbers to form compounds .

6. Relative number and the kinds of atoms are constant in a given compound.

What is an Atom?

An atom a fundamental piece of matter. (Matter is anything that can be touched physically.) Everything in the universe (except energy) is made of matter, and, so, everything in the universe is made of atoms. An atom itself is made up of three tiny kinds of particles called subatomic particles: protons, neutrons, and electrons.

Three major parts of an atom !

• Proton • Neutron• Electron

Atomic Mass

Atomic mass or atomic weight is the average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally-occurring element.

Atomic number

The Atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus.

MoleculeA molecule is in general a group of two or more atoms that are chemically bonded together, that is , tightly held together by attractive forces. A molecule 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.

AtomicityThe number of atoms in a given molecule. For example, oxygen (O2) has an atomicity of 2, ozone (O3) an atomicity of 3, benzene (C6H6) an atomicity of 12, etc.

• A Monoatomic molecule contains only one atom .

EX - Ar, He

• A Diatomic molecule contains only two atoms .

EX - H2, N2, O2, Br2, HCl, CO

EX - O3, H2O, NH3, CH4

• A Polyatomic molecule contains more than two

atoms .

IonCompounds compose of metals and non- metals contain charged species . The charged species are known as ions. An ion is an atom, or group of atoms, that has a net positive or negative charge.

Cation – Ion with a positive charge .If a neutral atom loses

one or more electrons it becomes a Cation.

Anion – Ion with a negative charge. If a neutral atom

gains one or more electrons it becomes an Anion.

Na+ 11 protons10 electrons

Cl 17 protons17 electrons Cl- 17 protons

18 electrons

Na 11 protons11 electrons

• A monatomic ion contains only one atom .

Ex - Na+, Cl-, Ca2+, O2-, Al3+

• A polyatomic ion contains more than one

atom . Ex - OH-, CN-, NH4+, NO3-