2. Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction

3. “In a chemical substance the elements are always present in definite proportions by mass”

4. 1. All matter is made of very 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 elements have different masses and chemical properties. 5. Atoms combine in the ratio of small whole numbers to form compounds. 6. The relative number and kinds of atoms are constant in a given compound.

5. 1. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3g of hydrogen gas? 2. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass? 3. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

6. Atomic radius is measured in nanometers 1\10 9 m=1 nm 1 m=10 9 nm

7. One atomic mass unit (amu) is a mass unit equals to exactly one twelfth the mass of one atom of Carbon -12.

8. A molecule is 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.

9. The molecules of an element are constituted by the same type of atoms. Example: Argon (Ar), Helium (He), Oxygen The number of atoms constituting a molecule is known as its atomicity. Example: The atomicity of oxygen is diatomic because the molecule of oxygen element has two oxygen atoms.

10. Atoms of different elements join together in definite proportions to form molecules of compounds Example: Water (H 2 O).The two atoms of element hydrogen and one atom of element oxygen combined together to form one molecule of compound water

11. 1.What is the atomicity of Helium? 2.A molecule of Nitrogen consists of two atoms of Nitrogen. What is its atomicity? 3.One molecule of water consists of how many atoms?