Question

1- Explain daton’s atomic theory and modern atomic theory ?

2- Explain cathode ray tube experiment and gold foil experiment?

3- Name various scientists and their accomplishments ?

4- Define subatomic particles and state their location?

Answer 1

According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms. The postulates of this theory may be stated as follows:

• All matter is made up of very tiny particles. These particles are called atoms.
• An atom cannot be divided further i.e., atoms are indivisible.
• Atoms can neither be created nor destroyed in a chemical reaction.
• All atoms of an element are identical in all respects, including the mass, chemical properties, etc.
• Atoms of different elements have different masses and chemical properties.
• Atoms of different elements combine in small whole number ratios to form compounds.
• In a given compound, the relative number and types of atoms are constant.
•   

• The modern atomic theory is described as follows.

  i) All forms of matter are made upon of atoms which cannot be created or destroyed.

  ii) Atom is made upon mainly of electrons, protons and neutrons. Therefore, atom no longer remains indivisible. However, atom is still the smallest particle which takes part in the chemical reactions. During chemical reactions, atoms do not split into smaller particles.

  iii) Atoms of an element can have different masses. The atoms of an element which have similar chemical properties but differ in their masses are called isotopes of that element. For example , hydrogen have the following three isotopes

• Hydrogen (protium) Mass number 1

• Deuterium Mass number 2

• Tritium Mass number 3

• iv) Atoms of different elements may sometimes have equal mass numbers. The atoms of different elements having equal mass numbers are called isobars. For example, an atom of organ (Mass no. 40), and that of Potassium (Mass no 40), are isobars.

  v) Dalton’s compound atom is now called a molecule. A molecule is the smallest unit of a substance which can exist independent under ordinary conditions.

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