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Revision notes
- In ancient India, several Indian philosophers tried to give the idea about the divisibility of matter.
- Greek philosophers Democritus gave the concept of 'atom' i.e. something which is indivisible.
- By the end of the 18th century, scientists recognized the difference between elements and compounds.
- Antoine L. Lavoisier laid the foundation of chemical sciences by giving two important laws of chemical combination. The laws were (1) Law of conservation mass and (2) Law of constant proportion.
Laws of chemical combination
- This law states that (in a chemical reaction) mass can neither be created nor destroyed.
- Scientist Proust stated this law as "In a chemical substance the elements are always present in definite proportions by mass".
- it was John Dalton who in his atomic theory gave the explanation for the law of conservation of mass and law of definite proportions.
Atom
- Everything in this world is made up of very tiny particles known as atoms.
- An atom can be defined as 'The smallest particle of an element that can participate in a chemical
- reaction'.
- The world and everything around us is made up of elements and elements are further made up of atoms.
- An element is represented by its symbol. For example, hydrogen (H), silicon (Si), etc.
Atomic Mass
- Dalton in his atomic theory gave the concept of atomic mass i.e. mass of an atom.
- Atomic mass is decided to take the mass of carbon-12 atom as the base. Carbon-12 atom is that the atom of carbon has 6 protons and 6 neutrons in its nucleus so that its mass number is 12.
- Mass of atom is measured in the atomic mass unit 'amu' or simply 'a'.
- 1 amu or 1 u is defined as exactly one-twelfth (1/12th) mass of an atom of carbon-12.
- Generally, atoms do not exist individually. They exist in the form of ions and molecules.
Molecule
- A molecule is a group of two or more atoms that are chemically bonded together.
- Atoms of the same element or even different elements can join together to form molecules.
- The molecule of an element will be formed of the same type of atoms.
- For example, molecules of oxygen will be formed only from oxygen atoms.
- Atoms of different elements join together in definite proportions to form molecules of compounds.
- For example, water (H₂O), a compound, is made up of atoms of two different elements hydrogen (H) and oxygen (O).
Ion
- An atom or a molecule that has gained or lost one or more of its valence electrons, giving it a net positive or a net negative charge is known as ion.
Chemical formulae:
- A chemical formula represents the composition of a molecule of the substance but in terms of the symbols of the elements present in the molecule.
- It is also known as molecular formulae.
Molecular mass and mole concept:
- The sum of atomic masses of all the atoms in a molecule of a substance is called the molecular mass of that substance.
- One mole of atoms, molecules, ions, or particles is the quantity in number having a mass equal to its atomic or molecular mass in grams.
- The number of particles (atoms, molecules, or ions) present in 1 mole of any substance is equal to 6.022 x 10^23. This number is called the Avogadro constant.
Other Important Points
1. A compound is composed of two or more elements chemically combined in definite proportions.
2. During chemical changes, the sum of masses of all the reactants remains equal to the mass of products. This is known as the Law of conservation of mass.
3. In a compound, the elements are always present in a definite proportion of mass. This is known as the Law of definite proportions.
4. According to Dalton's atomic theory, all matter whether element or compound is composed of small particles called atoms or molecules.
5. Each element has a name and a unique chemical symbol.
| Symbols of Elements |
6. An atom is the smallest particle of an element and retains its identity throughout all physical or chemical changes.
7. A molecule is a group of two or more atoms that are chemically bonded together and form the smallest part of an element or a compound. This can exist independently and shows all the properties of that substance.
8. ion is a charged particle and can be positively or negatively charged.
9. The combining capacity of an element is known as its valency.
10. The chemical formula of a molecular compound is determined by the valency of the combining elements.
11. In ionic compounds, the charge on positive ions and negative ions is taken into account so that the overall chemical formula gives a neutral compound.
12. To bring uniformity, relative masses of all atoms are given in atomic mass unit (u) taking the mass of one atom of a particular isotope of carbon, i.e., (C-12) as 1/12 u.
13. The number of atoms present in 12 g of carbon -12 is 6.023 x 10²³ which is called the Avogadro number or Avogadro constant.
14. A mole denotes 6.023 x 10²³ atoms, molecules, or ions.
15. Mass of 1 mole of a substance expressed in grams is called its molar mass.
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