CBSE NOTES CLASS 9 SCIENCE CHATER 3

ATOMS AND MOLECULES

CBSE NOTES CLASS 9 SCIENCE CHATER 3

ATOMS AND MOLECULES

1. Law of conservation of mass:

This law was stated by Lavoisier in 1789. It states that

“In all physical and chemical changes, the total mass of reactants is equal to the total mass of products.”

Or

Mass can neither be created nor be destroyed in a chemical reaction.

2. Law of constant proportions (or constant composition):

This law was first stated by Proust in 1797. According to the law

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

For example the ratio of hydrogen and oxygen in pure water is always 1:8 by mass.

This law is also called law of definite proportions.

3. Dalton’s Atomic Theory : Postulates of Dalton Atomic Theory

(i) All matter is made of very tiny particles called atoms.

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

(iii) Atoms of a given element are identical in mass and chemical properties.

(iv) Atoms of different elements have different masses and chemical properties.

(v) Atoms combine in the ratio of small whole numbers to form compounds.

(vi) The relative number and kinds of atoms are constant in a given compound.

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. It cannot be seen by naked eye.

Atomic radius is measured in anometers.

1/109 m =10-9 m = 1 nm & 1 m = 109 nm

 Relative Sizes Radii (in m) Example 10–10 Atom of hydrogen 10–9 Molecule of water 10–8 Molecule of haemoglobin

5. Names of elements: Names of elements in the past were derived from the name of the place where they were found for the first time. For example, the name copper was taken from Cyprus.

Some names were taken from specific colours. For example, the name of gold was taken from the English word meaning yellow.

Sometimes the names are derived from the names of their discoverers, e.g., Rutherfordium.

Now-a-days, IUPAC (International Union of Pure and Applied Chemistry) approves names of elements.

6. Symbols of Elements: The abbreviations 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 from English name or Latin name of the element. While writing a symbol, the first letter is always capital and the second is always small.

For example

(i) Hydrogen, H

(ii) Aluminum, Al and not AL

(iii) Cobalt, Co and not CO

Symbols for some elements

 Element Symbol Element Symbol Aluminium Al Iodine I Argon Ar Iron Fe Barium Ba Lead Pb Boron B Magnesium Mg Bromine Br Neon Ne Calcium Ca Nitrogen N Carbon C Oxygen O Chlorine Cl Potassium K Cobalt Co Silicon Si Copper Cu Silver Ag Chromium Cr Sodium Na Fluorine F Sulphur S Gold Au Uranium U Hydrogen H Zinc Zn

7. Atomic Mass Unit (amu or u) - One atomic mass unit is a mass equal to exactly one-twelfth ($\frac{1}{12}$th) the mass of one atom of carbon-12.

8. Atomic mass of an element: The atomic mass of an element is the number which indicates how many times an atom of an element is heavier than $\frac{1}{12}$th of mass of an atom of carbon-12.

Atomic masses of a few elements

 Name of Element Symbol Atomic Number Atomic mass Hydrogen H 1 1 Helium He 2 4 Lithium Li 3 7 Beryllium Be 4 9 Boron B 5 11 Carbon C 6 12 Nitrogen N 7 14 Oxygen O 8 16 Fluorine F 9 19 Neon Ne 10 20 Sodium Na 11 23 Magnesium Mg 12 24 Aluminium Al 13 27 Silicon Si 14 28 Phosphorus P 15 31 Sulphur S 16 32 Chlorine Cl 17 35.5 Argon Ar 18 40 Potassium K 19 39 Calcium Ca 20 40

8. Molecule: It is the smallest particle of an element or compound that is capable of independent existence and shows all the properties of that substance.

[The molecule of an element is made up of same type of atoms, while the molecule of a compound is made up of different types of atoms]

9. 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.

Atomicity of some elements

Carbon dinoxide

 Type of Element Name Atomicity Non-Metal Argon Monoatomic Helium Monoatomic Oxygen Diatomic Hydrogen Diatomic Nitrogen Diatomic Chlorine Diatomic Ozone Triatomic Phosphorus Tetra-atomic Sulphur Poly-atomic Metals Sodium Monoatomic Iron Monoatomic Aluminium Monoatomic Copper Monoatomic Compounds Hydrochloric Acid Diatomic Carbon monoxide Diatomic Triatomic

Sulphuric Acid

Poly-atomic

10. Molecules of Compounds: Atoms of different elements join together in definite proportions to form molecules of compounds.For example we can tabulate the diffrent atoms in water as follows,

 Element Ratio by mass Atomic mass (u) Mass ratio /atomic mass Simplest ratio H 1 1 $\frac{1}{1}$ 2 O 8 16 $\frac{8}{16}=\frac{1}{2}$ 1

11. Ion: It is an electrically charged atom or group of atoms. It is formed by the loss or gain of electrons by an atom. Ions are of two types :

(i) Cation: It is positively charged ion and is formed by the loss of electron from an atom e.g. H+, Na+, Ca2+, Al3+, NH4+ etc.

(ii) Anion: It is negatively charged ion and is formed by the gain of electrons by an atom, e.g. Cl-, O2-, F-, CO32-, PO43- etc.

A compound made by combination of ions is called ionic compound.

Some common, simple and polyatomic ions

 Val Metalic ion Symbol Non-metallic ions Symbol Polyatomic ions Symbol 1. Sodium Na+ Hydrogen H+ Ammonium NH4+ Potassium K+ Hydride H- Hydroxide OH– Silver Ag+ Chloride Cl- Nitrate NO3– Copper (I)* Cu+ Bromide Br- Hydrogen carbonate HCO3– Iodide I– Nitrite NO2- 2. Magnesium Mg2+ Oxide O2- Carbonate CO32– Calcium Ca2+ Sulphide S2- Sulphite SO32– Zinc Zn2+ Sulphate SO42– Iron(II)* Fe2+ Copper(II)* Cu2+ 3. Aluminium Al3+ Nitride N3- Phosphate PO43– Iron(III)* Fe3+

13. Valency: The combining power (or capacity) of an element or ion is known as its valency.

14. Formulae of simple compounds: Binary compounds are those compounds which are made up of two different elements e.g. NaCl, KBr, and CaO etc.

Following rules are to be followed for writing the formula.

(i) The valencies or charges on the ions must be balanced.

(ii) For a compound made up of a metal and a non-metal, the symbol of metal is written first.

(iii) In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.

15. Gram atomic mass: The atomic mass of an element expressed in grams is known as gram atomic mass. (Gram atomic mass is also known as gram atomic weight).

16. Molecular mass: The number of times a molecule of a compound is heavier than $\frac{1}{12}$th of the mass of C-12 atom, is known as its molecular mass.

The molecular mass is equal to the sum of the atomic masses of all atoms present in one molecule of the substance. For example H2S contains two atoms of hydrogen and one atom of S, so molecular mass of H2S is 2×1 + 32 = 34.

Gram Molecular mass is the molecular mass expressed in grams.

17. Formula unit mass: It is equal to the sum of atomic masses of all the atoms in a formula unit of ionic compounds. Formula unit mass of NaCl is 23 + 35.5 = 58.5.

18. Mole Concept: One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams. Irrespective of the substance under consideration 1 mole is equal to 6.022 × 1023 a specie.

The mass of 1 mole of particles (or simply is equal to its mass in grams.

1 mole = 6.023 × 1023 particles

1 mole atoms = 6.023 × 1023 atoms

1 mole electrons = 6.023 × 1023 electrons

1 mole protons = 6.023 × 1023 protons

1 mole ions = 6.023 ×1023 ions

1 mole molecules = 6.023 × 1023 molecules

19. Avogadro’s constant or Avogadro’s number: The number of particles present in one mole (i.e. 6.023 × 1023) is called Avogadro’s number or Avogadro’s constant.

20. Mole

1 mole = 6.023 × 1023 particles.

= mass of 1 mole particles in grams

= 22.4L of a gas at N.t.p.

1 mole atoms = gram atomic mass

1 mole molecules = gram molecular mass

N.t.p. stands for normal temperature (0°C) and normal pressure (1 atmosphere or 760 mm of Hg)

21. Formulae for mole concept :

Number of moles =

Number of moles =

Number of moles =

Number of atoms (or molecules or ions)