3. Atoms and Molecules | Class 9 Science

Chapter 3: ATOMS AND MOLECULES

The idea of matter's divisibility was developed in India around 500 BC.

Maharishi Kanad postulated that dividing matter would yield the smallest particles. He called them Parmanu.

Pakudha Katyayama, elaborated this doctrine and said that these particles exist in a combined form which gives various matters.

Similarly, Greek philosophers Democritus & Leucippus suggested that matter could be divided until it ultimately gives indivisible particles. Democritus called them atoms (meaning indivisible).

These ideas were philosophical, with little experimental evidence.

By the end of the 18^th century, scientists distinguished between elements and compounds.

Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.

LAWS OF CHEMICAL COMBINATION

Proposed by Lavoisier and Joseph L. Proust.

LAW OF CONSERVATION OF MASS

It states that mass can neither be created nor destroyed in a chemical reaction.

Activity

• Take one of the following sets, X and Y of chemicals:

X	Y
(i) copper sulphate	sodium carbonate
(ii) barium chloride	sodium sulphate
(iii) lead nitrate	sodium chloride

• Prepare separately a 5% solution of one pair of substances (X and Y) each in 10 mL of water.

• Take some solution of Y in a conical flask and some solution of X in an ignition tube.

• Hang the ignition tube in the flask carefully; ensuring the solutions do not mix. Put a cork on the flask.

Ignition tube containing solution of X, dipped in a conical flask containing solution of Y

• Weigh the flask with its contents.

• Now tilt and swirl the flask to mix the solutions X and Y. A chemical reaction occurs in the reaction flask.

• The cork prevents the escape of gases and ensures accurate mass measurement.

• Weigh the flask again. It shows that mass of the flask and its contents do not change.

LAW OF CONSTANT PROPORTIONS

- Lavoisier and others noted that many compounds are made of elements in fixed proportions. E.g.,

· In water, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8, whatever the source of water. Decomposition of 9 g water yields 1 g hydrogen and 8 g oxygen.

· In ammonia, nitrogen and hydrogen are always in the ratio 14:3 by mass, whatever the method or the source from which it is obtained.

- This observation led to the Law of constant proportions (Law of definite proportions) stated by Proust:

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

- John Dalton proposed Atomic Theory (1808) based on the laws of chemical combination. It provided an explanation for the Law of conservation of mass and the Law of definite proportions.

- He said that the smallest particles of matter are atoms.

Postulates of Dalton’s atomic theory:

i. All matter is made of very tiny particles called atoms, which participate in chemical reactions.

ii. Atoms are indivisible 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.

WHAT IS AN ATOM?

- The building blocks of all matter are atoms.

- Atoms are very small. Even if millions of atoms were stacked, it would be only the thickness of a sheet of paper.

Atomic radius is measured in nanometres.

1/10⁹ m = 1 nm

1 m = 10⁹nm

Relative Sizes
Radii (in m)	Example
10^–10	Hydrogen atom
10^–9	Water molecule
10^–8	Haemoglobin molecule
10^–4	Grain of sand
10^–3	Ant
10^–1	Apple

- It is possible to produce magnified images of surfaces of elements showing atoms.

An image of the surface of silicon

WHAT ARE THE MODERN-DAY SYMBOLS OF ATOMS OF DIFFERENT ELEMENTS?

- Dalton was the first scientist to use symbols for elements with specific meaning, representing one atom of that element. Some symbols are given below:

- Berzelius suggested using one or two letters from the element's name as its symbol.

- Initially, element names were derived from locations where they were first discovered (e.g., copper from Cyprus) or from colours (e.g., gold from the English word for yellow).

- International Union of Pure and Applied Chemistry (IUPAC) approves element names, symbols, and units. Symbols often use the first one or two letters of the element’s name in English.

- The first letter of a symbol is always written as a capital letter and the second letter as a small letter. E.g., hydrogen - H, aluminium - Al, cobalt - Co.

- Some symbols are formed from first letter and a letter appearing later in the name. E.g., chlorine - Cl, zinc - Zn.

- Other symbols are derived from Latin, German, or Greek names (e.g., iron - Fe from Latin ferrum, sodium - Na from natrium, potassium - K from kalium).

Symbols for some elements

Element

Symbol

Element

Symbol

Element

Symbol

Aluminium

Argon

Barium

Boron

Bromine

Calcium

Carbon

Chlorine

Cobalt

Copper

Fluorine

Gold

Hydrogen

Iodine

Iron

Lead

Magnesium

Neon

Nitrogen

Oxygen

Potassium

Silicon

Silver

Sodium

Sulphur

Uranium

Zinc

ATOMIC MASS

- The concept of atomic mass in Dalton's atomic theory was remarkable in explaining law of constant proportions. It prompted scientists to measure atomic masses.

- Measuring the mass of an individual atom is difficult, so relative atomic masses are determined using the laws of chemical combinations and the compounds formed.

- E.g., carbon monoxide (CO) is formed by combining 3 g of carbon with 4 g of oxygen. i.e., carbon combines with 4/3 times its mass of oxygen.

- Suppose the atomic mass unit (amu or unified mass unit, u according to IUPAC) is defined as the mass of one carbon atom, giving carbon an atomic mass of 1.0 u and oxygen 1.33 u. However, it is more convenient to express these values as whole numbers or near-whole numbers. To simplify, scientists initially used 1/16 of the mass of an oxygen atom as the atomic mass unit due to 2 reasons:

· Oxygen reacts with a large number of elements.

· It gives whole-number masses for most elements.

- In 1961, carbon-12 isotope was chosen as the standard reference for measuring atomic masses. It defines one atomic mass unit as a mass unit equal to exactly 1/12^th the mass of one atom of carbon-12.

- Now relative atomic masses of all elements are based on this standard. It can be understood by an analogy:

A fruit seller says, “a watermelon has a mass equal to 12

units” (12 watermelon units or 12 fruit mass units). He cuts it into 12 equal pieces and finds the mass of other fruits, relative to the mass of one piece. i.e., he sells fruits by relative fruit mass unit (fmu).

(a) Watermelon (b) 12 pieces

(c) 1/12 of watermelon, how the fruit seller can weigh the fruits using pieces of watermelon

- Similarly, the relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12^th the mass of one carbon-12 atom.

Atomic masses of a few elements
Element	Atomic Mass (u)
Hydrogen	1
Carbon	12
Nitrogen	14
Oxygen	16
Sodium	23
Magnesium	24
Sulphur	32
Chlorine	35.5
Calcium	40

HOW DO ATOMS EXIST?

- Atoms of most elements cannot exist independently.

- Atoms form molecules and ions. They aggregate in large numbers to form the matter.

WHAT IS A MOLECULE?

- A molecule is a group of two or more atoms chemically bonded together by attractive forces.

- It is defined as the smallest particle of an element or a compound that can exist independently and shows all the properties of that substance.

- Molecules can consist of atoms of the same element or different elements.

MOLECULES OF ELEMENTS

- Molecules of an element are made up of the same type of atoms.

- Some elements, like argon (Ar) and helium (He), are made up of only one atom. Molecules of most of the nonmetals have more than one atom. E.g.,

· Oxygen (O₂) is a diatomic molecule, consisting of two oxygen atoms.

· Ozone (O₃) is a molecule with three oxygen atoms.

- Number of atoms in a molecule is called its atomicity.

- Metals & elements like carbon consist of a large, indefinite number of atoms bonded together.

Atomicity of some elements
Type of Element	Name	Atomicity
Non-Metal	Argon	Monoatomic
	Helium	Monoatomic
	Oxygen	Diatomic
	Hydrogen	Diatomic
	Nitrogen	Diatomic
	Chlorine	Diatomic
	Phosphorus	Tetra-atomic
	Sulphur	Poly-atomic

MOLECULES OF COMPOUNDS

- Atoms of different elements join together in definite proportions to form molecules of compounds. E.g.,

Compound	Combining Elements	Ratio by Mass
Water (H₂O)	Hydrogen, Oxygen	1:8
Ammonia (NH₃)	Nitrogen, Hydrogen	14:3
Carbon dioxide (CO₂)	Carbon, Oxygen	3:8

The ratio by number of atoms for a water molecule is H:O = 2:1. It can be found as follows:

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

WHAT IS AN ION?

- Compounds composed of metals and nonmetals contain charged species called ions.

- Ions may consist of a single charged atom or a group of atoms that have a net charge on them.

- A negatively charged ion is called an anion and the positively charged ion, a cation. E.g., In NaCl, sodium ions (Na⁺) are cations and chloride ions (Cl^–) are anions.

- A group of atoms carrying a charge is known as a polyatomic ion.

Some ionic compounds
Ionic Compound	Constituting Elements	Ratio by Mass
Calcium oxide	Ca and O	5:2
Magnesium sulphide	Mg and S	3:4
Sodium chloride	Na and Cl	23:35.5

WRITING CHEMICAL FORMULAE

- The chemical formula of a compound is a symbolic representation of its composition.

- The combining power (or capacity) of an element is known as its valency. It is used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound.

- Valency can be imagined as the "arms" of an atom.

- E.g., if an octopus (O) with eight arms were to hold four humans (H), each with two arms, all arms would be locked. This can be represented as the formula OH₄. The subscript 4 indicates the number of humans held by the octopus.

Names and symbols of some ions
Valency	Name of ion	Symbol	Non- metallic element	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^–
2	Magnesium	Mg²⁺	Oxide	O^2-	Carbonate	CO₃^2–
	Calcium	Ca²⁺	Sulphide	S^2-	Sulphite	SO₃^2–
	Zinc	Zn²⁺	Sulphate	SO₄^2–
	Iron (II)*	Fe²⁺
	Copper (II)*	Cu²⁺
3	Aluminium	Al³⁺	Nitride	N^3-	Phosphate	PO₄^3–
3	Iron (III)*	Fe³⁺

* Some elements show more than one valency. A Roman numeral shows their valency in a bracket.

Rules of writing a chemical formula:

• The valencies or charges on the ion must balance.

• For compounds consisting of a metal and a non-metal, the metal's name or symbol is written first. E.g., calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO), etc.

• In compounds formed with polyatomic ions, the number of ions is indicated by enclosing the formula of ion in a bracket and number of ions outside the bracket. E.g., Mg (OH)₂. If only one polyatomic ion is present, no bracket is needed. E.g., NaOH.

FORMULAE OF SIMPLE COMPOUNDS

- The simplest compounds, which are made up of two different elements are called binary compounds.

- Method of writing the chemical formulae for some compounds is given below:

Formula of hydrogen chloride: HCl

Formula of hydrogen sulphide: H2S

Formula of carbon tetrachloride: CCl4

Formula of magnesium chloride: MgCl2

The positive and negative charges balance each other and the overall structure is neutral.

Formula for aluminium oxide: Al₂O₃

Formula for calcium oxide: CaO

Here, the valencies of the two elements are the same. Instead of Ca₂O₂, it is simplified as CaO.

Formula of sodium nitrate: NaNO₃

Formula of calcium hydroxide: Ca(OH)₂

Formula of calcium hydroxide is Ca(OH)₂ and not CaOH₂. It is due to the presence of more two polyatomic ions (OH). It indicates that there are two atoms each of oxygen and hydrogen in calcium hydroxide.

Formula of sodium carbonate: Na₂CO₃

Formula of ammonium sulphate: (NH₄)₂SO₄

MOLECULAR MASS

- It is the sum of the atomic masses of all the atoms in a molecule of the substance.

- It is expressed as the relative mass of a molecule in atomic mass units (u).

Example

(a) Calculate the relative molecular mass of water (H₂O).

(b) Calculate the molecular mass of HNO₃.

Solution:

(a) Atomic mass of hydrogen = 1u, oxygen = 16 u

So the molecular mass of water, which contains two atoms of hydrogen and one atom of oxygen is

= 2 × 1+ 1×16 = 18 u

(b) The molecular mass of HNO₃ = the atomic mass of H + the atomic mass of N+ 3 × the atomic mass of O

= 1 + 14 + 48 = 63 u

FORMULA UNIT MASS

- It is the sum of the atomic masses of all atoms in a formula unit of a compound.

- It is calculated similarly to molecular mass. The difference is that it is used for compounds with ions as constituent particles. E.g.,

· Formula unit mass of sodium chloride (NaCl):

1 × 23 + 1 × 35.5 = 58.5 u

· Calculate the formula unit mass of CaCl₂:

Atomic mass of Ca + (2 × atomic mass of Cl)

= 40 + 2 × 35.5 = 40 + 71 = 111 u.

👉 View PDF

👉 Class 9 All chapters

3. Atoms and Molecules | Class 9 Science | PDF and Web notes