Atoms and Molecules

Atoms and Molecules

Class 9

Main topics covered in this Lesson, are:

Laws of Chemical Combination

Dalton’s Atomic Theory

Atoms and their Symbols

Atomic Mass

Gram Atomic Mass

Ions Molecule Formula of Simple and Molecular Compounds

Molecular Mass Formula

Unit Mass Mole Concept

You'll be able to understand this all in a very easy manner!

So, let's begin...

Laws of chemical combination

• 1. Law of conservation of mass - This law states that mass can neither be created nor destroyed in a chemical reaction.

• 2. Law of constant proportions - This law states that 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 weight.

  This law is also called law of definite proportions or law of constant proportions.

Dalton’s Atomic Theory

Earlier, many philosophers stated that Parmanu cannot be divided further into smaller pieces. Later, According to Dalton’s atomic theory, it was proved that all matter, whether an element, a compound or a mixture is composed of small particles called atoms.

i.e., an atom is the smallest building block of matter.

For example: Sodium (Na), Hydrogen (H), Oxygen (O), etc.

Postulates of Dalton’s atomic theory:

Matter is made up of extremely small indivisible particles called atoms that can neither be created nor destroyed. Atoms of the same substance are identical in all aspects, i.e., they possess same size, shape, mass, chemical properties etc. Atoms of different substances are different in all aspects, i.e., they possess different size, shape, mass etc. Atom is the smallest particle that takes part in a chemical reaction. Atoms of different elements combine with each other in a simple whole number ratio to form compound. The relative number and kinds of atoms are constant in a given compound.

Try the following questions:

• Q. 3 g of carbon on burning in 8 g oxygen produces 11 g of carbon dioxide. What mass of carbon dioxide will be formed when 3 g of carbon is burnt in 50 g of oxygen?

• Q.. 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 3 g of hydrogen gas? Which law of combination will govern the answer?

• Q. Which postulate of Dalton’s atomic theory explains the law of definite proportions?

• Q. 12 g of magnesium powder was ignited in a container having 20 g of pure oxygen. After the reaction was over, it was found that 12 g of oxygen was left unreacted. Show that it is according to law of constant proportions.

  2Mg + O₂ → 2MgO

Names of Atoms or Elements and Their Symbols:

• IUPAC (International Union of Pure and Applied Chemistry) approves names of elements.

• The abbreviation used for lengthy names of elements are termed as their symbols.

• The symbol of an element is formed by writing only the first letter or first letter followed by the second or some other letter of English name or Latin name of the element. 

• While writing a symbol, the first letter is always capital and the second is always small.

Atomic Mass

Atomic mass of an element may be defined as the average relative mass of an atom of the element as compared with the mass of an atom of carbon (C-12 isotope) taken as 12 amu.

Gram Atomic Mass

The atomic mass of an element expressed in grams is known as gram atomic mass.

Molecules

A group of two or more than two atoms of the same or different elements that are chemically bonded together is called a molecule.

For example: Two atoms of hydrogen (H₂₎ and one atom of oxygen (O₂) react with each other and form one molecule of water.

Atomicity

The number of atoms present in a molecule of an element or a compound is known as its atomicity.

For example, atomicity of oxygen (O₂) is 2 while atomicity of ozone (O₃) is 3.

Molecules of Elements

The molecules of an element are constituted by the same type of atoms.

For example, a molecule of oxygen consists of two atoms of oxygen to form a diatomic molecule O₂.

Molecules of Compounds

Atoms of different elements join together in definite proportions to form molecules of compounds.

For example, a molecule of water consists of two atoms of hydrogen and one atom of oxygen to form a triatomic molecule H₂O.

Ion

It is an electrically charged atom or group of atoms. It is formed by the loss or gain of one or more electrons by an atom.

Ions are of two types:

• (i) Cation: It is positively charged ion and is formed by the loss of one or more electrons from an atom

  For example: sodium atom, loses one electron to form a sodium ion Na

  Na – e⁻ → Na⁺

• (ii) Anion: It is a negatively charged ion and is formed by the gain of one or more electrons by an atom.

  For example: a chlorine atom gains one electron to form a chloride ion Cl⁻.

  Cl + e⁻ → Cl⁻

  
  

Valency:

It is defined by the combining power (or capacity) of an element.

Depending on their Valency, elements can be classified as following:

• (i) Monovalent cation: Having cationic valency of 1.

  For example: Sodium ion (Na⁺ ). Potassium ion (K⁺ ), Hydrogen ion (H⁺ ).

  Monovalent anion: Having anionic valency of -1.

  For example: Chloride ion (Cl^-), Bromide ion (Br^-)

• (ii) Divalent cation: Having cationic valency of 2.

  For example: Magnesium ion (Mg²⁺), Ferrous ion (Fe²⁺).

  Divalent anion: Having anionic valency of −2.

  For example: Oxide ion (O²⁻), Sulphide ion (S²⁻).

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  (iii) Trivalent cations: Having cationic valency of 3.

  For example: Aluminium ion (Al³⁺), Ferric ion (Fe³⁺).

  Trivalent anion: Having anionic valency of -3.

  For example: Nitride ion (N^-3), Phosphate ion (PO₄³⁻) etc.

Formula of Simple and Molecular Compounds:

Steps to construct the chemical formula of a compound:

• (i) While writing the chemical formulae for compounds, write the constituent elements with their valencies written down the respective elements.

• ii) Then crossover the valencies of the combining atoms as shown in the following examples

• Steps to represent the chemical formula of a compound: 

  (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.

Formula unit mass

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

Formula unit mass is used for those substances whose constituent particles are ions.

For example, formula unit mass of ionic NaCl = 23 + 35.5 = 58.5 u.

Molecular Mass

The number of times a molecule of a compound is heavier than the 1/12 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, Molecular mass of H₂O= 2 × Mass of one H-atom + Mass of one O-atom = 2 ×1 + 16 = 18 u.

Mole Concept

Mole: A collection of 6.023 × 1023particles is named as one mole.

       1 mole = 6.023 × 10²³ particles = Mass of 1 mole particles in grams

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

       1 mole atoms = gram atomic mass

       1 mole molecules = gram molecular mass

Avogadro’s constant or Avogadro’s number

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

Try the following questions:

Q. Distinguish between atoms and molecules.

Q. Define the mole concept and molar mass.

Q. Represent the following molecules with the help of chemical formula:

(a) Aluminium chloride

(b) Calcium carbonate

(c) Copper nitrate

(d) Nitric acid

Q. Convert 20 g of water into moles.

Q. Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.

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Molecular Mass:

The number of times a molecule of a compound is heavier than the 1/12 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, Molecular mass of H₂O= 2 × Mass of one H-atom + Mass of one O-atom = 2 ×1 + 16 = 18 u.