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CHAPTER 3–Atoms and Molecules GRADE 9 –Chemistry Laws of Chemical Combination: The process of combination of two or more elements to form new compounds is governed by certain laws called laws of chemical combination. These are:

1. Law of conservation of mass.

2. Law of constant proportions.

1. Law of conservation of mass (by Lavoisier in 1744): This law states that mass can neither be created nor destroyed in a chemical reaction.

2. Law of constant proportions (by Proust in 1797):

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 According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms

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.

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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.

Atoms: The smallest tiny a particle of matter which can't be divided further is called atom, i.e., an atom is the smallest building block of matter.

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

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 is 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.

Symbols used for some common elements are given below: Atomic mass: Atomic mass of an element is defined as the average mass of one atom of the element compared with 1/12th the mass of a carbon -12 atom.

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Molecule

A molecule is a collection of various atoms that combine chemically with each other.

These atoms are bound together by certain forces of attraction.

Atoms of the same elements or different elements can bind together to form molecules.

Therefore, a molecule is the smallest particle of a substance that can exist independently and shows all the properties of that substance.

Molecules of Elements

The molecules of an element are formed by combinations of similar types of atoms. For example, Helium (He) is made up of only one atom while oxygen is made up of two atoms.

Atomicity – the number of atoms in a molecule of an element is called its atomicity. For example, helium is monoatomic and oxygen is diatomic.

Element Atomic Mass

Hydrogen 1 µ

Carbon 12 µ

Nitrogen 14 µ

Oxygen 16 µ

Sodium 23 µ

Magnesium 24 µ

Sulphur 32 µ

Chlorine 35.5 µ

Calcium 40 µ

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Monoatomic – when an element comprises of a single atom. Example – all metals

Diatomic – when an element comprises of two atoms. Example – all gases

Triatomic – when an element comprises of three atoms Molecules of Compounds

Molecules of compounds constitute atoms of different elements that combine together in a fixed proportion. For example, water comprises of two atoms of hydrogen and one atom of oxygen.

Atomicity of some Elements

Name Atomicity Formula

Argon Monoatomic Ar

Helium Monoatomic He

Oxygen Diatomic O2

Hydrogen Diatomic H2

Nitrogen Diatomic N2

Chlorine Diatomic Cl2

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Ions

Compounds contain metals as well as non-metals. These elements include charged species which are known as ions.

Thus, ion is a particle that has a positive or negative charge.

Anion – negatively charged ion Cation – positively charged ion

There can be a single charged atom in an ion or there may be a group of charged atoms in an ion that have a net charge on the compound.

When a group of atoms carries a charge in a compound it is called as a polyatomic ion.

Chemical Formula

We use a chemical formula to represent the composition of a compound in the form of symbols. To write a chemical formula you must know two things –

1. Symbols of elements

2. Valency

Rules of writing a Chemical Formula

Valencies of on the ions must balance.

In a case where both metal and non-metal substances are present in a compound, the name of the metal is always written first in the chemical formula. For example, Sodium Chloride is written as NaCl

In case of polyatomic ions, the ion is written in brackets before writing the number of ions associated to it. In case of a single ion, there is no need to mention the ion in brackets

Writing the Formulae of Simple Compounds Binary compounds – compounds that consist of two different elements

Write the symbols of the corresponding elements of the compound as explained above

Write the valencies of the elements of the compound

Crossover the valencies of the elements

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Here are a few examples of writing the chemical formula

Molecular Mass and the Mole Concept

Molecular Mass – summation of all the atomic masses in a molecule

Molecular mass is expressed in atomic mass units (amu).

For example, the molecular mass of HNO3 can be calculated as:

Atomic mass of H =1u

Atomic mass of N =14u

Atomic mass of O =16u

Molecular mass of HNO3 = 1 + 14 + (16*3) = 63u

Formula Unit Mass

The sum of atomic masses of all atoms in a formula unit of a compound is called as its formula unit mass. The formula unit mass is used in case of substances that constitute ions. For example, formula unit mass of Sodium Chloride (NaCl) can be calculated as: (1*23) + (1*35.5) = 58.5u

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Mole Concept

2C + O2 = 2CO2

We say that two molecules of carbon combine with one molecule of oxygen to form two molecules of carbon dioxide.

We can also say that 24u of Carbon molecules combine with 32u of oxygen molecules to form 56u of carbon dioxide molecules.

Therefore, we can characterize the quantity of a substance by its mass or by its number of molecules.

A chemical equation directly indicates the number of molecules participating in the reaction. Thus, it is convenient for us to refer to the number of substances in a chemical reaction as numbers of molecules or atoms.

Mole

Mole is a numerical quantity that has a mass equal to the atomic or molecular mass of species (atoms, molecules, ions or particles).

1 mole of any substance = 6.022 X 1023 number of particles (atoms, ions or molecules)

This is called the Avogadro number or Avogadro Constant which is represented as N0

The mass of 1 mole of a substance is the same as that its atomic mass or molecular mass expressed in grams.

Gram atomic mass of a substance – the atomic mass of a substance when expressed in grams is known as its gram atomic mass. Gram molecular mass of a substance – the molecular mass of a substance when expressed in grams is known as its gram molecular mass.

For example, the atomic mass of Sulphur is 32u. Gram atomic mass of Sulphur is 32g.

Also, 32u of Sulphur has 1 atom of Sulphur. 32g of Sulphur has 1 mole atoms, that is, 6.022 X 1023 atoms of Sulphur.

Similarly, we can say that the gram molecular mass of Carbon Dioxide is 56g.

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But we know that in the case of chemical equation mole is the measuring unit.

Therefore, 1 mole = 6.022 × 1023 number = Relative mass in grams

Wilhelm Ostwald introduced the word ‘mole’ which actually means a heap or a pile. Therefore, we consider a substance as a heap of atoms or molecules.

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CHAPTER 3–Atoms and Molecules GRADE 9 –Chemistry The Structure of an Atom

Matter is made up of tiny particles called atoms. Atoms are further made of three fundamental particles or sub – atomic particles called electron, proton and neutron.

Earlier Dalton postulated that atom is indivisible i.e. cannot be further divided which proved to be wrong by discovery of sub – atomic particles.

Discovery of Sub- atomic Particles

(a) Electron (e-):

These negatively charged particles were discovered by J. J. Thomson in 1897. In the experiment, a gas at low pressure was taken in a discharge tube made up of glass. At the ends of the discharge tube two electrodes were placed, connected to a battery for high voltage supply. The electrode connected to the negative end was known as cathode and that to the positive as anode. He then observed a stream of negatively charged particles coming out of cathode towards the anode. These particles were called electrons.

The collection of negatively charged particles emitted from cathode of discharge tube is called cathode rays.

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(b) Proton (p+):

In 1886, Goldstein observed in the same experiment, with different situations that the anode emitted positive particles which were called proton.

Canal rays: The positively charged radiations produced in the discharge tube from anode are called Canal rays.

(c) Neutron (n):

These neutrally charged particles were discovered by James Chadwick in 1932. Neutrons are present in atoms of all elements except Hydrogen.

Thomson’s Model of an atom

It is also known as Thomson’s plum pudding model. In 1903 J. J. Thomson proposed the structure of an atom similar to that of a Christmas pudding.

He proposed:

An atom is a positively charged sphere and the electrons are embedded in it.

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The Magnitude of positive and negative charge is same inside atom, so net charge inside an atom is zero.

Rutherford’s “α- particle scattering experiment” Rutherford designed an experiment, He used: A lot of fast moving α - particles were bombarded on thin gold foil. After passing the foil the α – particles, hit the screen.

Following are the observations and inferences made:

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Rutherford on the basis of this experiment concluded The Nuclear model of an atom. According to Nuclear model of an atom:

1. Most of the space inside the atom is empty.

2. At the Centre of an atom, a small, heavy, positively charged nucleus is present.

3. Electrons revolve around the nucleus.

4. Total positive charge in nucleus is same as total negative charge on all electrons of atom as atom has net zero charge.

Drawbacks:

According to Rutherford model, electrons revolve around positive charged nucleus. According to the, Theory of Maxwell, if any charged particle does accelerated motion it must radiate energy. Therefore, if a charged body (e-) rotates around another charged body (nucleus), it will radiate energy. Due to energy loss through the radiations, speed of electrons will decrease and eventually it will fall into the nucleus. But such collapse does not occur and atoms were found to be quite stable.

Advantages

Rutherford after this experiment gets the credit of discovery of nucleus.

The protons and neutron, collectively known as ‘Nucleons’ were present in the nucleus.

It was found that almost all mass of an atom resides in nucleus.

Bohr’s Model of Atom

In 1913, Neil Bohr overcame the limitations of Rutherford model and proposed a model of atomic structure.

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Following are the postulates:

1. Electrons revolve around a centrally located heavy small and positively charged nucleus in certain discrete orbits.

2. While revolving in discrete orbits the electrons do not radiate energy.

3. These discrete orbits or shells are called energy levels. These orbitals or shells are represented by the letters K, L, M, N… or the numbers, n = 1, 2, 3, 4

Arrangement of electrons in an atom:

The arrangement of electrons in various shells (energy levels) of an atom of the element is known as Electronic configuration.

The Maximum number of electrons that could be put in a particular shell, i.e., energy levels, was given by Bohr and Bury. According to Bohr-Bury Scheme:

The maximum number of electrons that can be accommodated in any energy level is given by 2n2where n = 1, 2, 3, 4, …. (for K, L, M, N…..)

The maximum number of electrons in the outermost orbit will be 8 electrons even if it has capacity to accommodate more electrons.

Electrons, are not accommodated in a given shell. Unless earlier shells are filled, that is stepwise filling of shells is followed. Orbit The circular path around the nucleus is called orbit, energy level or shell. Energy level are represented by English alphabets: K, L, M, N, …. and so o,

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1st orbit is denoted by K 2nd orbit is denoted by L 3rd orbit is denoted by M, and so on.

Distribution of Electrons in a Orbit or Shell The distribution of electrons in an orbit can be obtained by using formulae 2n 2 where ‘n’ is number of that orbit. For example:

Number of electrons in K-shell i.e. in 1st orbit Here n = 1 Therefore, 2n2 = 2 × 12 = 2

Thus, maximum number of electrons in K-shell, i.e. 1st shell = 2 Number of electrons in L-shell, i.e. in 2ndorbit Here n = 2, therefore,

2n2 = 2 × 22 = 8 Thus, maximum number of electrons in L-shell = 8

In similar way maximum number of electrons in any shell can be calculated.

Valency

The electrons present in the outermost shell of an atom are known as the valence electrons.

The combining capacity of the atoms or their tendency to react and form molecules with atoms of the same or different elements is known as valency of the atom.

Atoms of elements, having a completely filled outermost shell show little chemical activity.

Their combining capacity or valency is zero.

For example, we know that the number of electrons in the outermost shell of hydrogen is 1, and in magnesium, it is 2.

Therefore the valency of hydrogen is 1 as it can easily lose 1 electron and become stable.

On the other hand, that of magnesium is 2 as it can lose 2 electrons easily and also attain stability.

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Atomic number (Z) Atomic number of an element is equal to the number of protons present in the nucleus of an atom.

Since an atom is electrically neutral, thus number of protons and number of electrons are equal, therefore:

Atomic number (z) = number of protons = number of electrons.

Mass Number (A) The total number of the protons and neutrons present in the nucleus of an atom is called mass number. It is denoted by A. The protons and neutrons together are called nucleon.

The number of neutrons present in the nucleus of an atom is reperesented by N.

Isotopes Atoms of the same element having same atomic number but different mass numbers are known as Isotopes.

For example:

Applications of Isotopes Isotope of uranium is used as a fuel in nuclear reactor Isotope of cobalt is used in treatment of cancer Isotope of iodine is used in treatment of goitre.

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Isobars Atoms of different elements having same mass numbers are known as Isobars,

For example:

Potassium, K and Argon. Ar both have the mass numbers equal to 40.