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(b) incorrect since it is not balanced

An atom or a group of atoms when they either lose or gain electrons, get converted into ions or radicals.

1. Valency with respect to Chlorine:

• Since valency of chlorine is one, the number of chlorine atoms with which one atom of an element can combine is called valency.
• In sodium chloride (NaCl) molecule, one chlorine atom combines with one sodium atom. So, the valency of sodium is one.
• But, in magnesium chloride (MgCl₂) valency of magnesium is two because it combines with two chlorine atoms.

2. Valency with respect to oxygen:

• Valency can be defined as double the number of oxygen atoms with which one atom of an element can combine because valency of oxygen is two. 
• For example, in magnesium oxide (MgO) valency of magnesium is two.

Law of constant proportions states that in a pure chemical compound the elements are always present in definite proportions by mass.

1. True

2. False

3. False

4.False

5. False

6. True

7. True

Reactant is the substance that take part in a chemical reaction.

(c) number of electrons

The law states that during any chemical change, the total mass of the products is equal to the total mass of the reactants.

Helium is zero.

The total number of protons present in an atom is equal to the total number of electrons, thus balancing the total charge of the atom.

• Monovalent ions – Li⁺ , Cs⁺
• Divalent ions – Ni²⁺, Cu²⁺ , Ba²⁺, Zn²⁺ , Cd²⁺ , Hg²⁺ , Pb²⁺ , Mn²⁺ , Fe²⁺ , CO²⁺ , Sr²⁺ 
• Trivalent ions – Fe³⁺ ,Cr³⁺, Al³⁺ 

1. Aluminium sulphate = Al₂(SO₄)₃

2. Silver nitrate = AgNO₃

3. Magnesium oxide = MgO 

4. Barium chloride = BaCl₂

Step 1: 

Write down the symbols of elements/ions side by side so that the positive radical is on the left and the negative radical is on the right hand side.

Step 2: 

Write the valencies of the two radicals above their symbols to the right in superscript (Signs ‘+’ and ‘-‘ of the ions are omitted).

Step 3:

Reduce the valencies to simplest ratio if needed. Otherwise interchange the valencies of the elements/ions. Write these numbers as subscripts. However, ‘1’ appearing on the superscript of the symbol is omitted

(a) Both assertion and reason are true and the reason is the correct explanation of the assertion

1. d 

2. c 

3. e 

4. a 

5. b

J J Thomson scientists observed that cathode rays consist of negatively charged particles.

(d) valance shell

1. Water obtained from different sources like a well, a pond, a river and underground water  will always consist of the same two elements hydrogen and oxygen in the ratio 1 : 8 by mass. 

2. It obeys the law of constant proportion.

1. Physical state of the reactants and the products.

2. Heat changes (heat liberated or heat absorbed) accompanying the chemical reaction. 

3. Conditions such as temperature, pressure, catalyst etc., under which the reaction takes place.

4. Concentration (dilute or concentrated) of the reactants and products. 

5. Speed of the reaction.

1. In naming a compound containing a metal and a non-metal, the name of the metal is written first and the name of the non-metal is written next after adding the suffix-ide to its name.

Examples:

NaCl – Sodium chloride

AgBr – Silver bromide

2. In naming a compound containing a metal, a non-metal and oxygen, name of the metal is written first and name of the non-metal with oxygen is written next after adding the suffix- ate (for more atoms of oxygen) or -ite (for less atoms of oxygen) to its name.

Examples:

Na₂SO₄ – Sodium sulphate

Na₂ – Sodium nitrite

3. In naming a compound containing two non – metals only, the prefix mono, di, tri, tetra, penta etc. is written before the name of non – metals. 

Examples: 

SO₂ – Sulphur dioxide

N₂O₅ – Dinitrogen pentoxide.

1. b 

2. a 

3. d 

4. c

1. Initially the number of times an element occurs on both sides of the skeleton equation should be counted.

2. An element which occurs least number of times in reactant and product side must be balanced first. Then, elements occuring two times, elements occuring three times and so on in an increasing order must be balanced.

3. When two or more elements occur same number of times, the metallic element is balanced first in preference to non – metallic element. If more than one metal or nonmetal is present then a metal or non-metal with higher atomic mass (refer periodic table to find the atomic mass) is balanced first.

4. The number of molecules of reactants and products are written as coefficient.

5. The formula should not be changed to make the elements equal.

6. Fractional method of balancing must be employed only for molecule of an element (O₂ , H₂ ,O₃ , P₄ ,…) not for compound (H₂O, NH₃ ,…).

(b) J.J. Thomson

It is because the small particles of the cathode rays (electrons) have mass and energy. This energy is used in rotating the paddle wheels.

(c) negatively charged particles

Wilhelm Ostwald introduced the term mole (Latin ‘moles’ means heap), in 1896 which was accepted as a unit to represent a large number of atoms in 1967. The Mole is the amount of a substance which contains as many particles as in 12g of C12. Thus, one mole of any species is that quantity in number which has a mass equal to its atomic or molecular mass in grams. For example, 1 mole of carbon isotope (atomic mass = 12) is equal to 12g. 1 mole of oxygen (O₂, molecular mass = 2 x 16) is equal to 32 g. 1 mole of water (H₂O, molecular mass = 2×1 + 1×16) is equal to 18 g

Advantages of Dalton’s Atomic Theory: 

1. Dalton’s theory explains most of the properties of gases and liquids. 

2. This explains the laws of chemical combination and the law of conservation of mass.

3. This theory helps to recognize the molecular differences of elements and compounds. 

Limitations of Dalton’s Atomic Theory: 

1. Atom is no longer considered as the smallest indivisible particle. 

2. Atoms of the same element have different masses (Isotopes). 

3. Atoms of the different elements may have same masses (Isobars). 

4. Substances made up of same kind of atoms may have different properties 

5. Example: Coal, Graphite and Diamond are made up of carbon atoms but they differ in their properties

1. Atom is no longer considered as the smallest indivisible particle. 

2. Substances made up of same kind of atoms may have different properties.

1. Dalton’s theory explains most of the properties of gases and liquids.

2. This explains the law of chemical combination and the law of conservation of mass.

Postulates of Dalton’s atomic theory:

1. All matter is made up of minute particles called atoms.

2. Atom is indivisible. They can neither be created nor destroyed.

3. All atoms of a given element are identical in mass and size.

4. The atoms of one element are different from the atoms of other elements.

5. During a chemical reaction, the atoms of an element combine with atoms of another element in a fixed ratio.

The nain postulates of Dalton’s atomic theory are: 

1. All the matters are made up of extremely small particles called atoms (Greek philosopher Democritus used the same name for the smallest indivisible particles). 

2. Atoms of the same element are identical in all respects (size, shape, mass and properties).

3. Atoms of different elements have different sizes and masses and possess different properties. 

4. Atoms can neither be created nor be destroyed, i.e., atom is indestructible. 

5. Atoms of different elements may combine with each other in a fixed simple ratio to form molecules or compounds. 

6. An atom is the smallest particle of matter that takes part in a chemical reaction

1. c 

2. d 

3. a 

4. e 

5. b

Protons and neutrons.

The answer is (B) Proton and Neutron

The answer is (B) 2, 8, 1

α-particles were bombarded by Rutherford on a very thin gold foil

An ionic compound is defined as the compound formed by the oppositely charged ions which attract each other to form a chemical bond e.g. Sodium chloride.

The answer is (D) Electron (- 1), Proton (+ 1) and Neutron (0)

The jumping of an electron from one energy level to another is called electronic transition.

When electricity passes through air, remove the electrons*from the gaseous atoms and produces ions. This is called electrical discharge.

If the atomic number of an atom is 17 and mass number is 35 then number of neutrons will be 18.

Nucleus consisting of neutrons and protons.

The number of valence electrons present in magnesium is two

The subatomic particle with negative charge is electron.

A discharge tube is a cylindrical glass tube in which a gas can be filled and which is fitted with metal plates called electrodes at each end.

It is the sum of protons and neutrons in the nucleus of an atom.

β-particles are actually a stream of electrons.

The outermost shell of an atom is known as valence electrons.

liquid sodium is used as a coolant in nuclear reactor.