Worksheet Solutions: Atoms and Molecules

# Worksheet Solutions: Atoms and Molecules - Science Class 9

Q.1. The maximum number of electrons in L shell is
(a) 8
(b)
18
(c)
28
(d)
38

Q.2. Which of the following statements is correct?
(a) Cathode rays travel in straight line and have momentum.
(b) Cathode rays travel in straight line and have no momentum
(c) Cathode rays do not travel in straight line but have Momentum.
(d) Cathode rays do not travel in straight line and have no momentum.

Q.3. Oxygen is –
(a) Monovalent
(b) Bivalent
(c) Trivalent
(d) Tetravalent

Q.4. Molecular mass of water (H2O) is
(a) 18g
(b) 8g
(c) 33g
(d) 34g

Q.5. Atomicity of chlorine and Argon is
(a) Diatomic and Monoatomic
(b) Monoatomic and Diatomic
(c) Monoatomic and Monoatomic
(d) Diatomic and Diatomic

Q.6. Stat the properties of cathode rays?

Properties of cathode rays are:
(1) They travel in straight line
(2) They have momentum and energy
(3) They are deflected by electric and magnetic fields.

Q.7. Define the terms:
(a) Atomic number
(b) Mass number

Atomic Number is defined as the total number of protons present in an atom. Mass number is defined as the sum total of number of protons and the number of neutrons present in an atom.

Q.8. State the law of constant Proportion?

According to law of constant proportion, whatever the method of its formation, a chemical compound in its pure state will always contain the same elements combined together in the fixed ratio by mass.

Q.9. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

The postulate of Dalton’s atomic theory which explains the law of definite proportions is “Atoms combine in the ratio of small whole numbers to form compounds and the relative number and kinds of atoms are constant in a given compound.”

Q.10. Which element will be more reactive and why → the element whose atomic number is 10 or the one whose atomic number is 11?

Element with atomic number 11 is more reactive than the one with atomic number 10 because electronic configuration of atomic number 11 will be 2, 8, 1 so, it has to loose only 1e- from its outermost shall to be stable which is more easy than the element with atomic number 10 because its electronic configuration is 2, 8 and has 8e- in the outermost shell and hence is already stable.

Q.11. What is the mass of 0.2 mole of oxygen atoms?

0.2 mole of oxygen atoms
Mass of 1 mole of oxygen (O) atoms = 16 u
Mass of 0.2 mole of oxygen (O) atoms = 0.2 × 16 = 3.2 u

Q.12. The mass of an atom of element (X) is 2.0 × 10–23 g. Calculate its atomic mass.

One atom of element (X) has mass = 2.0 × 10–23 g.
6.022 × 1023 atoms of element (X) will have mass
= (2.0 × 10–23 g) × (6.022 × 1023)
= 12.044 ≈ 12.0 g

Q.13. Calculate the molar mass of Nitric acid.

Chemical formula of Nitric acid is (HNO3)
Molar mass of HNO3 = (1 × atomic mass of H) + (1 × atomic mass of N) + (3 × atomic mass of O)
= (1 × 1u) + (1 × 14u) + (3 × 16u) = 63u

Q.14. Explain why the number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas.

Hydrogen gas is diatomic in nature (H2) while helium gas is monoatomic (He). As a result, the number of atoms in one mole of hydrogen is (2 × NA) which is double as compared to number of atoms in one mole of helium (NA)

Q.15. If the valency of carbon is 4 and that of sulphur is 2, what is the chemical formula and name of the compound formed between carbon and sulphur atoms?

The chemical formula of compound can be written by exchanging the valencies (cross-over). Therefore, the expected formula is C2S4 or CS2. The compound is called carbon disulphide.

The document Worksheet Solutions: Atoms and Molecules | Science Class 9 is a part of the Class 9 Course Science Class 9.
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## Science Class 9

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## FAQs on Worksheet Solutions: Atoms and Molecules - Science Class 9

 1. What is an atom?
Ans. An atom is the smallest unit of matter that retains the chemical properties of an element. It consists of a nucleus, which contains protons and neutrons, surrounded by electrons.
 2. How are atoms and molecules related?
Ans. Atoms are the building blocks of molecules. Molecules are formed when two or more atoms chemically bond together. The atoms can be of the same element or different elements.
 3. What is the difference between an atom and a molecule?
Ans. An atom is a single particle of an element, while a molecule is a combination of two or more atoms. Atoms cannot exist independently in a stable form, whereas molecules are stable and can exist independently.
 4. How do atoms combine to form molecules?
Ans. Atoms combine to form molecules through chemical bonding. This can occur through various types of bonding, such as covalent bonding, where atoms share electrons, or ionic bonding, where atoms transfer electrons to form charged particles.
 5. Can atoms and molecules be seen with the naked eye?
Ans. No, atoms and molecules cannot be seen with the naked eye as they are extremely small. They are on the nanoscale, which is much smaller than what can be observed without the use of specialized tools like microscopes.

## Science Class 9

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