KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

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Students can download Class 8 Science Chapter 4 Atoms and Molecules Important Questions, KSEEB Class 8 Science Important Questions and Answers helps you to revise the complete Karnataka State Board Syllabus and to clear all their doubts, score well in final exams.

Karnataka State Syllabus Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 1.
Name the unit that is commonly used to express the mass of an atom or a molecule.
Answer:
The international unit commonly used to express the mass of an atom or molecule is called atomic mass unit. It is abbreviated as ‘amu’.

Question 2.
Which atom was chosen as standard for developing ‘atomic mass unif?
Answer:
The International Committee of Chemists chose carbon-12 atom as standard for developing ‘atomic mass unit’.

Question 3.
Define atomic mass.
Answer:
Atomic mass can be defined as the total mass of its nucleons (protons and neutrons together are called nucleons) present in it.

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 4.
What is the atomic mass of carbon?
Answer:
The atomic mass of carbon is 12.01 u.

Question 5.
What is the atomic mass of oxygen? (Given: Mass of proton is 1.0078 and mass of neutron is 1.0087)
Answer:
An atom of oxygen has 8 protons and 8 neutrons in its nucleus. The atomic mass is the total mass of all the nucleons present in the atom. Therefore,
Atomic mass of oxygen = 8 × 1.0078 + 8 × 1.0087 = 8.0624 + 8.0696 = 16.132 amu.

Question 6.
Define relative atomic mass of an element.
Answer:
The relative atomic mass of an element is defined as the ratio between mass of one atom of an element and one-twelfth the mass of carbon-12 atom.
KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules 1

Question 7.
What does the relative atomic mass of an element express?
Answer:
Relative atomic mass of an element shows how many times an atom of an element is heavier than one-twelfth the mass of carbon-12 atom.

Question 8.
The relative atomic mass of magnesium is 2431. What is the meaning of this statement?
Answer:
The relative atomic mass of magnesium is 24.31. This means that one atom of magnesium is 24.31 times heavier than one-twelfth the mass of carbon-12 atom.

Question 9.
Give symbols and relative atomic masses of aluminium, calcium, carbon, chlorine, hydrogen, magnesium, nitrogen, oxygen, phosphorus and silver.
Answer:
KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules 2

Question 10.
What is molecular mass?
Answer:
Molecular mass is the sum-total of atomic masses of the elements present in a molecule.

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 11.
Define relative molecular mass of a substance.
Answer:
The relative molecular mass of a substance is defined as the ratio of the mass of one molecule of the substance to one-twelfth the mass of carbon-12 atom. The relative molecular mass of a substance is also known as molecular weight of that substance.

Question 12.
Can we use ‘relative molecular mass’for chemical elements or chemical compounds?
Answer:
Both elements and compounds exist in molecular state. Therefore, ‘relative molecular mass’ can be used for both chemical elements and chemical compounds.

Question 13.
What does the ‘relative molecular mass of a substance’ actually state?
Answer:
The relative molecular mass of a substance is actually a number that states how many times one molecule of the substance is heavier than one-twelfth the mass of carbon-12 atom.
KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules 3

Question 14.
How do you calculate the relative molecular mass of a substance?
Answer:
The relative molecular mass of a substance is a number which is equal to the sum of the atomic masses of the atoms present in one molecule of the given substance. Therefore, the molecular mass of a substance is calculated by adding the atomic masses of all the atoms present in one molecule of the substance.

Question 15.
The atomic mass of carbon is 12 amu and that of oxygen is 15.99 amu. Calculate the relative molecular mass of carbon dioxide.
Answer:
The molecular formula of carbon dioxide is CO2. This means, a molecule of carbon dioxide contains one atom of carbon and two atoms of oxygen. The relative molecular mass of carbon dioxide is equal to sum of the atomic masses of one atom of carbon and two atoms of oxygen.
Relative molecular mass of CO2 = Mass of 1 carbon atom + Mass of two atoms of oxygen
= 12 + (2 × 15.99)
= 12 + 31.98 = 43.98.

Question 16.
The atomic mass of hydrogen is 1.008 amu and that of oxygen 15.99 amu. Find the relative molecular mass of water.
Answer:
The molecular formula of water is H2O. This means, a molecule of water contains two atoms of hydrogen and one atom of oxygen. The relative molecular mass of water, is equal to sum of the relative atomic masses of two atoms of hydrogen and one atom of oxygen.
Relative molecular mass of H2O = Relative mass of (2 hydrogen atoms + one atom of oxygen)
= (2 × 1.008) + 15.99 = 2.016 + 15.99 = 18.006.

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 17.
Define ‘gram atomic mass’ of an element.
Answer:
Gram atomic mass of an element is its relative atomic mass expressed in gram.

Question 18.
The relative atomic mass of phosphorus is 3037. What is its gram atomic mass? How much phosphorus is present in 1 gram atomic mass of phosphorus?
Answer:
Gram atomic mass of phosphorus is its relative atomic mass expressed in gram. The relative atomic mass of phosphorus is 30.97. One gram atomic mass of phosphorus is 30.97 g of phosphorus. Therefore, 1 gram atomic mass of phosphorus contains 30.97 g of phosphorus.

Question 19.
How much oxygen is present in 1 gram atomic mass of oxygen?
Answer:
The atomic mass of oxygen is 15.99 amu. One gram atomic mass of oxygen means 15.99 g of oxygen. Therefore, 1 gram atomic mass of oxygen contains 15.99 g of oxygen.

Question 20.
Define ‘Gram molecular mass’ of a substance.
Answer:
One gram molecular mass of a substance (element or compound) is defined as its relative molecular mass expressed in gram.

Question 21.
How much mass of water is present in 1 gram molecular mass of water?
Answer:
The relative molecular mass of water is 18. One gram molecular mass of water means 18 g of water. Thus, 1 gram molecular mass of water contains 18 g of water.

Question 22.
The formula of sulphuric acid is H2SO4. The relative atomic mass of hydrogen is 1, relative atomic mass of sulphur is 32 and that of oxygen is 16. How many gram of sulphuric acid is present in 1 gram molecular mass of it?
Answer:
Relative molecular mass of H2SO4 = (2 × 1) + 32 + (4 × 16)
= 2 + 32 + 64 = 98.
Gram molecular mass of sulphuric acid is its relative molecular mass expressed in gram. Therefore, 1 gram molecular mass of H2SO4 means 98 g of sulphuric acid. Thus, one gram molecular mass of sulphuric acid contains 98 g of sulphuric acid.

Question 23.
Define ‘one mole’ of a substance.
Answer:
Mole represents the number of atoms or molecules in 22.4 litres of any gas at standard temperature and pressure (STP).

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 24.
What is ‘mole’ used in science? What is the symbol of mole?
Answer:
The mole is an international unit of measurement used in Chemistry to express amounts of a chemical substance. The unit ‘mole’ is expressed by the symbol mol.

Question 25.
What is the number of atoms molecules!ions in a mole called?
Answer:
The number of atoms/molecules/ions present in a mole is exactly 6.023 × 1023. This number is called Avogadro number.

Question 26.
What is the importance of mole concept?
Answer:
Mole concept gives us a method of calculating the number of atoms present in a given mass of a substance. It helps in the calculation of the ratio of reactants consumed to products formed quantitatively.

Question 27.
What is Avogadro number?
Answer:
The number of fundamental units present in one mole of a substance is always constant and this number is equal to 6.023 × 1023. This number is known as Avogadro number.

Question 28.
State Avogadro’s law. Explain with an example.
Answer:
Avogadro’s law states that ‘equal volumes of all gases under identical conditions of temperature and pressure contain equal number of molecules’.

Let us consider equal volumes of molecular hydrogen and molecular nitrogen. Both will contain the same number of molecules when they are at the same temperature and pressure. This is Avogadro’s law.

Question 29.
What is the volume of one mole of any gas at 0°C and 1 atm pressure?
Answer:
One mole of any gas has a volume of 22.4 litre at 0°C and 1 atm pressure.

Question 30.
What is meant by gram molecular volume of a gas? What is its value?
Answer:
The volume occupied by one mole of a gas at STP (standard temperature and pressure) is called its gram molecular volume. One mole of all gases at STP occupy the same volume. Its value is 22.4 litre.

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 31.
What is the volume of 16 g of oxygen at STP?
Answer:
One mole of oxygen has mass 16 g. Therefore, one mole of oxygen at STP has volume of 22.4 litre.

Question 32.
What is the number of molecules present in 22.4 litre of any gas at standard temperature (0°C) and pressure (1 atm)?
Answer:
All gases of volume 22.4 litre under standard conditions of temperature and pressure will contain 6.023 × 1023 molecules.

Question 33.
One litre of hydrogen and one litre of chlorine are kept inside two identical boxes under same conditions of temperature and pressure. Which one of these boxes will have more molecules inside it?
Answer:
Both boxes will contain same number of molecules.

Question 34.
How many molecules are present in one mole of hydrogen?
Answer:
One mole of hydrogen (or any substance) contains 6.023 x 1023 molecules.

Question 35.
A girl, during inhaling, has taken one mole of oxygen in a certain period. How many molecules has she taken in during this period? How many gram of oxygen has she inhaled?
Answer:
One mole of oxygen (or any substance) contains exactly 6.023 × 1023 molecules. Therefore, the girl has inhaled 6.023 × 1023 molecules during the period. The mass of one mole of oxygen is equal to its relative molecular mass expressed in gram. The relative molecular mass of oxygen is 32. Thus, she has inhaled 32 g of oxygen during the period.

Question 36.
There are two containers of which one contains 44 g of carbon dioxide and the other contains 71 g of chloride. Which container has more number of molecules?
Answer:
Container with carbon dioxide has 1 mole of carbon dioxide and the container with chlorine has 1 mole of chlorine. We know that one mole of any substance has the same number of molecules which is equal to the Avogadro number. Therefore, both containers have exactly the same number of molecules which is equal to 6.023 × 1023 molecules.
KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules 4

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 37.
Compare the number of atoms of chlorine present in 1 gram atomic mass of chlorine and the number of chlorine molecules present in 1 gram molecular mass of chlorine.
Answer:
The number of chlorine atoms present in 1 gram atomic mass of chlorine is equal to the number of molecules present in 1 gram molecular mass of chlorine. This number is the Avogadro number which is equal to 6.023 × 1023.

Question 38.
A green tree has used 22 g of carbon dioxide in an hour for photosynthesis. How many molecules of carbon dioxide did the tree use during the period?
Answer:
44 g of carbon dioxide contains 6.023 × 1023 molecules.
22 g of carbon dioxide contains 3.0115 × 1023 molecules.
Thus, the tree has used 3.0115 × 1023 molecules of carbon dioxide in one hour.

Question 39.
A person has drunk 90 g of water in a day. How many molecules of water has she consumed?
Answer:
18 g of water contains 6.023 × 1023 molecules.
90 g of water contains 5 × 6.023 × 1023 molecules = 30.115 × 1023 molecules. Thus, the person has consumed 30.115 x 1023 molecules of water.

Question 40.
Give the symbol, relative atomic mass, gram atomic mass and the number of atoms present in one mole of hydrogen, oxygen, phosphorus and magnesium.
Answer:
KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules 5

Question 41.
Give the molecular formula, relative molecular mass, gram molecular mass and the number of atoms present in one mole of water, carbon dioxide, hydrogen chloride and sulphuric acid.
Answer:
KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules 6

Question 42.
How many moles of Fe are in 5.6 g Fe? How many Fe atoms are contained in the sample? [Given: The relative atomic mass of iron is 56]
Answer:
We know by definition, 1 mole of Fe is 56.0 g.
5.6 g Fe is therefore 0.1 mole of Fe.
The number of Fe atoms in the sample = 0.1 mole × 6.023 × 1023 atoms
= 6.023 × 1022 atoms.
Thus, 5.6 g of iron contains 6.023 × 1022 atoms.

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 43.
The symbol of zinc is Zn and the formula of ammonia is NH3. How much of zinc and ammonia do they represent respectively?
Answer:
Zn represents 1 mole [1 gram atomic mass] of zinc = 65.39 g
NH3 represents [1 mole 11 gram molecular mass] of ammonia = 17 g

Question 44.
An artist desires to make a copper bust of Gandhiji. If she uses 13.12 kg of pure copper for the bust, how many atoms of copper is used by the artist? [Given: Relative atomic mass of copper is 63.6]
Answer:
We know, by definition, that 1 mole of copper is equal to 63.6 g
Thus, 63.6 g of copper contains 6.023 × 1023 atoms
13.12 × 1023 g of copper contains = \(\frac{6.023 \times 10^{23}}{63.6 \mathrm{g}}\) × 13.12 × 103 g
= 1.2424805 × 1026 atoms.

Question 45.
In a chemical reaction hydrogen combines with oxygen to form water. This is represented as: 2H2 + O2 → 2H2O. Explain this reaction using mole concept.
Answer:
In this reaction 2 moles of hydrogen have combined with 1 mole of oxygen to form 2 moles of water. The reaction also means that 4 g of hydrogen combines with 32 g of oxygen to form 36 g of water. Thus, to produce 36 g of water we must use 32 g of oxygen and 4 g of hydrogen.

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 46.
In the reaction between zinc and hydrochloric acid, how many gram of zinc has to be taken to produce 1 g of hydrogen?
Answer:
The reaction between zinc and hydrochloric acid resulting in zinc chloride and hydrogen is represented as follows:
Zn + 2 HCl → ZnCl2 + H2
In the above reaction, 2 g of hydrogen is produced by 65 g of zinc. Therefore, to produce 1 g of hydrogen, we need 32.5 g of zinc.

Question 47.
Calculate the number of moles of water in 1000 cm3 of water.
Answer:
1000 cm3 of water weighs 1000 g (one cubic centimetre of water weighs 1 g)
We know that the molecular mass of water is 18 g.
Thus, 18 g of water means 1 mole of water
1000 g of water means = \(\frac{1 \text { mole } \times 1000 \mathrm{g}}{18 \mathrm{g}}\)
= 55.55 mol.

Question 48.
Calcium carbonate decomposes on heating to give calcium oxide and carbon dioxide. This reaction is represented as follows: CaCO3 → CaO + CO2. Calculate the amount of carbon dioxide in gram liberated by heating 25 g of calcium carbonate.
Answer:
The relative molecular mass of CaCO3 =40 + 12 + 3 × 16 = 100
The relative molecular mass of CO2 = 12 + 2 × 16 = 44
44 g of carbon dioxide is liberated by heating 100 g of CaCO3
To produce 25 g of carbon dioxide we need to heat = \(\frac{100 g \times 25 g}{44 g}\) = 56.82 g

Question 49.
What is meant by valency of an element? Explain with an example.
Answer:
Valency is the combining capacity of an element with atoms of other elements. The number of univalent atoms with which one atom of the given element can combine is called valency of that element.

For example, consider a molecule of water. The formula of water is H2O. Here, one atom of oxygen has combined with two univalent atoms of hydrogen. Therefore, valency of oxygen in water is 2.

Question 50.
What is the valency of iron in ferric chloride (FeCl3)? Explain.
Answer:
The valency of iron in ferric chloride is 3. Chlorine is a monovalent atom. In ferric chloride, one atom of iron has combined with three atoms of chlorine. Therefore, the valency of iron in ferric chloride is 3.

Question 51.
What is the valency of calcium in calcium chloride (CaCl2)? Explain.
Answer:
The valency of calcium in calcium chloride is 2. Chlorine is a monovalent atom. In calcium chloride, one atom of calcium has combined with two atoms of chlorine. Therefore, the valency of calcium in calcium chloride is 2.

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 52.
What are valence electrons? Why are they called so?
Answer:
The number of electrons present in the outermost orbit of an atom of an element are called valency electrons. They are called so because only these electrons take part in chemical reactions.

Question 53.
Give the modern definition of ‘valency’. Explain with an example.
Answer:
The modern concept of valency can be defined as the number of electrons of an element that take part in chemical reactions. For example, valency of chlorine is 1, because it can accept or share one electron to form a chemical bond. The valency of calcium is 2, because it can donate two electrons. The valency of oxygen is also 2, because it can accept or share 2 electrons to form a bond. The valency of carbon is 4, because it can share 4 electrons with other atoms.

Question 54.
What is the stable structure for an atom?
Answer:
Any atom that has either 2 or 8 electrons in its outermost shell is stable.

Question 55.
Why are chemical elements such as helium, neon, argon, krypton, xenon and radon called inert gases or noble gases?
Answer:
Gaseous elements such as helium, neon, argon, krypton, xenon and radon show very little inclination to take part in chemical reactions. This is why these elements are called noble gases or inert gases.

Question 56.
Why do inert gases show very little affinity for chemical reactions?
Answer:
Atoms of noble gases have a stable structure. They have either 2 or 8 electrons in their outermost orbit. Hence such elements show very little affinity for chemical reactions.

Question 57.
Why do atoms of elements take part in chemical reactions?
Answer:
The structure of atoms of elements are generally unstable. Therefore, atoms of one element combine with atoms of others to form a stable configuration similar to that of noble gases. This is why atoms of elements take part in chemical reactions.

Question 58.
What is octet structure?
Answer:
The arrangement of having 8 electrons in the outermost orbit or shell of an atom is called the octet structure.

Question 59.
How does the participation of an atom in chemical reaction give it a stable configuration?
Answer:
Atoms of elements try to acquire the stable octet structure by donating, accepting or sharing electrons with other atoms during a chemical reaction. In doing so they acquire the configuration of the nearest noble gas.

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 60.
The atomic number of aluminium is 13. Find out its valency and the number of valence electrons.
Answer:
The electronic configuration of aluminium is 2, 8, 3. This means there are 3 electrons in the outermost orbit of an aluminium atom. Thus, the number of valence electrons in aluminium atom is 3. Aluminium can attain the octet structure by donating 3 electrons. Hence, its valency is also 3.

Question 61.
QWhat is the valency of noble gases?
Answer:
The valency of noble gases is considered to be zero.

Question 62.
Why is the valency of elements such as argon and krypton considered as zero?
Answer:
Elements such as argon and krypton have 8 electrons in their outermost shell. This is chemically a stable configuration. This is why their valency is taken as zero.

Question 63.
What is variable valency? Give an example.
Answer:
Some atoms show more than one valency during chemical reactions. The chemical elements which show different valencies are said to show variable valency.

For example, iron exhibits variable valency. It shows valency 2 in some reaqtions and valency 3 in some other situations. For instance, the valency of iron in ferric chloride [FeCl3] is 3. Its valency is 2 in ferrous chloride [FeCl2].

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 64.
Complete the following table by writing the number of valence electrons in the atoms of the elements given:
KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules 7
Answer:
KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules 8

Question 65.
Give the name, symbol, atomic number, distribution of electrons, the valence electrons and the valency of the inert gases.
Answer:
KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules 9

Question 66.
What is meant by mass of an element?
Answer:
The percentage composition of an element in a compound is the mass of an element present in 100 g of that compound. It tells us how much percentage of a particular element is present in a given compound.
KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules 10

Question 67.
What is the percentage of each of the elements in calcium carbonate (CaCO3)?
Answer:
Total weight of CaCO3 = 1(Ca) + 1(C) + 3(0) = 1(40) + 1(12) + 3(16)
= 40+ 12 + 48= 100
% mass of calcium in CaCO3 = \(\frac{40}{100}\) × 100 = 40%
% mass of carbon in CaCO3 = \(\frac{12}{100}\) × 100=12%
% mass of oxygen in CaCO3 = \(\frac{48}{100}\) × 100 = 48%

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 68.
What is the percentage composition of hydrogen and oxygen in water?
Answer:
Mass of hydrogen monoxide(water) = 2(H) + 1(O) = 2(1) + 1(16) = 18
% mass of hydrogen = \(\frac{2}{18}\) × 100 = 11.18 %
% mass of oxygen = \(\frac{16}{18}\) × 100 = 88.82 %

Question 69.
Calculate the percentage composition of oxygen in potassium chloride (KClO3). (Atomic mass of K = 39, Cl = 35.5 and O = 16)
Answer:
Mass of a molecule of potassium chloride (KClO3) = 39 + 35.5 + 3 × 16 = 12.5
KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules 11

Question 70.
Calculate the percentage composition of carbon and oxygen in CO2. (Give: Atomic masses: Carbon = 12 and oxygen = 16).
Answer:
Mass of a molecule of carbon dioxide (CO2) = 12 + 2 × 16 = 44
KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules 12

Question 71.
A student has a piece of copper sulphate (CuSO4) crystal. Explain thie method of calculating the number of CuSO4 molecules in that crystal. (Given: Atomic masses: Calcium = 40, Sulphur = 32 and Oxygen = 16)
Answer:
First find the molecular mass of copper sulphate. This can be done by adding the masses of all the atoms present in copper sulphate. Mass of copper sulphate (CuSO4) piolecule = 40 + 32 + 4 × 16 = 136.

Express this mass ip gram, ft will be 136 g. This is the gram atopic mass of copper sulphate. This is also called ‘mole’. We know that one mole of a substance (CuSO4 in this case) contains 6.023 × 1023 molecules (Ayogadfo’s number). How weigh the given copper sulphate crystal. Let its mass be ‘th’ gram. Now the number (n) of copper sulphate molecules present in the given crystal can be found out by using the formula.
n = \(\frac{6.023 \times 10^{23}}{136}\) × m

Multiple Choice Questions

Question 1.
Some elements Have atomic mass in fractions. It is due to the presence of
(a) fractions of protons
(b) fractions of neutrons
(c) isotopes
(d) valence electrons
Answer:
(c) isotopes

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 2.
The atomic number of chlorine is 17. The number of valence electrons in an atom of chlorine is
(a) 7
(b) 8
(c) 9
(d) 10
Answer:
(a) 7

Question 3.
Plants in a garden make use of 22.4 litres of carbon dioxide under certain conditions. This much gas contains
(a) 6.023 × 1023 molecules
(b) 22.4 molecules
(c) 6.023 × 1023 atoms
(d) 11.2 moles of molecules
Answer:
(a) 6.023 × 1023 molecules

Question 4.
The atomic mass of deuterium isotope of hydrogen is 2. The atomic mass of oxygen is 16. The molecular mass of heavy water (D2O) is
(a) 16
(b) 18
(c) 20
(d) 24
Answer:
(c) 20

Question 5.
Which one of the following statements is true regarding Sodium?
(a) 1 g Sodium has 1 mole of atoms
(b) 23 g Sodium has 1 mole of atoms
(c) 1 g Sodium has 6.023 × 1023 atoms
(d) 23 g Sodium has 6.023 × 1022 atoms
Answer:
(b) 23 g Sodium has 1 mole of atoms

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 6.
Some elements can have different valencies. It can happen due to the change in the number of
(a) valence electrons
(b) neutrons
(c) protons
(d) atomic mass
Answer:
(a) valence electrons

Question 7.
Which one of the following has the octet structure?
(a) an atom with atomic number 11
(b) an atom with atomic number 15
(c) an atom with atomic number 18
(d) an atom with atomic number 20
Answer:
(c) an atom with atomic number 18

Question 8.
The relative atomic mass of silicbn is 28 and that of carbon is 12. The amount in moles of carbon atoms contained in 160 g silicon carbide(SiC) is
(a) 2
(b) 4
(c) 6
(d) 8
Answer:
(b) 4

Question 9.
How many grams are there in 2.1 moles of sodium?
(a) 48.3 gram
(b) 0.0913 gram
(c) 11.0 gram
(d) 23.3 gram
Answer:
(a) 48.3 gram

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Question 10.
The standard chosen by scientists for developing the international unit for expressing the masses of atoms and molecules is
(a) Oxygen – 16 atom
(b) Carbon – 12 atom
(c) Hydrogen – 1 atom
(d) Helium – 4 atom
Answer:
(b) Carbon – 12 atom

Question 11.
Which one of the following phrases is incorrect to use?
(a) one mole of an element
(b) one mole of a compound
(c) an atom of a compound
(d) one atom of an element
Answer:
(c) an atom of a compound

Question 12.
The atomic number of carbon is 6. The mass of one atom of carbon-12 is
(a) 1 amu
(b) 12 amu
(c) 6 amu
(d) 18 amu
Answer:
(b) 12 amu

KSEEB Class 8 Science Important Questions Chapter 4 Atoms and Molecules

Fill In The Blanks

1. 6.023 × 1023 molecules of a compound is called mole
2. The number of electrons in the outermost shell of most of the inert gases is eight
3. The mass of one mole of an element is equal to its gram atomic mass
4. The combining capacity of an element with atoms of other elements is known as its valency
5. The number 6.023 × 1023 is called Avogadro number
6. The valency of an atom with atomic number 10 is zero