MOECDC Class 8 Matter
MOECDC Class 8 Science and Technology | Matter | Nepal Class 8 | Exercise | Question Answer
MOECDC Class 8 English Medium Science Chapter 9 Matter Exercise Question Answer Solution.
| Board | Government of Nepal. |
| Class | Eight (8). |
| Subject | Science and Technology. |
| Chapter | Life process |
| Topic | Exercise Solve / Question Answer / Notes. |
Exercise 1 :
(1) Choose the best alternatives :
(a) What is the symbol of the element given in the figure?
(अ) S
(आ) Si
(इ) Na
(ई) C
Answer :
(iv) C – is the symbol of the element given in the figure.
(b) Which of the following elements has zero valency?
(i) Hydrogen
(iii) Neon
(ii) Sodium
(iv) Calcium
Answer :
(iii) Neon – has zero valency.
(c) What is the atomic weight of Argon?
(i) 40 amu
(ii) 42 amu
(iii) 44 amu
(iv) 46 amu
Answer :
(i) 40 amu – is the atomic weight of Argon.
(d) Which of the following is the mass of negatively charged subatomic particles of an atom?
(i) 1/1834 amu
(ii) 1/1835 amu
(iii) 1/1836 amu
(iv) 1/1837 amu
Answer :
(iv) 1/1837 amu – is the mass of negatively charged subatomic particles of an atom.
(e) What are the subatomic particles in the nucleus of an atom?
(i) Proton and Electron
(ii) Electron and Neutron
(iii) Neutron and proton
(iv) Proton. Neutron and Electron
Answer :
(iii) Neutron and proton – are the subatomic particles in the nucleus of an atom.
(f) What is the molecular formula of calcium oxide?
(i) CaO
(iii) CaO
(ii) CaO,
(iv) CaCO3
Answer :
(i) CaO – is the molecular formula of calcium oxide.
(g) The number of protons (p+), electrons (e-) and neutrons (nº) of various elements are given in the table below.
Which of these has the highest atomic weight?
(i) Potassium
(ii) Carbon
(iii) Oxygen
(iv) Argon
Answer :
(iv) Argon – has the highest atomic weight.
(2) Differentiate between :
(a) Proton and electron
Answer :
Proton and electron :
(i) A positivity charged sub-atomic particle in the nucleus of an atom is called a proton. On the other hand, a negatively charged sub-atomic particle that revolves around the nucleus of an atom is called an electron.
(ii) The mass of a proton is called 1 atomic mass unit. But the mass of an electron is equal to 1/1837 atomic mass unit.
(b) K shell and M shell
Answer :
K shell and M shell:
(i) K shell is the first energy level of an atom, while the third energy level is M shell.
(ii) According to the 2n2rule, the k shell has 2 electrons, while M shell has 18 electrons.
(c) Valence shell and valence electron
Answer :
Valence shell and valence electron: The outmost shell of an atom is called valence shell. On the other hand, the electrons in the valence shell are called valence electrons.
(d) Atomic weight and molecular weight
Answer :
Atomic weight and molecular weight: The sum of the number of protons and neutrons in the nucleus of an atom is called the atomic weight of the element. On the other hand, the molecular weight of any element or compound is the sum of the atomic weights of the elements present in the molecule of the element or compound.
(3) Give reasons :
(a) An atom is electrically neutral.
Answer :
An atom is electrically neutral because the positive and negative charges are balanced in an atom as the number of electron and proton are equal there.
(b) The valency of Chlorine is 1.
Answer :
The valency of Chlorine is 1 because chlorine gains 1 electron to attain stability.
(4) Answer the following questions:
(a) What is an atom?
Answer :
An atom is an extremely minute particle of matter that cannot be seen with the naked eye.
(b) Write the name, symbol and charge of sub-atomic particles in an atom.
Answer :
Proton (p+), positively charged.
Neutron (n0), charge less.
Electron (e-), negatively charged
(c) Write the molecular formula of the given compound by the crisscross method.
(i) Magnesium oxide
(iii) Potassium oxide
(ii) Sodium Chloride
(iv) Aluminium Chloride
(v) Ammonia
Answer :
(d) What is a shell?
Answer :
The electrons revolve around the nucleus in a fixed path which is called a shell.
(e) What isthe 2n² rule used for?
Answer :
2n2 rule is used to tell how many electrons are located in the shell of an atom.
(f) What is a valence shell? What is called the electron in the valence shell?
Answer :
The outmost shell of an atom is called valence shell.
The electron in the valence shell is called valence electron.
(g) What is valency? What are the factors on which the valency of an element depends?
Answer :
Valency is the number of electrons donated, shared or gained by an atom of an element to attain stability.
An atom of an element with valence electrons 1 to 3 loses electrons, an atom with 4 valence electrons shares electrons and an atom 5 to 7 valence electrons gains electrons to attain stability. An atom of an element with valence electron 8 does not donate, share or lose any electron.
(h) What is meant by the valency of an element being zero?
Answer :
An atom of an element with valence electron 8 does not donate, share or lose any electron. Therefore, the valency of such elements is 0.
(i) What will be the valency of an element if the electronic configuration of an element is 2.8.8.1? Write with reasons.
Answer :
If the electronic configuration of an element is 2, 8, 8, 1, then the valency of that element is 1 because this element contains 1 valence electron in its valence shell which it loses to attain stability.
(j) What is the molecular formula? What arethe information required to write the molecular formula of a compound?
Answer :
The symbolic representation of a molecule of an element or a compound is called molecular formula.
While writing the molecular formula of a compound, it is necessary to know the type of elements and their valency involved in the formation of the compound.
(k) What is the atomic weight of Potassium? Calculate the molecular weight of Calcium Chloride.
Answer :
The atomic weight of potassium is 39. [No. of proton + no. of electron = 19 + 20 = 39]
The molecular weight of Calcium Chloride = CaCl2 = Ca + 2 * Cl = 40 + 2 * 35 = 110 amu. [Atomic weight of Ca = 40 amu and Atomic weight of Cl = 35 amu]
(l) Complete the table given below:
| Element | Symbol | Atomic number | Atomic mass | No. of proton | No. of neutron | No. of electron |
| Oxygen | 16 | |||||
| C | 6 | |||||
| 11 | 12 | |||||
| 24 | 12 | |||||
| Chlorine | 17 | |||||
| K | 20 | |||||
| 14 | 14 | |||||
| Argon | 40 |
Answer :
(m) Can an atom exist without electrons? Give reasons.
Answer :
An atom can exist without electron because an atom does not fully depended on electron, it has proton and neutron in the central part which called nucleus of an atom.
(n) Anju has drawn the atomic structure of Sodium as follows. Find any three mistakes in the atomic structure drawn by her.
Answer :
Three mistakes that Anju done are,
(i) Anju draw 6 Protons and 8 Neutrons which are wrong because Sodium has 11 Protons and 12 Neutrons.
(ii) Sodium has 11 electrons but Anju has drawn 10 electrons.
(iii) M shell of Sodium has 8 electrons but Anju has drawn 7 electrons.
(0) What is the element to which the given nucleus of an atom belongs? Draw the atomic structure of an atom of the elements given below. What are the information required to calculate the atomic weight of the elements? Write with reasons.
Proton=19
Neutron=20
Answer :
The element is Potassium.
Number of proton and number of neutron in the atom of the element are required to calculate the atomic weight of the element because atomic weight of an element is the sum of the number of protons and neutrons in the nucleus of the atom of this element.
(p) Complete the table by the number of atoms in a molecule of Sulphuric acid (H,SO)
Answer :
| Element | Number of atoms |
| Hydrogen | 2 |
| Sulphur | 1 |
| Oxygen | 4 |
Exercise 2 :
(1) Choose the correct alternative :
(a) Who discovered the modern periodic table?
(i) Dmitri Mendeleev
(ii) Henry Moseley
(iii) John Dalton
(iv) Bohr
Answer :
(ii) Henry Moseley – discovered the modern periodic table.
(b) On which basis are the elementsarranged in the modern periodic table?
(i) atomic weight
(ii) atomic mass
(iii) molecular mass
(iv) atomic number
Answer :
The elements are arranged in the modern periodic table basis on – (iv) atomic number.
(c) Which of the following statements is true regarding the characteristics of the group?
(i) Elements in the same group have different chemical and physical properties.
(ii) Atomic size of elements decreases from top to bottom of the group.
(iii) Elements in the same group have the same valency.
(iv) The non-metallic properties of the elements increase from top to bottom of the group.
Answer :
(iii) Elements in the same group have the same valency – is true regarding the characteristics of the group.
(d) Which metal is found in the liquid state at normal room temperature?
(i) Sodium
(ii) Magnesium
(iii) Aluminum
(iv) Mercury
Answer :
(iv) Mercury – is found in the liquid state at normal room temperature.
(e) Which of the following elements belongs to group 18 of the modern periodic table?
(a) Sodium
(ii) Magnesium
(iii) Phosphorus
(iv) Argon
Answer :
(iv) Argon – belongs to group 18 of the modern periodic table.
(f) What are the similarities between argon and chlorine?
(i) Both elements are inert gases.
(ii) Both elements belong to the same period.
(iii) Chemical reactivity of both elements isthe same.
(iv) Both elements lie in the same column
Answer :
The similarities between argon and chlorine are – (ii) both elements belong to the same period.
(g) Which of the following statements is correct about the characteristics of groups in the periodic table?
(i) The physical properties of the elements are different in the same group.
(ii) Elements belonging to the same group have the same chemical properties.
(iii) Elements belonging to the same group have the different physical and chemical properties.
(iv) Elements belonging to the same group have the same physical properties but different chemical properties.
Answer :
(ii) Elements belonging to the same group have the same chemical properties – is correct about the characteristics of groups in the periodic table.
(2) Differentiate between :
(a) Group and period
Answer :
Group and period:
(i) In the modern periodic table, the elements with similar properties fell in the same vertical columns called groups, whereas elements with different properties lie in horizontal rows called periods.
(ii) The physical and chemical properties of elements in the same group are similar, whereas the physical and chemical properties of elements gradually change in the same period.
(b) Metals and non-metals
Answer :
Metals and non-metals:
(i) Elements that donate electrons during a chemical reaction are called metals, whereas elements that gain electrons during a chemical reaction are called non-metal.
(ii) Metals are good conductors of heat and electricity, whereas non-metals are poor conductors of heat and electricity.
(c) Group 1 and Group 17 elements
Answer :
Group 1 and Group 17 elements:
(i) Elements in group 1 have one valence electron which they easily donate during a chemical reaction to attain stability, whereas elements in group 17 attain stability gaining one electron.
(ii) Elements in group 1 are called very reactive metals, whereas elements in group 17 are called very reactive non-metals.
(d) Word equation and formula equation
Answer :
Word equation and formula equation: Chemical equation in which reactors and products are expressed in words are called word equation, whereas the chemical equation in which reactors and products are expressed in molecular formulae are called formula equation.
(e) Reactants and products
Answer :
Reactants and products:
(i) The substance that take part in a chemical reaction are called the reactors, whereas the substance formed at the end of the chemical reaction are called products.
(ii) While writing a chemical equation, the reactors are written on the left, whereas the products on the right of the arrow symbol.
(3) Give reason :
(a) Classification of elements is necessary.
Answer :
Classification of elements is necessary because the study of the elements would be easier and faster to memorize for a long time.
(b) Sodium and Chlorine belong to different groups but they have valency 1.
Answer :
Sodium and Chlorine belong to different groups but they have valency 1 because they attain stability by donating and gaining one electron respectively.
(c) The study of elements becomes easier using a periodic table.
Answer :
The study of elements becomes easier using a periodic table because it is faster to memorize for a long time as the elements with similar properties fell in the same vertical column and different elements with different properties lie in a horizontal row.
(d) The products are different from the reactants.
Answer :
The products are different from the reactants because the substance that take part in a chemical reaction are called the reactors, whereas the substance formed at the end of the chemical reaction are called products.
(e) Metals and non-metal sare kept separately in the periodic table.
Answer :
Metals and non-metals are kept separately in the periodic table because their chemical properties are dissimilar.
(4) Answer the following questions :
(a) What is periodic table?
Answer :
While classifying the elements, they are arranged in a table having rows and columns which is called the periodic table.
(b) Write the modern periodic law.
Answer :
The modern periodic law – ‘The physical and chemical properties of elements are the periodic function of their atomic number’.
(c) Explain the position of metals in the modern periodic table.
Answer :
The position of metals in the modern periodic table – Elements that donate electrons during a chemical reaction are called metals. Metals are good conductors of heat and electricity. If there are 1 to 3 electrons in the outer shell of an element, the elements show their reactivity by donating electrons. There are metals from groups 1 to 13. Elements in group 1 have one valence electron which they easily donate during a chemical reaction to attain stability. Hence elements of this group are very reactive. They are also called alkali metals. Group 2 elements are called alkaline earth metals.
(d) What change in the metallic properties of the elements occurs while moving from top to bottom in any group of the modern periodic table?
Answer :
While moving from top to bottom in any group of the modern periodic table, the electron-donating ability or metallic character of the elements increases whereas the electron-gaining capacity or non-metallic character decreases and the atomic size of the elements also increases.
(e) What change in metallic properties of elements occurs while moving from left to right in any period of the modern periodic table?
Answer :
While moving left to right in any period of the modern periodic table, the electron-donating capacity of the elements or metallic character decreases, whereas the electron-gaining capacity or the non-metallic character increases and the atomic size of the elements also decreases.
(f) What is a chemical reaction?
Answer :
The changes in which the composition of the substance varies and cannot be reversed directly to its original state is called a chemical reaction.
(g) What is a chemical equation?
Answer :
A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and chemical formulas.
(h) How can we balance the formula equation? Write with examples.
Answer :
The number of atoms of every element on the left and right sides of the formula equation must be equal. So the reactants and products are balanced by placing coefficients in front of reactants and products as per necessity.
For example, the chemical reaction between sodium and chlorine to form sodium chloride can be expressed into a balanced chemical equation as: Sodium + Chlorine → Sodium chloride
2Na + Cl2 → 2NaCl
(i) Express the given chemical reaction into a balanced chemical equation:
(i) Magnesium oxide + Hydrogen → Magnesium + water
(ii) Calcium carbonate + Hydrochloric acid→ Calcium Chloride +Water+Carbon dioxide
(iii) Potassium + Chlorine → Potassium chloride
(iv) Sodium + Oxygen→ Sodium oxide
(vi) Hydrogen + Oxygen Water
Answer :
(a) MgO + H2 → Mg + H2O
(b) CaCO3 + 2HCl → CaCl2 + H2O + CO2
(c) K + Cl → KCl
(d) 4Na + O2 → 2Na2O
(e) 2H2 + O2 → 2H2O
(j) The chemical equation for the chemical reaction between Zinc and hydrochloric acid is given below:
Zn + 2HCl → ZnCl + H
Write the chemical equation if sulfuric acid is used instead of hydrochloric acid.
Answer :
Zn + H2SO4 → ZnSO4 + H2
(k) Part of a group of the periodic table is given below:
| Be |
| Mg |
| Ca |
Write any four characteristics of this group.
Answer :
Four characteristics of this group are,
(i) Group 2 elements are called alkaline earth metals.
(ii) They are good conductors of heat and electricity.
(iii) They all have 2 valence electrons.
(iv) They show their reactivity by donating electrons.
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