WBBSE Class 8 School Science Chapter 2 Element Compound And Chemical Reaction Structure Of Matter SAQs

WBBSE Chapter 2 Element, Compound, And Chemical Reaction Structure Of Matter Short Answer Type Questions

Question 1. Why an atom is uncharged?
Answer:

Atom is uncharged:

The atom consists of three types of subatomic particles – neutron (uncharged), proton (positively charged), and electron (negatively charged).

Since the number of protons and several electrons in an atom is the same, so there> is no net charge in the atom and the atom is uncharged.

Question 2. What do you mean by nuclear force? Positively charged protons and uncharged
Answer:

Nuclear Force:

neutrons are present within the nucleus. We know that “like” charges repel each other. If that is the case then positively charged protons within the nucleus must repel each other with great repulsive force and this can destabilize the structure of the nucleus.

But a strong attractive force exists within the nucleus, which strongly holds neutrons and protons together within the nucleus. This nuclear force overcomes the repulsive forces existing between the positively charged protons.

This force has such a great magnitude that to separate the nucleons (i.e., protons and neutrons) very high energy would be required. This force is known as nuclear force.

WBBSE Class 8 Structure of Matter short answer questions

Question 3. What is an isotope? Give example.
Answer:

Isotope:

Atoms of the same elements (having the same atomic number) having different numbers of neutrons (i.e., having different mass numbers) are called isotopes.

So two isotopes have the same number of protons (i.e., the same atomic number) but different numbers of neutrons. For example, hydrogen (\H), Deutch (2H), and tritium (H) are isotopes.

Question 4. What is isobar? Give example.
Answer:

Isobar:

Atoms of different elements that have the same mass number are called isobars. This means that in the case of isobars, the number of protons and several neutrons are different, but the total number of neutrons and protons are the same. Examples of isobars are 4C and N; Hand He.

Short answer questions on elements and compounds for Class 8

Question 5. What is isotone? Give example.
Answer:

Isotone:-

Atoms of different elements having the same number of neutrons in the nucleus are called isotones. In the case of isotones, the number of protons in the nucleus is different. Examples of isotones are: \({ }_{14}^{30} \mathrm{Si},{ }_{15}^{31} P,{ }_{16}^{32} S\) In,, three atoms the number of neutrons is 16.

Question 6. What do you mean by the atomic number and the mass number of an element? Give a suitable example.
Answer:

Atomic number and a mass number of an element:

Total number of protons in the nucleus of an atom is called the atomic number of the atom. The total number of protons and neutrons in the nucleus of an atom is called its mass number.

For example, in the case of a sodium atom, there are 11 protons and 12 neutrons in its nucleus. So the atomic number of sodium is 11 and its mass number is (11 + 12) = 23.

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Question 7. What is the relation between the atomic number and the mass number of an element? The atomic number of an atom is the number of protons present in the atom and the mass number is the total number of protons and neutrons present in the atom.
Answer:

Relation Between The Atomic Number And A Mass Number Of An Element:

So, Mass number = a number of protons + a number of neutrons

or, mass number = atomic number + a number of neutrons.

or, mass number – atomic number = number of neutrons

Question 8. Calculate the number of electrons, protons, and neutrons in¹⁶₈O and ²³⁸₉₂ U
Answer:

Given:

¹⁶₈O And ²³⁸₉₂ U.

The atomic number of ¹⁶₈O  is 8 and its mass number is 16. So the number of protons is 8 and the number of electrons is also 8 (because the number of protons is the same as the number of electrons in an atom). The number of neutrons is (16-8) = 8.

Its atomic number is 92 and its mass number is 238. So the number of protons is 92 and the number of electrons is also 92 (because the number of protons is the same as the number of electrons in an atom). The number of neutrons is (238 – 92) = 146.

WBBSE Chapter 2 structure of matter answers

Question 9. Which one is heavier – an atom of gold or an atom of iron? Then predict 1 mg of which element will have more atoms in it.
Answer:

An atom of gold is heavier than an atom of iron. Suppose, the mass of one atom of gold is MAu mg and that of iron is MFe mg. So 1 mg gold contains (1/MAu) a number of atoms and 1 mg iron contains (1/MFe) a number of atoms. Since, MAu > MFe, hence, 1/MAu < 1/MFe. This means 1 mg of iron will contain more atoms.

Question 10. Give an idea regarding the size of an atom with respect to its nucleus.
Answer:

The average diameter of a nucleus is approximately 1011 cm and the average diameter of an atom is approximately 10* cm. So the diameter of an atom is lCf times (i.e., 1 lakh times) more than the diameter of its nucleus. This means that if the nucleus is a marble of diameter 1 cm then the atom is a sphere of diameter 1 km.

Question 11. What is the relation between ³⁵₁₇cl and ³⁵₁₇cl? Can you comment on their chemical properties?
Answer:

The relation between ³⁵₁₇cl and ³⁵₁₇cl

The atomic number of both ³⁵₁₇cl and ³⁵₁₇cl is the same (17) but their mass numbers are different. So they are isotopes.

Since the chemical properties of an element depend on the atomic number, it is expected that the chemical properties of both of them will be the same.

Question 12. Predict the number of proton/neutrons and give the number of electrons in each orbit for the atom f3AI.
Answer:

The number of protons is 13. The number of neutrons is (27 – 13) = 14 and the number of electrons is 13. The first orbit has 2 electrons, 2nd orbit has 8 electrons and the 3rd orbit has 3 electrons.

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Question 13. Which one is more stable Na or Na+? The atom is diagrammatically represented as:
Answer:

Na has 11 electrons. It has 2 electrons in the 1st orbit, 8 electrons in the 2nd orbit, and 1 electron in the 3rd orbit. It loses one electron from its outermost orbit and achieves the electronic configuration of its nearest inert gas neon. This electronic arrangement is stable. So Na+ is more stable than the Na atom.

Question 14. “In ionic compounds, no molecules exist”. Explain with a suitable example.
Answer:

“In ionic compounds, no molecules exist”.

In ionic compounds, no molecules exist. For example, in solid sodium chloride, there are only properly arranged Na+ and Cl’ ions; no entity like “NaCI” exists in it.

When dissolved in water or in a molten state, these ions come apart readily and move from one place to another and are able to conduct electricity whenever current flows through an aqueous solution of sodium chloride or molten sodium chloride.

So, basically what we are writing is the number of cations per anion and in this way, we are representing the ionic compound.

Structure of matter concepts with short answers for Class 8

Question 15. Mention briefly the main drawbacks of Dalton’s atomic theory.
Answer:

Limitations of Dalton’s atomic theory:

With the discovery of subatomic particles (ie., the electron, proton, and neutron), it was concluded that atoms can be further divided.

The Discovery of isotopes proved that atoms of the same element may possess different atomic weights, ie., atoms of same elements may not be identical in all respects.

In some cases, atoms of different elements are found to have the same mass number, ie., atoms of different elements may be identical in this respect.

Question 16. Write the formula of ferrous sulfate. The atomic number and a mass number of an element are 6 and 12 respectively. Represent the atom of the element diagrammatically.
Answer:

Ferrous cation = Fe²⁺

Sulphate anion = SO²₄

∴Ferrous sulfate = FeSO₄.

An atomic number of the element = 6.

∴ The number of protons and that of extranuclear electrons = 12-6=6.

 

Examples of structure of matter short answer questions for Class 8

Question 17. An atom of gold is heavier than an atom of copper. In which atom do you think the total number of protons and neutrons is more?
Answer:

An atom of gold is heavier than an atom of copper.

Mass number indicates the sum of protons and neutrons present inside the nucleus. The whole mass of the atom may be supposed to be concentrated at its nucleus, ie., it depends on the mass number of the atom.

Since an atom of gold is heavier than an atom of copper, hence the total number of protons and neutrons is more in a gold atom than that in a copper atom.

Question 18. What are the cations and anions in aluminum nitrate and calcium phosphate?
Answer:

Aluminium nitrate \(\left[\mathrm{Al}\left(\mathrm{NO}_3\right)_3\right]=\mathrm{Al}{ }^{3+}+3\left(\mathrm{NO}_3\right)^{-}\)

⇒ \(\left[\mathrm{Ca}_3\left(\mathrm{PO}_4\right)_2\right]=3 . \mathrm{Ca}^{2+}+2\left(\mathrm{PO}_4\right)^{3-}\)

Question 19. Find out the isotopes and isobars among the following: \({ }_1^2 H,{ }_7^{14} N,{ }_1^3 H,{ }_6^{14}\) C
Answer: 

²₁H and ³₁H  have the same atomic number, Z = 1, but different mass numbers. Hence they are isotopes.

¹⁴₇N and ¹⁴₆C have the same mass number, A -14, but different atomic numbers. Hence they are isobars.

WBBSE Class 8 Science practice short answer questions on elements and compounds

Question 20. Isotopes exhibit similar chemical properties- justify.
Answer:

Isotopes exhibit similar chemical properties

The chemical property of an element depends on the number of electrons present in the outermost or valence shell of an atom of the element Also, how many electrons will be there in the outermost shell depends on the total number of electrons or the atomic number of the element.

So, the chemical properties of an element depend on its atomic number. Since isotopes have the same atomic number, hence they exhibit a similar set of chemical properties.