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Science Answer the questions.[Che-2M]

Structure of the Atom · CBSE STD 9 (CBSE - English Medium). 37 questions.

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37 questions · timed · auto-graded

Question 12 Marks
Why is an atom neutral inspite of the presence of charged particles in it?
Answer
There are equal no. of positive and negative particles in an atom, so it is neutral as a whole.
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Question 22 Marks
Which of the following pairs are isotopes? Give reasons for your choice:

  1. $^{58}_{26}\text{A},\ ^{58}_{28}\text{B}$

  2. $^{79}_{35}\text{X},\ ^{80}_{35}\text{Y}$

Answer

  1. Atoms of the same element can have different numbers of neutrons, the different possible versions of each element are called isotopes. ${^{58}\text{A}}_{26}\text{ and }^{58}\text{B}_{28}$ are not isotopes because they have different atomic numbers.

  2. ${^{79}\text{X}}_{26}\text{ and }^{80}\text{Y}_{35}$ are isotopes as they have same atomic number.

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Question 32 Marks
Which of the following pair is are isobars?

  1. $^{58}_{26}\text{A},\ ^{58}_{28}\text{B}$

  2. $^{79}_{35}\text{X},\ ^{80}_{35}\text{Y}$

Give reasons for your choice.

Answer
$^{58}_{26}\text{A}\text{ and }\ ^{58}_{28}\text{B}$ are isobars because they have same number of nucleons i.e., their mass numbers are same but atomic number is different.
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Question 42 Marks
What valencies will be shown by the elements A, B, C, D and E having atomic numbers 2, 4, 8, 10, and 13 respectively.
Answer
Valency shown by A(atomic no. 2) – 0
Valency shown by B(atomic no. 4) – 2
Valency shown by C(atomic no. 8) – 2
Valency shown by D(atomic no. 10) – 0
Valency shown by E(atomic no. 13) – 3
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Question 52 Marks
What is the relation between the valency of an element and the number of valence electrons in its atoms? Explain with examples.
Answer
The valency of an element is either equal to the number of valence electrons in its atom or equal to the number of electrons required to complete eight electrons in the valence shell.
Valency of metal = No. of valence electron in its atom
Valency of a non-metal = 8 - No. of valence electron in its atom
For example: Valency of sodium (metal) is 1 and that of chlorine (non-metal) is also 1.
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Question 62 Marks
What is the number of valence electrons in:
  1. Sodium ion, Na+
  2. Oxide ion, O2-
Answer
  1. Atomic number of sodium atom = 11

In Na+ ion, there are only 10 electrons. The electronic configuration is 2, 8.

Therefore, the number of valence electrons in sodium ion is 8.

  1. Atomic number of oxygen atom = 8

In O2- ion, there will be 10 electrons, as it has gained two electrons. The electronic configuration is 2, 8.

Therefore, the number of valence electrons in O2- is 8.

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Question 72 Marks
What is meant by (i) atomic number, and (ii) mass number, of an element? Explain with the help of an example.
Answer
  1. Atomic no. is the number of protons in one atom of an element.
  2. Mass no. is the total number of protons and neutrons present in one atom of the element.

Example: The total no. of protons in a carbon atom is 6, so its atomic no. is 6.

Also, one atom of Na contains 11 protons and 12 neutrons, so its mass no. is 23.

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Question 82 Marks
What is a proton? State its relative mass and charge.
Answer
Protons are positively charged particle found in the atoms of all the elements.
Relative mass of proton is 1u.
Relative charge of proton is +1C.
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Question 92 Marks
What is an electron? State its relative mass and charge.
Answer
Electron is a negatively charged particle found in the atoms of all elements.
The relative mass of an electron is $\frac{1}{1840\text{u}}$
A charge of -1 is carried by an electron.
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Question 102 Marks
What important information is furnished about the nucleus of an atom by the alpha particle scattering experiment of Rutherford?
Answer
Important information furnished about nucleus in Rutherford’s alpha-particle scattering experiment is:
  • Nucleus of an atom is positively charged.
  • Nucleus of an atom is very hard and dense.
  • Nucleus of an atom is very small as compared to the size of an atom as a whole.
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Question 112 Marks
What do you understand by the tern "electronic configuration" of an element? Write down the electronic configuration of oxygen (At. No. = 8).
Answer
The arrangement of electrons in various shells of an atom of the element is known as electronic configuration of the element. Electronic configuration of oxygen (atomic no. = 8) is (2, 6).
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Question 122 Marks
What are radioactive isotopes? Give two examples of radioactive isotopes
Answer
The isotopes which are unstable due to presence of extra neutrons in their nuclei and emit various types of radiations, are called radioactive isotopes or radioisotopes.
For example: Carbon - 14 , Arsenic - 74
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Question 132 Marks
What are cathode rays? What is the nature of charge on cathode rays?
Answer
The stream of particles coming out from cathode (negative electrode) are called cathode rays. Cathode rays are negatively charged.
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Question 142 Marks
The nucleus of an atom has 5 protons and 6 neutrons. What would be the:
  1. Atomic number.
  2. Mass number.
  3. The number of electrons.
  4. The number of valence electrons, per atom of this element?
Answer
  1. Atomic no. = 5
  2. Mass no. = 6 + 5 = 11
  3. No. of electrons = 5
  4. No. of valence electrons, per atom = 3
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Question 152 Marks
The mass number of an element is 23 and it contains 11 electrons. What is the number of protons and neutrons in it? What is the atomic number of the element?
Answer
Given,
Mass no. = 23
No. of electrons = 11
Then, no. of protons = 11
No. of neutrons = 23 - 11 = 12 
Atomic no. = 11
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Question 162 Marks
The mass number of an element is 18. It contains 7 electrons. What is the number of protons and neutrons in it?
Answer
When mass no. is 18 and no. of electrons is 7 then:
  • No. of protons = 7
  • No. of neutrons = 18 - 7 = 11
  • Atomic no. = 7
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Question 172 Marks
The composition of two atomic particles is given below:
X
Y
Protons
8
8
Neutrons
8
9
Electrons
8
8
  1. What is the mass number of X?
  2. What is the mass number of Y?
  3. What is the relation between X and Y?
  4. Which element/ elements do they represent?
Answer
  1. Mass number of X = 8 + 8 = 16
  2. Mass number of Y = 8 + 9 = 17
  3. X and Y are isotopes, as they have the same number of protons but different number of neutrons.
  4. X and Y represent the isotopes of oxygen.
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Question 182 Marks
The atomic species A and B have different number of protons but the same number of nucleons. On the other hand, the atomic species X and Y have the same number of protons but different number of nucleons. Which pair is an example of isobars? Why?
Answer
Atoms of different elements with different atomic numbers but same mass number are known as isobars. Here, A and B are the examples of isobars, as they have same number of nucleons.
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Question 192 Marks
The atomic number of an element X is 16.
What will be the valency of X?
Answer
Valency of X = 8 - no. of valence electrons
Valency of X = 8 - 6 = 2
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Question 202 Marks
State two observations which show that atom is not indivisible.
Answer
Two observations which shows that atom is not indivisible are:
  1. In J. J. Thomson’s experiment, the stream of cathode rays in the gas discharge tube shows the presence of negatively charged subatomic particles called electrons.
  2. In Goldstein’s experiment, the faint red glow in the gas discharge tube shows the presence of positively charged subatomic particles called protons.
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Question 212 Marks
State the location of electrons, protons and neutrons in an atom.
Answer
Protons and neutrons are collectively present in the nucleus at the center while electrons revolve rapidly round the nucleus in fixed circular orbits called energy levels.
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Question 222 Marks
Hydrogen has three isotopes written as:

$^1_1\text{H},\ ^2_1\text{H},\ ^3_1\text{H}$

Explain why:

  1. These isotopes have almost identical chemical properties.
  2. They are electrically neutral.
Answer
  1. Due to identical electronic configuration containing the same no. of valence electrons, these isotopes have almost identical chemical properties.
  2. All of them have 1 electron and 1 proton, so, they are electrically neutral.
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Question 232 Marks
How was it shown that an atom has a lot of empty space within it?
Answer
Most of the alpha-particles passed straight through the gold foil without any deflection in Rutherford’s alpha- particle scattering experiment, this shows that most of the space in an atom is empty.
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Question 242 Marks
How does a proton differ from an electron?
Answer
Difference between proton and neutron:
  • Proton is positively charged while electron is negatively charged.
  • Proton is much heavier than electron.
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Question 252 Marks
How does an electron differ from a neutron?
Answer
Electron has relative charge of -1u whereas neutron has no relative charge. Also, electron has relative mass of $\frac{1}{1840\text{u}}$ and neutron has relative mass of 1u.
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Question 262 Marks
Give the evidence for the existence of nucleus in an atom.
Answer
The deflection of fast moving alpha-particles through small and large angles in Rutherford’s scattering experiment is the evidence for the presence of nucleus in an atom.
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Question 272 Marks
Give one use each of the following radioactive isotopes:
  1. Uranium-235
  2. Cobalt–60
Answer
  1. Uranium-235: This isotope is used as a fuel in the reactors of nuclear power plants for generating electricity.
  2. Cobalt-60: This is used in the treatment of cancer.
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Question 282 Marks
Give any two uses of radioactive isotopes.
Answer
Uses of isotopes:
  1. They are used in the treatment of cancer.
  2. Radioactive isotopes are used as 'tracers' in medicine to detect the presence of tumors and blood clots in human body.
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Question 292 Marks
Explain how, cathode rays are formed from the gas taken in the discharge tube.
Answer
When electricity at high voltage is passed through a gas at very low pressure taken in discharge tube, stream of minute particles are given out by the cathode. These stream of particles are called cathode rays.
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Question 302 Marks
Describe Thomson’s model of the atom. Which subatomic particle was not present in Thomson’s model of the atom?
Answer
According to Thomson model of atom: An atom consists of a sphere of positive charge with negatively charged electrons embedded in it. The positive and negative charges in an atom are equal in magnitude. Neutron was not present in the Thomson model of atom.
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Question 312 Marks
Define valency of an element. What valency will be shown by an element having atomic number 14?
Answer
The capacity of an atom of an element to form chemical bonds is known as its valency. Valency of an atom with atomic no. 14 is 4.
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Question 322 Marks
Composition of the nuclei of two atomic species A and B is given as under:
A
B
Protons
18
20
Neutrons
22
20
Give the mass numbers of A and B. What is the relation between the two species and which element or elements they represent?
Answer
Mass number is the sum of the number of neutrons and the number of protons. So, mass number of A and B is 40. As the mass number of A and B is same, the two are isobars.
Therefore, A represents Argon $\Big({^{40}_{18}}\text{Ar}\Big)$ while B represents $\Big({^{40}_{20}}\text{Ca}\Big)$ calcium.
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Question 332 Marks
Compare an electron and a proton in respect of mass and charge.
Answer
Electron has relative mass of $\frac{1}{1840\text{u}}$ and proton has relative mass of 1u.
Electron has relative charge of -1u while proton has +1u of relative charge.
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Question 342 Marks
Atom A has a mass number 209 and atomic number 82.
Atom B has a mass number 209 and atomic number 83.
  1. How many protons atom A has?
  2. How many protons atom B has?
  3. Are atoms A and B isotopes of the same element?
Answer
$\text{Atom A}-{^{209}\text{A}}_{82}$

$\text{Atom B}-{^{209}\text{A}}_{83}$

  1. A has 82 protons as the atomic number is 82.
  2. B has 83 protons as the atomic number is 83.
  3. No, A and B are not isotopes of each other because their atomic numbers are not same.
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Question 352 Marks
An element Z contains two naturally occurring isotopes $^{35}_{17}\text{Z}\text{ and }^{37}_{17}\text{Z}.$ If the average atomic mass of this element be 35.5u, calculate the percentage of two isotopes.
Answer
Average atomic mass = 35.5u

Let % amount of 35Z17 be y, then amount of 37Z17 is (100 - y)

Then the % of isotope with mass number 37 will be 100 - x

Average atomic mass of chlorine $=35\Big[\frac{\text{x}}{100}\Big]+37\Big[\frac{100-\text{x}}{100}\Big]$

$35=\frac{35\text{x}}{100}+\frac{37(100-\text{x})}{100}$

$\text{x}=75$

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Question 362 Marks
An element has Z = 7. What is the valency of the element? Also name the element.
Answer
Atomic No. = 7
E.C = 2, 5
Valency of given element = 3
Given element is NITROGEN.
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Question 372 Marks
An element has an atomic number of 11 and its mass number is 23. What is the arrangement of electrons in the shells? State nuclear composition of an atom of the element.
Answer
Given,
Atomic no. = 11
Mass no. = 23
Then, electronic configuration -(2, 8, 1)
Nuclear composition is -11 protons and 12 neutrons.
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