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Structure of the Atom · CBSE STD 9 (CBSE - English Medium). 75 questions.

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Question 11 Mark
The total number of electrons in a nitrogen atom is $7.$ Find the number of valence electrons in it.
Answer
$E.C$ of Nitrogen $= 2, 5$ So, no. of valence electrons in Nitrogen atom $= 5$
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Question 21 Mark
All the gases form cathode rays and anode rays when electricity is passed through them:
What does the formation of anode rays tell us about the atoms?
Answer
Formation of anode rays tells about the presence of positively charged protons in all the atoms.
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Question 31 Mark
What are isotopes? Explain by giving an example.
Answer
Isotopes are the atoms of the same element having the same atomic number but different mass numbers. For example: $^{35}\text{Cl}_{17}\text{ and }^{37}\text{Cl}_{17}$ are isotopes of chlorine.
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Question 41 Mark
What is a neutron? State its relative mass and charge.
Answer
The neutron is a neutral particle found in the nucleus of an atom. Its relative mass is $1u.$ It has no charge.
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Question 51 Mark
Which part of an atom was discovered by Rutherford’s alpha particle scattering experiment?
Answer
Nucleus was discovered by Rutherford in the alpha particle scattering experiment.
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Question 61 Mark
Explain why, the atomic masses of many elements are in fractions and not whole numbers.
Answer
The fractional atomic masses of elements are due to the existence of their isotopes having different masses.
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Question 71 Mark
The atomic number of an element $X$ is $16.$ Write down the electronic configuration of $X.$
Answer
Atomic No. of $X = 16$ $E.C.$ of $X = (2, 8, 6)$
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Question 81 Mark
Name the scientist who discovered the nucleus of atom.
Answer
Rutherford discovered nucleus of an atom.
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Question 91 Mark
Write the distribution of electrons in an atom of element whose atomic number is $18.$ What is special about the outermost electron shell (or valence shell) of the atom of this element?
Answer
Atomic No: $18$ Electronic configuration: $(2, 8, 8)$ The special thing about the outermost shell is that it is completely filled with the electrons.
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Question 101 Mark
What do we call this number which is characteristic of a particular element?
Answer
Atomic no. is characteristic for any particular element.
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Question 111 Mark
What is the relationship between an atom containing $11$ protons, $11$ electrons ad $11$ neutrons, and another atom containing $11$ protons, $11$ electrons and $12$ neurtrons$?$
Answer
Isotopes.
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Question 121 Mark
What are valence electrons? Where are valence electrons situated in an atom?
Answer
The number of electrons present in the valence shell are called valence electrons. Valence electrons are situated in the outermost shell.
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Question 131 Mark
Name the particles used by Rutherford in his experiment on the discovery of nucleus. Also state the charge on these particles.
Answer
Alpha-particles were used by Rutherford in his experiment on the discovery of nucleus. Alpha-particles have $+2$ units of charge.
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Question 151 Mark
Which shell of an atom can accommodate a maximum of:
  1. $8$ electrons$?$
  2. $32$ electrons$?$
Answer
  1. L shell can accommodate maximum of $8 \mathrm{e}^{-}$
  2. N shell can accommodate maximum of $32 \mathrm{e}^{-}$
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Question 161 Mark
What is the number of valence electrons in the atoms of an element having atomic number $13?$ Name the valence shell of this atom.
Answer
There are $3$ valence electrons present in the element with atomic no. $13.$
Valence shell of this atom is $M.$
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Question 171 Mark
An element has an atomic number $12.$ How many electrons will be present in the $K, L$ and $M$ energy shells of its atom$?$
Answer
Electronic configuration of element with atomic no. $12-(2, 8, 2)$ So, $K-2, L-8, M-2$
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Question 181 Mark
What is the nucleus of an atom and what is the nature of charge on it?
Answer
Nucleus is a small positively charged part at the center of an atom. Nucleus is positively charged.
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Question 191 Mark
What name is given to those isotopes which have unstable nuclei and emit various types of radiations?
Answer
Radioactive isotopes have unstable nuclei and emit various types of radiations.
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Question 201 Mark
What is the reason for the different atomic masses of the isotopes of an element?
Answer
The difference in the masses of isotopes of an element is due to different number of neutrons in their nuclei.
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Question 211 Mark
Which of the following are isotopes and which are isobars? Argon, Deuterium, Calcium, Tritium, Protium.
Answer
Deuterium, Protium and Tritium are isotopes. Argon and Calcium are isobars.
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Question 221 Mark
What conclusion is obtained from the fact that all the gases form cathode rays?
Answer
The conclusion is that all the atoms contain negatively charged particles called electrons.
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Question 231 Mark
What is the absolute mass and charge of an electron?
Answer
Absolute mass of electron is $9 \times 10^{-28} \mathrm{Kg}$
Absolute charge on electron is $1.6 \times 10^{-19} \mathrm{C}$
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Question 241 Mark
What is the general name of the elements having 8 electrons in the valence shell of their atoms?
Answer
Noble gases.
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Question 251 Mark
Name the gas which produces anode rays consisting of protons in the discharge tube experiment.
Answer
Hydrogen gas produces anode rays consisting of protons in the discharge tube experiment.
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Question 261 Mark
Name the central part of an atom where protons and neutrons are held together.
Answer
Nucleus is the central part of an atom in which protons and neutrons are held together.
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Question 271 Mark
All the gases form cathode rays and anode rays when electricity is passed through them: What does the formation of cathode rays tell us about the atoms?
Answer
Formation of cathode rays tells about the presence of negatively charged electrons in all the atoms.
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Question 281 Mark
For the symbols $H, D$ and $T,$ write the subatomic particles $($protons, neutrons and electrons$)$ found in each one of them.
Answer
$H–1$ proton, $1$ electron and no neutron. $D–1$ proton, $1$ electron and $1$ neutron. $T–1$ proton, $1$ electron and $2$ neutrons.
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Question 291 Mark
What is the relation between the atomic number and mass number of an element?
Answer
Mass No. = Atomic no. + No. of neutrons
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Question 301 Mark
If an element $M$ has mass number $24$ and atomic number $12,$ how many neutrons does its atom contain$?$
Answer
No. of neutrons $=$ Mass No. $-$ Atomic no.
$= 24 - 12$
$= 12$
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Question 311 Mark
Which of the nuclear particles is present in the same fixed number in the atoms of any particular element?
Answer
Proton is present in same fixed no. in the atoms of any particular element.
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Question 321 Mark
Compare an electron, a proton and a neutron in respect of their relative masses and charges.
Answer
Electron has relative mass of $\frac{1}{1840\text{u}}$ proton has $1u$ and neutron also has $1u.$
Electron has relative charge of $-1u,$ proton has $+1u$ and neutron has $0$ relative charge.
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Question 331 Mark
What is the maximum number of electrons which can be accommodated in the:
Innermost shell of an atom?
Answer
Maximum of $2 \mathrm{e}^{-}$ can be accommodated in innermost shell of an atom.
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Question 341 Mark
Name the particles which actually determine the mass of an atom.
Answer
Protons and neutrons actually determine the mass of atom.
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Question 351 Mark
What name is given to the pair of atoms such as $^{14}_\text{ 7}\text{N}\ \text{and }^{15}_\text{ 7}?$
Answer
The given pair are isotopes.
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Question 361 Mark
Explain why $^3_1\text{H}\text{ and }^3_2\text{He}$ are not considered isotopes.
Answer
$^{3}{\text{H}_1}\text{ and }^3\text{He}_2$ are not consideredas isotopes because they do not have same atomic number.
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Question 371 Mark
Give one similarity and one difference between a pair of isotopes.
Answer
Similarity: A pair of isotopes have same atomic number. Difference: A pair of isotopes have different mass numbers.
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Question 381 Mark
Given that the percentage abundance of the isotope is $90\%$ and that of the isotope is $10\%,$ calculate the average atomic mass of neon.
Answer
$\text{Atomicmass}=\sum\text{Mass no.}\times\%\text{ of that isotope}$$\text{Atomicmass}=20\times\frac{90}{100}+2\times\frac{10}{100}$
$\text{Atomicmass}=20.2\text{u}$
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Question 391 Mark
Which radioactive isotope is used to determine the activity of thyroid gland$?$
Answer
lodine$-131$ radioisotope is used to determine the activity of thyroid gland.
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Question 401 Mark
Give one example of a radioactive isotope which is used as a fuel in the reactors of nuclear power plants.
Answer
Uranium$-235$ isotope is used as a fuel in the reactors of nuclear power plants for generating electricity.
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Question 411 Mark
Which noble gas has less than 8 electrons in the valence shell of its atom$?$ What is this number$?$
Answer
Helium has less than $8$ electrons in the valence shell of an atom. Its atomic no. is $2$
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Question 421 Mark
The number of protons, neutrons and electrons in particles $A$ to $E$ are given below:
Particle
Protons
Neutrons
Electrons
$A$
$17$
$18$
$17$
$B$
$3$
$4$
$2$
$C$
$18$
$22$
$18$
$D$
$17$
$20$
$17$
$E$
$9$
$10$
$10$
Give reasons, find a pair of isotopes from the above particles.
Answer
Here, $A$ and $D$ are isotopes as they have the same number of protons but different number of neutrons.
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Question 431 Mark
What is the maximum number of electrons an $M$ shell of the atom can accommodate$?$
Answer
Maximum of $18 \mathrm{e}^{-}$ can be accommodated in $M$ shell of an atom.
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Question 441 Mark
State one use of radioactive isotopes in medicine.
Answer
Radioactive isotopes are used in the treatment of cancer. One such isotope is Cobalt$-60.$
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Question 451 Mark
What name is given to those atoms which contain the same number of protons and electrons but different number of neutrons$?$
Answer
Atoms containing same number of protons and electrons but different number of neutrons are called $ISOTOPES.$
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Question 471 Mark
What is the maximum number of electrons which can be accommodated in the $K$ shell of an atom$?$
Answer
Maximum of $2 \mathrm{e}^{-}$ can be accommodated in $K$ shell of an atom.
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Question 481 Mark
What is the electronic configuration of a hydrogen atom$?$
Answer
Electronic configuration of hydrogen $: 1$
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Question 491 Mark
Compare a proton and a neutron in respect of mass and charge.
Answer
Proton has relative mass of $1u$ and neutron also has relative mass of $1u.$ Proton has relative charge of $+1u$ and neutron has no relative charge.
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Question 501 Mark
Name the radioactive isotope which is used in the treatment of cancer.
Answer
Cobalt$-60$ radioisotope is used in the treatment of cancer.
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