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Question 11 Mark

What is the number of valence electrons in the chlorine atom$?$

Answer

You know this by looking at the groups (vertical) of the periodic table. Chlorine is in the Halogens $($Group $17)$ so it has a $- 1$ charge $($meaning $7$ valence electrons; one short of $8,$ which makes a full octet$)$

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Question 21 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 31 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 41 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 51 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 61 Mark

What is the number of sub-atomic particles in this isotope$?$

Answer

Sub particles in Uranium $- 235$ are: Protons $= 92$ Neutrons $= 143$ Electrons $= 92$

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Question 71 Mark

Name the discoveror of electrons, protons and neutrons.

Answer

Neutron: The neutron was discovered in $1932$ by the English physicist James Chadwick.
Proton: Goldstein in $1886$
Electron: J.J Thomson n in $1897$

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Question 81 Mark

Is it possible for the atom of an element to have one electron, one proton and no neutron. If so, name the element.

Answer

Yes, it is true for the common, stable isotope of hydrogen (protium) which is represented as $1^{1\text{H}}.$

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Question 91 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 101 Mark

If an atom of an element has atomic number $= 15$ and, mass number $= 31,$ find the number of protons, electrons, neutrons in its atoms.

Answer

The atomic number for any element is the number of protons in its atomic nuclei. In a neutral atom, the number of electrons are the same as the number of protons. The atomic mass $($mass number$)$ is the sum of the protons and neutrons in the nucleus. Mass number can vary due to the fact that the number of neutrons can vary. Isotopes are named for their mass numbers. So, this element has $15$ protons $($atomic number$),$ $15$ electrons, and $16$ neutrons
$($mass number minus atomic number$).$ This isotope is phosphorus$-31,$ which has atomic number $15.$

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Question 111 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 121 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 131 Mark

Name the scientist who discovered the nucleus of atom.

Answer

Rutherford discovered nucleus of an atom.

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Question 141 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 151 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 161 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 171 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 181 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 191 Mark

Name the scientist who discovered electron.

Answer

J. J. Thomson discovered electron.

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Question 201 Mark

Which shell of an atom can accommodate a maximum of:

$8$ electrons$?$
$32$ electrons$?$

Answer

a. $L$ shell can accommodate maximum of $8 \mathrm{e}^{-}$
b. $N$ shell can accommodate maximum of $32 \mathrm{e}^{-}$

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Question 211 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 221 Mark

Does the nucleus contain neutrons also?

Answer

The nucleus, that dense central core of the atom, contains both protons and neutrons. Electrons are outside the nucleus in energy levels. Protons have a positive charge, neutrons have no charge, and electrons have a negative charge. Atoms of the same element that have differing numbers of neutrons are called isotopes.

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Question 231 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 241 Mark

How are the following pairs of atoms related? $ _8\text{X}^{16},\ _8\text{X}^{17},\ _{18}\text{Y}^{40},\ _{20}\text{Z}^{40}$

Answer

Elements $_8\text{X}^{16}$ and $_8\text{X}^{17}$ are isotopes because of a same number of protons.Elements $_{18}\text{Y} ^{40}$ and $_{20}\text{Z}^{40}$ are isobars because of the same mass number.

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Question 251 Mark

Make a schematic atomic structure of Magnesium of Phosphorus. (Given number of protons of Magnesium $= 12$, Phosphorus $= 15).$

Answer

Atomic structure of Mg:

Electronic configuration of Mg. $K L M _{12}Mg = 2, 8, 2$ Atomic structure of P:

 Electronic configuration of $P K L M _{15}P = 2, 8, 5$

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Question 261 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 271 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 281 Mark

What is the similarity in the electronic structure of the following set of atoms$?$

Lithium.
Sodium.
Potassium.

Answer

$Li: 2, 1$
$Na: 2, 8, 1$
$K: 2, 8, 8, 1$

Clearly, all of these have one electrons each in their outermost shells.
The main points of similarity are:

They all have an unpaired electron in an s orbital.
They all have an unpaired electron with a magnetic quantum number, $ml = 0.$
They all have an unpaired electron with an angular momentum quantum number, $l = 0.$

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Question 291 Mark

What are canal rays$?$

Answer

Canal rays are positively charged radiations. These rays consist of positively charged particles known as protons. They were discovered by Goldstein in $1886.$

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Question 301 Mark

The number of electrons in the outermost shell of an element is $5.$ What is its valency and why$?$

Answer

Valency is the combining power of an element, especially as measured by the number of electrons it has to share, loose or gain in order to form an octet. Since the given element has $5$ electrons in its outermost shell, then to complete octet, it has to gain $3$ electrons . Hence the valency of the element $= 3$

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Question 311 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 321 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 331 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 341 Mark

Name the three sub-atomic particles of an atom.

Answer

The three sub-atomic particles of an atom are:

Protons.
Electrons.
Neutrons.

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Question 351 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 361 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 371 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 381 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 391 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 401 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 411 Mark

An element $X$ has $5$ electrons in its $M-$shell. What is its atomic number$?$

Answer

If the element has $5$ electrons in the $M$ shell, it is in the $3rd$ period of Periodic Table and $15th$ group.
The element in $3rd$ period and $15th$ group is Phosphorous.
Hence $"X"$ is Phosphorous $(P).$

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Question 421 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 431 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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MCQ 441 Mark

Put tick $(\checkmark)$ against correct choice and cross $(\times)$ against wrong choice.
Rutherford’s alpha$-$particle scattering experiment was responsible for the discovery of

✓
Atomic Nucleus.
B
Electron.
C
Proton.
D
Neutron.

Answer

Correct option: A.

Atomic Nucleus.

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Question 451 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 461 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 471 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 481 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 491 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 501 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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