Welcome to the Chapter 4 - Structure of the Atom, Class 9 Science NCERT Solutions page. Here, we provide detailed question answers for Chapter 4 - Structure of the Atom. The page is designed to help students gain a thorough understanding of the concepts related to natural resources, their classification, and sustainable development.
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Canal rays are decidedly charged radiations which prompted the disclosure of emphatically charged subatomic particles called protons.
The particle will be electrically unbiased as one – ve charge adjusts one + ve charge.
As indicated by Thomson's model of a molecule
(I) A molecule comprises of an emphatically charged circle and the electrons are implanted in it,
(ii) The negative and positive charges are equivalent in greatness. So the iota is electrically unbiased.
According to Rutherford's model of a particle, the protons which are emphatically charged are available in the core of a nucleus.
On utilizing any metal foil, the perceptions of the a-molecule dispersing examination would continue as before as all particles would have a similar construction.
Three sub-atomic particles of an atom:
(i) Proton :- Positively charged
(ii) Electron :- Negatively charged
(iii) Neautron :- neutral
Atomic mass of Helium = 4u
Atomic mass of an atom = no. of protons + no. of neutrons
4 = 2 + no. of neutrons
no. of neutrons = 4 - 2 = 2
Helium has 2 neutrons.
The distribution of electrons in Carbon and Sodium atom :
Carbon : Atomic no. of Carbon = 6
no. of protons = 6
no. of protons = no. of electrons
Distritribution of electron = K L
2 4
Sodium : Atomic no. of Sodium = 11
no. of protons = 11
no. of protons = no. of electrons
Distritribution of electron = K L M
2 8 1
K shell can hold 2 electrons and L shell can hold 8 electrons.When both the shells are full, there will be (8 + 2) 10 electrons in the atom.
Valency of a particle is the quantity of electrons acquired, lost or shared to finish the octet of electrons in the valence shell.
Valency of chloride: It has electronic design = 2, 8, 7
In this manner, one electron is acquired to finish its octet thus its valency is 1.
Valency of sulfur: Its has electronic design = 2, 8, 6
In this manner, 2 electrons are acquired to finish its octet and henceforth its valency = 2
Valency of magnesium : It has electronic design = 2, 8, 2
Accordingly, it can lose two electrons to achieve octet and thus its valency = 2
H, D, and T are the three isotopes of hydrogen with similar nuclear number and distinctive mass quantities of 1, 2 and 3 separately.
| Element | Symbol | Number of Electrons | Number of Protons | Number of Neutrons |
| Hydrogen | H | 1 | 1 | 0 |
| Deuterium | D | 1 | 1 | 1 |
| Tritium | T | 1 | 1 | 2 |
(a) Isotopes: Isotopes are iotas which have similar number of protons however extraordinary number of neutrons. They have same nuclear number yet unique mass number.
Model: Carbon particle exists as 6C12 and 6C14 however when their electronic configuration is K-2; L-4
(b) Isobars: Isobars are particles which have a similar mass number however extraordinary nuclear number.
Model: Electronic configuration of 20Ca40is – K-2; L-8; M-8; N-2 and of 18Ar40 is – K-2; L-8; M-8
Electron:
(I) Electron are available external the core of an atom.
(ii) Electron are contrarily charged.
(iii) The mass of an electron is considered to immaterial.
Proton:
(I) Proton are available in the core of a molecule.
(ii) Proton are emphatically charged.
(iii) The mass of a proton is roughly multiple times as the mass of an electron.
Neutron:
(I) Neutron are available in the core of an atom.
(ii) Neutron are unbiased.
(iii) The mass of neutron is almost equivalent to the mass of a proton.
Given, the normal atomicr mass of an example of a component X is 16.2u.
Two isotopes of component = and
Presently, Let's percent of isotope be x and percent of
be 100 - x
In this way, According to the inquiry,
Average Atomic Mass :
Subsequently the level of isotope is 90 % and the percentage isotope
is 10%.
Z (atomic no.)= 3
∴ Electronic configuration = 2, 1
Valency = 1
Name of the element is lithium.
Mass number of X = Protons + Neutrons = 6 + 6 = 12
Mass number of Y = Protons + Neutrons = 6 + 8 = 14
As the atomic number is the similar i.e., = 6.
[atomic number = number of protons].
Both X and Y are isotopes of the similar element.
(a) Atomic Nucleus
(c) different number of neutrons
(d) 2, 8, 1
Complete the following table.
| Atomic Number |
Mass Number |
Number of Neutrons |
Number of Protons |
Number of Electrons |
Name of the Atomic Species |
| 9 | - | 10 | - | - | - |
| 16 | 32 | - | - | - | Sulphur |
| - | 24 | - | 12 | - | - |
| - | 2 | - | 1 | - | - |
| - | 1 | 0 | 1 | 0 | - |
| Atomic Number |
Mass Number |
Number of Neutrons |
Number of Protons |
Number of Electrons |
Name of the Atomic Species |
|---|---|---|---|---|---|
| 9 | 19 | 10 | 9 | 9 | Fluorine |
| 16 | 32 | 16 | 16 | 16 | Sulphur |
| 12 | 24 | 12 | 12 | 12 | Magnesium |
| 1 | 2 | 1 | 1 | 1 | Hydrogen Deuterium |
| 1 | 1 | 0 | 1 | 0 | Hydrogen |
As per J.J. Thomson's model of a particle, the electrons are implanted all over in the emphatically charged circles. In any case, tests done by different researchers showed that protons are available just in the focal point of the particle and electrons are dispersed around it.
As per Rutherford's model of a molecule the electrons are spinning in a round circle around the core. Any such molecule that spins would go through speed increase and emanate energy. The spinning electron would lose its energy lastly fall into the core, the particle would be profoundly temperamental. In any case, we realize that molecules are very steady.
Bohr's model of the molecule
(1) Atoms have a nucleus in the middle.
(2) Electrons rotate around the nucleus.
(3) Certain exceptional circles known as discrete circles of electrons are permitted inside the molecule.
(4) While rotating in discrete circles the electrons don't transmit energy.
(5) These circles or shells are called energy levels.
(6) These circles or shells are represented by the letters K, L, M, N or the numbers n = 1, 2, 3, 4
Thomson's model : A molecule comprises of an emphatically accused circle of electrons installed in it.
Rutherford's model : An atom comprises of a decidedly charged molecule assembled at the middle known as the nucleus. The size of nucleus exceptionally little when contrasted with the size of the atom. The electron spin around the nucleus in clear cut circles.
Bohr's model : There are just sure circles known as discrete circles inside the atom in which electrons rotate around the nucleus. Electrons don't emanate energy while rotating.
The standards for composing of appropriation of electrons in different shells for the initial eighteen components are:
(I) The most extreme number of electrons present in a shell is given by the formula 2 n2
∵ n = orbit number i.e., 1, 2, 3
∵ Maximum number of electrons in various shells are:
K shell n = 1 2n2 => 2(1)2 = 2
L shell n = 2 2n2 => 2(2)2 = 8
M shell n = 3 2n2 => 2(3)2 = 18
N shell n = 4 2n2 => 2(4)2 = 32
(ii) The most extreme number of electrons that can be obliged in the furthest orbit is 8.
(iii) Electrons are not obliged in a given shell except if the internal shells are filled.
Valency is the consolidating limit of a molecule.
Nuclear number of oxygen = 8 Atomic number of silicon = 14 K L M
Electronic configuration of oxygen = 2 6 –
Electronic configuration of silicon =2 8 4
In the molecules of oxygen the valence electrons are 6 (i.e., electrons in the outermost shell). To fill the orbit, 2 electrons are required. In the particle of silicon, the valence electrons are 4. To fill this orbit 4 electrons are required.
Subsequently, the joined limit of oxygen is 2 and of silicon is 4.
i.e., Valency of oxygen = 2
Valency of silicon = 4
(I) Atomic number: The atomic number of a component is equivalent to the quantity of protons in the nucleus of its molecule.
e.g., Oxygen has 6 protons thus atomic no. = 6.
(ii) Mass number: The mass number of a molecule is equivalent to the quantity of protons and neutrons in its nucleus.
Nucleons = number of protons + number of neutrons
Example: Protons + Neutrons = Nucleus = Mass number = 6 + 6 = 12
(iii) Isotopes: Isotopes are molecules of a similar component which have different mass numbers however same atomic number.
(iv) Isobars: Isobars are molecules having similar mass number however different atomic numbers.
Both calcium and argon have a similar mass number yet unique atomic number.
Two uses of isotopes are:
(I) An isotope of iodine is utilized in the treatment of goiter.
(ii) An isotope of uranium is utilized as a fuel in atomic reactors.
Sodium atom (Na), has atomic number =11
Number of protons =11
Number of electrons = 11
Electronic configuration of Na = K L M – 2 8 1
Sodium atom (Na) lose 1 electron to get steady and structure Na+ particles. Thus it has totally filled K and L shells.
