Chemistry Class 9 Notes | Short Question Answers of Chapter 2 | Structure of atom ~ Study Notes, Exams Guide, and Carrier Paths

Here in this article, students of Classes 9, 10, 11, and 12 will find the answers to some important short questions related to the structure of atoms. These question-answers can be used as study notes/study material for both teachers as well as for school and college-level students.

Besides this, these notes also provide help to those students or learners who are preparing themselves for some type of competitive examinations, such as entry test preparation, EATA tests, job interviews, and many more like these.

Write down a brief note on Dalton's atomic theory.

Dalton's atomic theory

In the year 1808 John Dalton presented his famous theory about the structure of the atom in his book named “A new system of chemical philosophy”.

Following are the main points of Dalton’s atomic theory.

The matter is composed of small indivisible and invisible particles called atoms.
Atom can neither be created nor destroyed.
Atoms of an element arm are identical in all respects i.e. in size, shape, mass, and also in other properties.
Atoms of different elements are different in their properties.
Atoms combine in a simple whole number ratio to form compounds.
All chemical reactions are due to the combination or separation of atoms.

This theory was accepted by the scientists of the 19^th century, but at the end of the 19^th century a lot of investigations were carried out and so the theory was modified, however, it still become the basis of modern Chemistry.

Describe Rutherford’s experiment. What did he conclude from his experiment?

Rutherford experiment

In the year 1911 Rutherford was studying the radiation given out by the radioactive element Polonium. He bombarded Alpha particles (20,000), which carry a double positive charge, on thin Gold foil (0.00004 cm). To detect the rays he was having a moveable microscope and Zinc Sulphate Alpha particles detector. Rutherford observed the following three types of rays

Some of the rays (2) were bounced

Conclusion. From these rays, he concluded that there are some positively charged particles in an atom, which are acutely Protons. It happened so because both the ∝-particles and protons have the same positive charge so they repel each other.

Some of the rays (8) deflected at certain angles

Conclusion. From the rays that were deflected at certain angles, he concluded that there are some negatively charged particles in an atom, which are acutely Electrons. It was observed because of the production of fluorescence at plate time by time.

Most of the rays (19,990) passed undeflected

Conclusion. From the rays that were passed undeflected, he concluded that most of the space in an atom is empty.

Describe Rutherford's atomic model.

Rutherford's atomic model

Based on his experiment Rutherford presented the following model for the structure of the atom.

Atoms consist of a positively charged central portion called nucleus. It contains protons and neutrons.
The negatively charged particles called Electrons revolve around the nucleus as planets revolve around the sun in our solar system.
The electrons revolving around the nucleus would require centripetal force. The attractive force of the nucleus on electrons provides centripetal force to the electrons.
The size of the nucleus of an atom is small as compared to the size of an atom and most of the mass of an atom is present in its nucleus.
An atom is a neutral particle. As the number of electrons is numerically equal to that of protons.

State the objections or shortcomings of Rutherford's atomic model.

Objections raised on Rutherford's atomic model

Following objections rose against Rutherford's atomic model.

Rutherford’s atomic model is based on the laws of motion and gravitation, which are applicable to neutral bodies and not to charged bodies.
According to Maxwell's theory, electrons being charged particles must radiate energy continuously and ultimately spiral into the Nucleus. But not happen so.
As the electrons revolve around the Nucleus continuously so the spectrum formed should be continuous but acutely we get a line spectrum.
It does not provide any explanation of the chemical properties of the elements.

Briefly discuss the fundamental particles of an atom.

Fundamental particles of an atom

Modern research showed that an atom consists of many subatomic particles. Three subatomic particles Protons, neutrons, and electrons are very important to chemists. These particles are the fundamental particles.

1. Electron

An electron is a negatively charged particle. Its mass is equal to 0.000548597 amu or 〖9.11 x 10-31 kg〗kg. The charge of an electron is 1.602 x 〖10-19〗coulomb with a negative sign. Electrons are very light small particles that revolve around the nucleus in orbits.

2. Proton

Proton is a positively charged particle. Its mass is equal to 1.0072766 amu or 〖1.6726 x 10-27 kg〗. The charge of the proton is 〖1.6022 x 10-19〗coulomb with a positive sign. A proton is 1837 times heavier than an electron. Proton is present in the nucleus of an atom.

3. Neutron

Neutron is a neutral particle because it has no charge. Its mass is equal to 1.0086654 amu or 〖1.6749 x 10-27〗kg. A neutron is 1842 times heavier than an electron. Neutrons are present in the nucleus of an atom.

Define and explain the term electronic configuration.

Electronic Configuration

The arrangement of electrons in shells and sub-shells of an atom is called Electronic Configuration. (OR)

The distribution or arrangement of electrons around the nucleus in orbits or shells is called electronic configuration.

Explanation

As we know from Bohr's atomic theory that electrons are distributed in shells these shells are designated by the letters K, L, M, N, O, and so on. For K-shell n=1 and for L-shell n=2 and so on.

Distribution of electrons

The distribution of electrons can be given by the 2n² rule. Here n means a number of shells. For example, if we have to find how many electrons are there in the 3^rd shell, we have to put n=3 in the above formula i.e.

2n²= 2(3)²= 2×9 =18.

So there are 18 electrons in 3^rd Shell. Similarly, we can find the number of electrons in any particular shell by changing the value of n in the above formula.

Define energy levels and energy sub-levels.

Energy levels

Energy levels, also known as electron shells, are concentric regions around the nucleus of an atom where electrons are likely to be found.

Explanation

The word energy level was first used by Bohr in his atomic theory, these energy levels are assigned by letters K, L, M, N, O, and so on, these are called energy levels and are the major paths for revolving electrons. For K-shell n=1, for L-shell n=2, for M-shell n=3, and so on. The maximum number of electrons that an energy level can accommodate is given by the 2n² rule. Here n means the number of shells.

Energy sub-levels

Energy sublevels, also known as orbital, are regions within an energy level where electrons are likely to be found. They are designated by the angular momentum quantum number (l) and have a specific shape, such as s, p, d, and f. The number of sublevels within each energy level is determined by the value of n.

Explanation

There are four sub-shells. These sub-shells are represented by small letters s, p, d, and f. these are the spectroscopic terms that stand for sharp, principal, diffused, and fundamental. The increasing order of energy of the sub-shells belonging to different shells is given below

1s<2s<2p<3s<3p<4s<3d<…and so on

Each Energy level has one or more energy sub-levels as given in the table.

Energy levels	Number of electrons in Energy levels	Energy sub-levels in each energy levels	Number of electrons in energy sub-levels
K	2	S	s =2
L	8	s and p	p = 6
M	18	s, p and d	d = 10
N	32	s, p, d and f	f = 14

Auf Bau principle

Electrons are accommodated in the different energy levels by using Auf Bau principle. According to this principle, electrons fill first the lowest energy sub-shell available to it and then the higher energy sub-shells. According to this principle, electrons will first fill 1s, 2s, 2p, 3s, 3p,4s and then 3d, because the energy of 4s is lower than the 3d sub-shell.

Write down the electronic configuration of the elements from atomic No 1 to Atomic No 20.

Electronic configuration of first 20 elements in the periodic table

Name	Symbol	Atomic number	Electronic configuration
Hydrogen	H	1	1s¹
Helium	He	2	1s²
Lithium	Li	3	1s², 2s¹
Beryllium	Be	4	1s², 2s²
Boron	B	5	1s², 2s², 2p¹
Carbon	C	6	1s², 2s², 2p²
Nitrogen	N	7	1s², 2s², 2p³
Oxygen	O	8	1s², 2s², 2p⁴
Fluorine	F	9	1s², 2s², 2p⁵
Neon	Ne	10	1s², 2s², 2p⁶
Sodium	Na	11	1s², 2s², 2p⁶, 3s¹
Magnesium	Mg	12	1s², 2s², 2p⁶, 3s²
Aluminum	Al	13	1s², 2s², 2p⁶, 3s², 3p¹
Silicon	Si	14	1s², 2s², 2p⁶, 3s², 3p²
Phosphorus	P	15	1s², 2s², 2p⁶, 3s², 3p³
Sulphur	S	16	1s², 2s², 2p⁶, 3s², 3p⁴
Chlorine	Cl	17	1s², 2s², 2p⁶, 3s², 3p⁵
Argon	Ar	18	1s², 2s², 2p⁶, 3s², 3p⁶
Potassium	K	19	1s², 2s², 2p⁶, 3s², 3p⁶, 4s¹
Calcium	Ca	20	1s², 2s², 2p⁶, 3s², 3p⁶, 4s²

Define and explain the term Isotopes.

Isotopes

The atoms of the same elements having the same atomic number but different atomic masses are called Isotopes of that element.

For example, there are three Isotopes of Carbon i.e.¹²C₆, ¹³C_6, and ¹⁴C₆, the atomic numbers of these Isotopes are the same i.e. 6 while atomic masses are different i.e. 12, 13, and 14 respectively.

Explanation

The following points are important for describing Isotopes.

In all Isotopes, the atomic masses are different because of the different numbers of Neutrons in them, while electrons and protons are the same.
Isotopes have the same chemical properties because of the same number of valence electrons.
Isotopes have different physical properties because of different atomic masses.

Describe the isotopes of Hydrogen with a diagram.

Isotopes of Hydrogen

There are three Isotopes of Hydrogen i.e. Protium, Deuterium, and Tritium, they are discussed below as

1. Protium

Common name Hydrogen

Symbol ¹H₁ or ¹P₁

Atomic number 1

Atomic mass 1

Number of electrons 1

Number of protons 1

Number of Neutrons 0

Percentage abundance

99.985 %

2. Deuterium

Common name Heavy Hydrogen

Symbol ²H₁ or ²D₁

Atomic number 1

Atomic mass 2

Number of electrons 1

Number of protons 1

Number of Neutrons 1

Percentage abundance 0.015 %

3. Tritium

Common name Heavy Hydrogen

Symbol ³H₁ or ³T₁

Atomic number 1

Atomic mass 3

Number of electrons 1

Number of protons 1

Number of Neutrons 2

Percentage abundance rarely found in Nature

Similarities among the isotopes. All the above Isotopes have the same chemical properties. They also have the same number of electrons and protons. Differences among the isotopes. All the above Isotopes have different physical properties like melting point, boiling points, and density, etc. They also have different numbers of Neutrons.

Write a short note on the Isotopes of Carbon.

Isotopes of Carbon

There are three Isotopes of Carbon i.e. C-12, C-13, and C-14. All of these have 6 electrons and protons because the atomic number of Carbon is 6, but the number of neutrons is different. Similarly, all these Isotopes have the same chemical but different physical properties.

Write a note on the isotopes of Chlorine and Uranium.

Isotopes of Chlorine and Uranium

Isotopes of Chlorine
Isotopes name	Symbol	At.No (Z)	At.mass (A)	No of e^-	No of P⁺	No of N (A - Z)	%age abundance
Chlorine-35	Cl-35	17	35	17	17	18	75.53%
Chlorine-37	Cl-37	17	37	17	17	20	24.47%
Isotopes of Uranium
Isotopes name	Symbol	At.No (Z)	At.mass (A)	No of e^-	No of P⁺	No of N (A - Z)	%age abundance
Uranium-234	U-234	92	234	92	92	142	0.05%
Uranium-235	U-235	92	235	92	92	143	0.75%
Uranium-238	U-238	92	238	92	92	146	99.245 %

Write down some of the uses of Isotopes.

Uses of Isotopes

Isotopes are used in chemical, agricultural, and medical research for the diagnosis and treatment of diseases. Isotopes of certain elements show radioactivity while others do not. Some of the uses of isotopes are given as

Iodine-131 (⁵³I₁₃₁) becomes concentrated in the Thyroid gland and is used to cure goiter.
Iodine-123 (⁵³I₁₂₃) is used for brain imaging.
Heavy hydrogen (Deuterium), heavy Carbon (C-13), heavy Nitrogen (N-15), Iodine-131, and heavy Oxygen (O-18) are also used as tracer elements in biochemical and physiochemical research to trace the path of the element to find the defective or obstructive part of the body.
Radium irradiation and Cobalt-60 are used in the treatment of cancer and for the diagnosis of tumors.
Sodium (Na-24) is used for the identification of blood circulation problems in patients.
Carbon-14 is used to trace the path of carbon in photosynthesis.
American-241 is used in many smoke detectors for homes and businesses and to help determine where oil wells should be drilled.
Californium-252 is used to inspect airline luggage for hidden explosives, and to measure the moister contents of soil in road construction and building industries.
Krypton-85 is used in indicator lights in appliances such as clothes washers and dryers, stereos, etc.
Uranium and Carbon-14 are used in the Carbon dating of plants and animals.
Estimation of the ages of plants and animal remains by measuring the amount of radioactive decay product is called Carbon dating.