bise Rawalpindi chapter 2 Structure of Atoms Chemistry Notes Class 9, short questions, long question,n and Mcqs.
Structure of Atoms Bise Rawalpindi Class 9 Chemsitry
Table of Contents
i) Which one of the following results in the discovery of proton
- A. cathode rays
- B. canal rays
- C. X-rays
- D. alpha rays.
ii) Which one of the following is the most penetrating.
- A. protons
- B. electrons
- C. neutrons
- D. alpha particles
iii) The concept of orbit was used by
- A. J. J. Thomson
- B. Rutherford
- C. Bohr
- D. Planck
iv) Which one of the following shells consists of three subshells.
- A. O shell
- B. N shell
- C. L shell
- D. M shell
v) Which radioisotope is used for the diagnosis of tumors in the body?
- A. cobalt-60
- B. iodine-131
- C. strontium-90
- D. phosphorus-32
vi) When U-235 breaks up, it produces:
- A. electrons
- B. neutrons
- C. protons
- D. nothing
vii) The p subshell has:
- A. one orbital
- B. two orbitals
- C. three orbitals
- D. four orbitals
viii) Deuterium is used to make:
- A. light water
- B. heavy water
- C. soft water
- D. hard water
ix) The isotope C-12 is present in the abundance of:
- A. 96.9 %
- B. 97.6 %
- C. 99.7 %
- D. none of these
x) Who discovered the proton:
- A. Goldstein
- B. J. J. Thomson
- C. Niels Bohr
- D. Rutherford
What is the structure of an atom?
An atom contains three basic particles namely protons, neutrons, and electrons. The nucleus of the atom contains protons and neutrons where protons are positively charged and neutrons are neutral. The electrons are located in the outermost region called the electron shell.
Why is the electronic structure of an atom important?
This allows us to determine which orbitals are occupied by electrons in each atom. The specific arrangement of electrons in orbitals of an atom determines many of the chemical properties of that atom.
Q.1) What is the nature of a charge on cathode rays?
Answer:
Cathode rays repel from the negatively charged cathode and are attracted to the positively charged anode. Therefore, they are negatively charged particles.
Q.2) Give five characteristics of cathode rays.
Answer:
Characteristics of cathode rays
i) They travel in straight lines from the cathode towards the anode.
ii) They cast shadows if some target is placed in their path.
iii) When subjected to an electrical field, they are deflected towards the positive plate.
iv) They possess a heating effect and raise the temperature of the body on which they fall.
v) They cause fluorescence in the phosphor-coated glass walls of the discharge tube.
Q.3) The atomic symbol of a phosphorus ion is given as 15P31 3-
a) How many protons, electrons, and neutrons are there in the ion?
b) What is the name of the ion?
c) Draw the electronic configuration of the ion.
d) Name the noble gas which has the same electronic configuration as the phosphorus ion.
Answer:
a)
Number of protons = 15
Number of electrons = 15 + 3 = 18
Number of neutrons = 31 -15 = 16
b)
P3- is called phosphide ion.
c)
P3- has 18 electrons arranged in the shells in the following order:
1s2 2s2 2p6 3s2 3p6
K L M
So, the electronic configuration is as follows:
d)
Argon (Ar) has the same electronic configuration as the phosphorus ion has.
Q.4) Differentiate between shell and subshell with examples of each.
Answer:
| Shells | Sub-shells |
| Shell is the main energy level where the electrons revolve around the nucleus. It makes up the electronic configuration of an atom. | Shell is made up of one or more sub-shells. |
| Shells are labeled K, L, M, N, and so on. | Sub-shells are labeled s, p, d, and f. |
Q.5) An element has an atomic number 17. How many electrons are present in the K, L, and M shells of the atom?
Answer:
Atomic number = 17
Electronic configuration
1s2 2s2 2p6 3s2 3p5
K L M
Thus,
K = 2 electrons
L = 8 electrons
M = 7 electrons
Q.6) Write down the electronic configuration of Al3+. How many electrons are present in its outermost shell?
Answer:
Al has 13 electrons. Al3+ ion is formed by losing 3 electrons. So Al3+ ion has 10 electrons.
Electronic configuration:
1s2 2s2 2p6
K L
Outermost shell ‘L’ has 8 electrons in it.
Q.7) Magnesium has an electronic configuration 2, 8, 2,
a) How many electrons are in the outermost shell?
b) In which subshell of the outermost shell electrons are present?
c) Why does magnesium tend to lose electrons?
Answer:
a)
There are 2 electrons in the outermost shell.
b)
Electronic configuration of Magnesium:
1s2 2s2 2p6 2s2
Electrons are present in the s-subshell of the outermost shell.
c)
Magnesium can get a full outermost shell by either losing two electrons or by gaining six. Losing electrons requires the transfer of fewer electrons so it is easier to lose electrons than to gain six electrons.
Q.8) What will be the nature of a charge on an atom when it loses an electron or when it gains an electron?
Answer:
When an atom loses an electron, it becomes a positively charged ion and is called a cation. For example, sodium (Na) loses an electron to become Na+.
When an atom gains an electron, it becomes a negatively charged ion and is called an anion. For example, chlorine (Cl) gains an electron to become Cl–.
Q.9) For what purpose U-235 is used?
Answer:
U-235 produces a tremendous amount of energy by nuclear fission reactions, which are used to generate electricity. It can be used as fuel for nuclear power plants and the nuclear reactors that run naval ships and submarines. It can be used in nuclear weapons too.
Q.10) A patient has a goiter. How will it be detected?
Answer:
The radioactive isotope of Iodine-131 can be used to detect goiter in the patient. It acts as a tracer to diagnose the presence of any tumor.
Q.11) Give three properties of positive rays.
Answer:
Properties of positive rays
i) These rays travel in a straight line opposite to cathode rays.
ii) These rays are positively charged because they deflect in an electric field towards the negatively charged plate.
iii) Mass of these particles is equal to that of a proton or a simple multiple of it.
Q.12) What are the defects of Rutherford’s atomic model?
Answer:
Defects of Rutherford’s atomic model
i) According to classical electromagnetic theory, being a charged particle electron must emit energy. Because of this loss of energy, the electron should ultimately fall into the nucleus. If it happens, the atom should collapse. But in actual practice, this does not happen. Atoms are stable.
ii) If the electrons emit energy continuously, they should form a continuous spectrum. But actually, the line spectrum is obtained.
Q.13) As long as an electron remains in an orbit, it does not emit or absorb energy. When does it emit or absorb energy?
Answer:
When an electron jumps from a higher orbit to a lower orbit it emits energy. When an electron jumps from a lower orbit to a higher orbit it absorbs energy.
Long Questions
Q.1) How are cathode rays produced? What are its five major characteristics?
Answer:
Production of cathode rays
In 1895, Sir William Crookes carried out a series of investigations by passing an electric current through glasses in a discharge tube at a pressure of 10-4 atm. He was the first scientist to design the discharge tube.
He took a long glass tube having two metal plates connected to oppositely charged poles of a battery. The pressure inside the tube could be adjusted by the vacuum pump. When a high voltage current was passed through the gas, shiny rays were emitted from the cathode traveling towards the anode. Since the rays originated from the cathode, they were given the name cathode rays.
Characteristics of cathode rays
i) They travel in straight lines from the cathode towards the anode.
ii) They cast shadows if some target is placed in their path.
iii) When subjected to an electrical field, they are deflected towards the positive plate.
iv) They possess a heating effect and raise the temperature of the body on which they fall.
v) They cause fluorescence in the phosphor-coated glass walls of the discharge tube.
Q.2) How was it proved that electrons are fundamental particles of an atom?
Answer:
It was observed that cathode rays cast a shadow if an opaque object is placed in its path. This led scientists to a conclusion that these are not rays but rather fast-moving material particles. These particles were given the name electrons. All the materials produce the same particles because the nature of cathode rays is independent of the nature of the gas used in the tube. This means all the materials consist of electrons. And materials are made up of atoms, so electrons are fundamental particles of an atom.
Q.7) What do you mean by electronic configuration? What are the basic requirements while writing the electronic center of an element (atom)?
Answer:
Electronic configuration
Every element consists of atoms, which are composed of protons, neutrons, and electrons. Electrons are negatively charged and revolve around the nucleus of the atom in different energy levels or shells. Shells further consist of sub-shells.
The electronic configuration of an atom is the representation of the arrangement of electrons distributed around the nucleus among the shells and subshells according to their increasing energy. Electronic configuration represents the orbitals of an atom in its ground state as well as an atom that has ionized into a cation or anion. Many of the physical and chemical properties of elements can be correlated to their unique electronic configurations.
The electrons fill the shells and subshells in order of their increasing energy. The lower energy level gets filled first, then the higher energy level, and so on. Shells can accommodate electrons as follows:
K shell can accommodate 2 electrons
L shell can accommodate 8 electrons
M shell can accommodate 18 electrons
N shell can accommodate 32 electrons
Sub-shells can accommodate electrons as follows:
s subshell can accommodate 2 electrons
p subshell can accommodate 6 electrons
Basic requirements while writing electronic configuration
The following things should be known:
(i) Number of electrons in atom/ion.
(ii) The maximum number of electrons a shell or subshell can accommodate.
(iiI) The order of shells and subshells according to the energy levels.
Q.8) Describe the electronic configuration of Na+, Mg2+center, and Al3+ ions. Do they have the same number of electrons in the outermost shell?
Answer:
i) Na+
Number of electrons = 11 – 1 = 10
Electronic configuration:
1s2, 2s2, 2p6
ii) Mg2+
Number of electrons = 12 – 2 = 10
Electronic configuration:
1s2, 2s2, 2p6
iii) Al3+
Number of electrons = 13 – 3 = 10
Electronic configuration:
1s2, 2s2, 2p6
Yes, they all have the same number of electrons in their outermost shell i.e. 8 electrons.
Q.9) Give the applications of isotopes in the field of radiotherapy and medicines.
Answer:
Applications of isotopes in radiotherapy
Isotopes are used for the treatment of various kinds of cancer. For example:
(i) Isotopes like P-32 and Sr-90 are used to treat skin cancer because they emit less penetrating beta radiations.
(ii) Cobalt-60 emits strongly penetrating gamma rays, therefore, is used to irradiate cancer cells in order to kill or shrink the tumors.
Applications of isotopes in medicine
Radioactive isotopes act as tracers to detect the presence of tumors in the human body. For example:
(i) Radioactive iodine-131 is used as a tracer in diagnosing thyroid problems.
(ii) Technetium is used to monitor bone growth.
What are the features of ‘The Thomas atomic model’?
J.J. Thomson’s experiments with cathode-ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom.
Who was ‘Ernest Rutherford’?
Ernest Rutherford was a New Zealand physicist who came to be known as the father of nuclear physics.
What did ‘Neils Bohr’ discover?
Niels Bohr proposed a theory for the hydrogen atom, based on quantum theory that some physical quantities only take discrete values.
