Structure of Atom Class 9 NCERT Solutions – The notes provided on our website for CBSE students are designed to align with CBSE and NCERT syllabus guidelines. By referring to these notes, students can save time and gain a comprehensive understanding of important topics and questions in each chapter. This can help them better retain key concepts and be well-equipped to answer any questions that may appear on exams.
Structure of Atom Class 9 NCERT Solutions
Q. What are canal rays?
Answer – Canal rays are a type of radiation that is positively charged. The discovery of canal rays was important in the discovery of protons, which are also positively charged subatomic particles.
Q. Will an atom have any charge if it has one proton and one electron?
Answer – The proton and electron carry opposite charges, with the proton being positively charged and the electron being negatively charged. Due to this opposite charge, when an atom contains one electron and one proton, the net charge of the atom becomes neutral. In other words, the positive charge of the proton is exactly balanced by the negative charge of the electron, resulting in a neutral charge for the atom.
Q. Explain how the atom is neutral as a whole using Thomson’s model of an atom as your foundation.
Answer – According to Thomson’s model of an atom:
(i) An atom consists of a positively charged sphere, and the negatively charged electrons are embedded within it.
(ii) Electrons and protons have equal but opposite charges, and they are uniformly distributed within the atom. This results in an atom having no overall charge, making it electrically neutral.
Q. Which subatomic particle is present in an atom’s nucleus according to Rutherford’s model of an atom?
Answer – The majority of an atom’s mass is contained in the nucleus, which contains positively charged protons, according to Rutherford’s model of an atom. Similar to how planets circle around the sun, the electrons in the nucleus move in clearly defined orbits.
Q. Draw a rough sketch of Bohr’s three-shelled atom.
Answer –
Q. What do you think the results of the α-particle scattering experiment would be if a foil made of a different metal than gold was used?
Answer – Regardless of which metal foil is used in the α-particle scattering experiment, the observations remain the same because the fundamental structure of an atom, when examined on an individual level, remains constant across different elements. In other words, the behavior of α-particles is determined by the structure of individual atoms rather than the specific properties of the metal foil used as a target.
Q. Identify the three atom’s subatomic components.
Answer – The general pattern of scattering in the α-particle scattering experiment would still be observed if a metal foil other than gold is used, but the exact scattering angles and intensities of the scattered particles would be different due to variations in the atomic structure of the foil.
Q. The nucleus of a helium atom contains two protons and has an atomic mass of 4 u. What is the number of neutrons in it?
Answer – The equation to calculate the number of neutrons in an atom is:
Number of neutrons = Atomic mass – Number of protons
Using the given information, we can substitute the values as follows:
Number of neutrons = 4 u – 2
Simplifying, we get:
Number of neutrons = 2
Therefore, the helium atom with an atomic mass of 4 u and two protons in its nucleus has 2 neutrons.
Q. Describe how the electrons are distributed among the carbon and sodium atoms.
Answer –
Carbon
Atomic number = 6
No. of protons = 6
and Number of protons = Number of electrons
Distribution of electrons = KL
24
Sodium
Atomic number = 11
No. of protons = 11 = No. of electrons
Distribution of electrons = K L M
2 8 1
Q. What would be the total number of electrons in an atom if the K and L shells were both filled?
Answer – If the K and L shells of an atom are full, then the total number of electrons in the atom would be:
2 in the K shell + 8 in the L shell = 10 electrons
The K shell can hold a maximum of 2 electrons, and the L shell can hold a maximum of 8 electrons. Therefore, if both shells are full, the total number of electrons in the atom would be 10.
Q. How do you determine the valencies of magnesium, sulphur, and chlorine?
Answer – The valency of an element can be determined by subtracting the number of valence electrons in the outermost shell from 8 (for elements in groups 1, 2, 13) or from 18 (for elements in groups 3-18) since the maximum number of valence electrons an atom can have is 8 or 18.
The valency of chlorine is 7 because it has 7 valence electrons in the third energy level (3s and 3p).
The valency of sulphur can be 2, 4, or 6, depending on the chemical reaction, because it has 6 valence electrons in the third energy level (3s and 3p).
The valency of magnesium is 2 because it has 2 valence electrons in the third energy level (3s).
Q. If number of proton in atom is 8 and number of electorns is also 8, then
(i) what is the atomic number of the atom
(ii) what is the charge on the atom?
Answer – If the number of electrons in an atom is 8 and the number of protons is also 8, then:
(i) The atomic number of the atom is equal to the number of protons, which is 8. This is because the atomic number of an element is defined as the number of protons in the nucleus of an atom of that element. Therefore, if an atom has 8 protons, its atomic number is 8, which indicates that it is an oxygen atom.
(ii) The charge on the atom would be neutral or zero. This is because the charge on an atom is determined by the number of protons and electrons. In a neutral atom, the number of protons is equal to the number of electrons, and their charges cancel out, resulting in a net charge of zero. Therefore, if an atom has 8 protons and 8 electrons, it has a neutral charge.
Q. With the help of below table find out the mass number of sulphur and oxygen atom.
Answer – To determine the mass number of an atom, we add the number of protons and neutrons present in the nucleus of the atom.
(a) For oxygen:
- Number of protons = 8
- Number of neutrons = 8
- Mass number = number of protons + number of neutrons = 8 + 8 = 16
Therefore, the mass number of oxygen is 16.
(b) For sulphur:
- Number of protons = 16
- Number of neutrons = 16
- Mass number = number of protons + number of neutrons = 16 + 16 = 32
Therefore, the mass number of sulphur is 32.
Q. List the three subatomic particles that are present in each of the symbols H, D, and T.
Answer – The symbols H, D, and T represent different isotopes of the element hydrogen. Here is the table for the three subatomic particles found in each isotope:
| Isotope | Number of protons (Z) | Number of neutrons (N) | Number of electrons (assuming neutral atom) |
|---|---|---|---|
| H | 1 | 0 | 1 |
| D | 1 | 1 | 1 |
| T | 1 | 2 | 1 |
Note:
- The number of protons determines the identity of the element, so all three isotopes are hydrogen and have one proton each.
- The number of neutrons can vary among isotopes of the same element, leading to different atomic masses.
- The number of electrons in a neutral atom is equal to the number of protons, so all three isotopes have one electron each.
Q. Any particular pair of isotopes and isobars’ electronic configuration should be written down.
Answer – Isotopes are atoms of the same element that have the same number of protons (atomic number), but different numbers of neutrons (and hence different atomic masses). Isobars are atoms that have the same mass number, but different atomic numbers.
Here are the electronic configurations of one pair of isotopes and one pair of isobars:
Pair of isotopes: Hydrogen-1 and Hydrogen-2 (also known as deuterium)
Hydrogen-1: 1s1
Hydrogen-2 (deuterium): 1s1 2s1
Both isotopes have one electron in the first shell, but deuterium has an additional neutron in the nucleus.
Pair of isobars: Carbon-12 and Nitrogen-12
Carbon-12: 1s2 2s2 2p2
Nitrogen-12: 1s2 2s2 2p3
Both isotopes have the same mass number of 12, but different atomic numbers (6 for carbon and 7 for nitrogen). They have the same electronic configuration up to the 2p subshell, but differ in the number of electrons in the 2p subshell.
Q. Write the difference between electrons, protons and neutrons.
Answer –
| Property | Electrons | Protons | Neutrons |
|---|---|---|---|
| Location | Orbit around nucleus | Nucleus | Nucleus |
| Charge | Negative (-1) | Positive (+1) | Neutral (0) |
| Relative Mass | 0.0005 u (negligible) | 1 u | 1 u |
| Symbol | e | p | n |
| Discovery | 1897 (J.J. Thomson) | 1917 (Rutherford) | 1932 (Chadwick) |
| Subatomic particle | Yes | Yes | Yes |
| Position in atom | Outer shells | Nucleus | Nucleus |
| Role in chemical | Participate in bonding | Determine the element | Determine the isotope |
Note: u refers to atomic mass unit.
Q. What are the drawbacks of J.J. Thomson’s atomic model?
Answer – According to J.J. Thomson’s atomic model, electrons are evenly dispersed across a positively charged sphere. However, later research by other scientists showed that protons are found in the atom’s core nucleus and that electrons orbit the nucleus at distinct energy levels. The Bohr model, a redesigned representation of the atom, is the cornerstone of contemporary knowledge of atomic structure.
Q. What are the drawbacks of Rutherford’s atomic model?
Answer – According to J.J. Thomson’s atomic model, electrons are evenly dispersed across a positively charged sphere. However, later research by other scientists showed that protons are found in the atom’s core nucleus and that electrons orbit the nucleus at distinct energy levels. The Bohr model, a redesigned representation of the atom, is the cornerstone of contemporary knowledge of atomic structure.
Q. Describe Bohr’s atomic model.
Answer – Bohr’s model of the atom proposed that electrons exist in discrete energy levels or shells around the nucleus, and that they can move between these shells by absorbing or emitting energy in the form of photons. The model also explains the spectral lines observed in the emission and absorption spectra of atoms.
Q. Compare each of the atomic model proposals presented in this chapter.
Answer –
| Model | Proposed by | Key Points |
|---|---|---|
| Dalton’s model | John Dalton | Atoms were solid, indivisible spheres with no internal structure. |
| Thomson’s model | J.J. Thomson | Atoms were composed of positively charged material with negatively charged electrons scattered throughout, like “plums in a pudding.” |
| Rutherford’s model | Ernest Rutherford | Atoms had a small, dense, positively charged nucleus surrounded by negatively charged electrons in circular orbits. |
| Bohr’s model | Niels Bohr | Electrons exist in discrete energy levels or shells around the nucleus, and they can move between these shells by absorbing or emitting energy in the form of photons. |
Q. Provide a brief summary of the guidelines for describing the distribution of electrons among the first 18 elements’ different shells.
Answer – The rules for writing the distribution of electrons in various shells for the first eighteen elements can be rephrased as follows:
The maximum number of electrons that can be present in a shell is determined by the formula 2 n2, where n is the orbit number (1, 2, 3, etc.). For example, the maximum number of electrons that can be present in the K shell (n=1) is 2, in the L shell (n=2) is 8, in the M shell (n=3) is 18, and in the N shell (n=4) is 32.
The maximum number of electrons that can be present in the outermost orbit (valence shell) is 8.
Electrons are not accommodated in a given shell unless the inner shells are filled first. In other words, shells are filled step-wise, with the lowest energy level being filled first before moving to higher energy levels. This is known as the Aufbau principle.
Q. Using silicon and oxygen as examples, define valency.
Answer – Valency is defined as the combining capacity of an atom to form chemical bonds with other atoms. It is determined by the number of electrons in the outermost shell or valence shell of an atom. The electronic configurations of oxygen and silicon are 2-6 and 2-8-4, respectively.
In the case of oxygen, the atomic number is 8, which means it has 8 electrons in total. Out of these, 6 electrons are present in the outermost shell. To fill the outermost shell, 2 more electrons are required. Therefore, the valency of oxygen is 2.
In the case of silicon, the atomic number is 14, which means it has 14 electrons in total. Out of these, 4 electrons are present in the outermost shell. To fill the outermost shell, 4 more electrons are required. Therefore, the valency of silicon is 4.
Q. Explain with examples
(i) Atomic number,
(ii) Mass number,
(iii) Isotopes and
(iv) Isobars. Give any two uses of isotopes.
Answer –
(i) Atomic number: Number of protons in the nucleus of an atom, denoted by symbol Z. Example: The atomic number of oxygen is 8.
(ii) Mass number: Sum of protons and neutrons in the nucleus of an atom, denoted by symbol A. Example: The mass number of carbon-12 is 12.
(iii) Isotopes: Atoms of the same element with the same atomic number but different mass numbers. Example: Carbon-12 and carbon-14 are isotopes of carbon.
(iv) Isobars: Atoms of different elements with the same mass number but different atomic numbers. Example: Carbon-14 and nitrogen-14 are isobars.
Uses of isotopes:
- Medical applications: Technetium-99m is used in medical imaging to diagnose various diseases, such as cancer and heart disease.
- Industrial applications: Cobalt-60 is used to irradiate food to extend its shelf life, while iridium-192 is used to detect flaws in welds and other materials.
Q. K and L shells are entirely filled with Na+. Explain.
Answer – Sodium (Na) has an atomic number of 11, which means it has 11 protons in its nucleus and 11 electrons surrounding the nucleus. The electronic configuration of sodium is 2, 8, 1, where 2 electrons occupy the first energy level or K-shell, 8 electrons occupy the second energy level or L-shell, and 1 electron occupies the third energy level or M-shell.
When sodium loses an electron, it forms a sodium ion (Na+). This is because sodium has a tendency to lose an electron to achieve the stable configuration of the previous noble gas, neon (2, 8). Thus, Na+ has 10 electrons, with 2 electrons in the K-shell and 8 electrons in the L-shell. Since both the K and L shells are completely filled, Na+ has achieved a stable electronic configuration and is thus an inert gas-like cation.
Q. Calculate the average atomic mass of the bromine atom if it exists in the form of, say, two isotopes: 79.35 Br (49.7%) and 81.35 Br (50.3%).
Answer – To calculate the average atomic mass of bromine, we need to take into account the relative abundance and mass of each of its isotopes. Given that the atomic masses of the two isotopes, 79Br and 81Br, are 78.9183 u and 80.9163 u respectively, we can use the following formula to calculate the average atomic mass:
Average atomic mass = (relative abundance of isotope 1 × mass of isotope 1) + (relative abundance of isotope 2 × mass of isotope 2)
Given that the relative abundance of 79Br is 49.7% and that of 81Br is 50.3%, we can substitute these values into the formula to get:
Average atomic mass = (0.497 × 78.9183 u) + (0.503 × 80.9163 u)
= 39.228 u + 40.683 u
= 79.911 u
Therefore, the average atomic mass of bromine is approximately 79.911 u.
Q. A sample of element X has an average atomic mass of 16.2 u. What proportions of the isotopes 16 8 X and 18 8 X are present in the sample?
Answer – The average atomic mass of the sample is 16.2 u. Let the percentage of the isotope 8X16 be ‘a’, and the percentage of the isotope 8X18 be ‘100 – a’.
Using the given equation, we get:
(16a/100) + (18(100 – a)/100) = 16.2
Simplifying this equation, we get:
16a + 1800 – 18a = 1620
-2a = -180
a = 90
Therefore, the percentage of the isotope 8X16 is 90% and the percentage of the isotope 8X18 is 10%.
Q. What would the element’s valency be if Z = 3? Name the element.
Answer – The given atomic number is 3, and the electronic configuration of the element is K-2 and L-1, which means that the first shell (K-shell) has 2 electrons, and the second shell (L-shell) has 1 electron. Since the outermost shell of the element has 1 electron, its valency is 1.
Therefore, the element with atomic number 3 and electronic configuration K-2; L-1, and a valency of 1 is Lithium.
The atomic number 3 is Lithium.
Q. The two atomic species X and Y’s nuclei’s composition is listed under X Y.
Protons = 6 6
Neutrons = 6 8
Please provide the masses of X and Y. What connection do the two species have to one another?
Answer – Two atomic species X and Y are given with the following composition of their nuclei:
- X has 6 protons and 6 neutrons
- Y has 6 protons and 8 neutrons
The mass numbers of X and Y can be calculated by adding the number of protons and neutrons in each species’ nucleus:
- Mass number of X = 6 protons + 6 neutrons = 12
- Mass number of Y = 6 protons + 8 neutrons = 14
X and Y are isotopes of different elements because they have the same number of protons but a different number of neutrons. Therefore, they have the same atomic number (6) but different mass numbers.
Q. Write T for True and F for False for the following statements.
(A) J.J. Thomson suggested that an atom’s nucleus is made up entirely of nucleons.
(b) The union of an electron and a proton results in the creation of a neutron. It is hence impartial.
(c) An electron has a mass that is around 1 2000 times more than a proton.
(d) Iodine tincture, a type of medicine, is created using an isotope of the element.
In questions, place a checkmark (ü) next to the correct answer and a cross (x) next to the incorrect answer.
Answer –
(a) The statement is False.
(b) The statement is False.
(c) The statement is True.
(d) The statement is False.
Q. The finding of was due to Rutherford’s alpha-particle scattering experiment.
(a) Atomic Nucleus
(b) Electron
(c) Proton
(d) Neutron
Answer – (a) Atomic Nucleus
Q. Isotopes of an element have
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers.
Answer – (c) different number of neutrons
Q. Number of valence electrons in Cl– ion are:
(a) 16 (b) 8 (c) 17 (d) 18
Answer – (b) 8