Frank Brothers Solutions for ICSE Class 9 Chemistry Chapter 7 Atomic Structure

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Question 1:
Answer:
S.N. 1. Properties: Position
Electron: Electrons revolve around the nucleus in orbits.
Proton: A proton resides in the central part of nucleus.
Neutron: Neutron reside in nucleus.

S.N. 2. Properties: Charge
Electron: An electron possesses a unit negative charge of magnitude - 1.602 ×10^-19 coulombs.
Proton: A proton possesses a unit positive charge of the value 1.602 ×10^-19 coulombs.
Neutron: A neutron possesses zero charge. So, it is electrically neutral particle.

S.N. 3. Properties: Mass
Electron: Electron has negligible mass. The mass of an electron is 1/1837 times the mass of a hydrogen atom i.e. 9.108 ×10^-28 g.
Proton: Mass of proton is nearly same as that of hydrogen atom i.e. 1{ amu} or 1.672 ×10^-24 g.
Neutron: Mass of neutron is slightly greater than proton i.e. 1.675 ×10^-24 g.

S.N. 4. Properties: Notation
Electron: An electron is indicated as - ₁⁰e where zero indicates that it has no mass and -1 indicates that it has one unit of negative charge.
Proton: A proton is indicated by ₊₁¹p or ₊₁¹H where 1 indicates that it has a mass equal to 1{ amu} and +1 stands for one unit positive charge.
Neutron: A neutron is indicated by ₀¹n where 0 stands for zero charge and 1 stands for 1{ amu} mass.
These fundamental particles are the building blocks of every atom in the universe. Protons and neutrons stay heavy and still in the center, while light electrons zoom around them in paths called orbits.
Teacher's Tip: Use the word "PEN" to remember the three particles: Protons, Electrons, and Neutrons.
Exam Tip: When writing the mass of an electron, always use the word "negligible" because it is so small compared to others.

Question 2:
Answer:
₁₂²⁴Mg
Number of electrons = 12
Number of protons = 12
Number of neutrons = 24 - 12 = 12

₁₉³⁹K
Number of electrons = 19
Number of protons = 19
Number of neutrons = 39 - 19 = 20

₂₀⁴⁰Ca
Number of electrons = 20
Number of protons = 20
Number of neutrons = 40 - 20 = 20

₁₅³¹P
Number of electrons = 15
Number of protons = 15
Number of neutrons = 31 - 15 = 16
In a neutral atom, the number of protons and electrons is always equal and matches the atomic number at the bottom. To find the neutrons, you simply subtract the smaller bottom number from the larger top number.
Teacher's Tip: Think of the top number as the "Total Weight" and the bottom as the "ID Number."
Exam Tip: Always show the subtraction step (Mass Number - Atomic Number) to ensure you get full calculation marks.

Question 3:
Answer:
1. (a) Mass of an atom - Protons and neutrons
2. (b) Size of an atom - Electrons
The mass is concentrated in the tiny center called the nucleus because protons and neutrons are very heavy. The size of the atom is determined by how far away the tiny electrons travel in their orbits.
Teacher's Tip: Imagine a marble (nucleus) in the middle of a football stadium (the orbits of electrons).
Exam Tip: Use the term "Nucleons" to refer to the protons and neutrons together when discussing mass.

Question 4:
Answer: Three fundamental particles of an atom are-
1. Proton
2. Electron
3. Neutron
These three particles determine all the physical and chemical behaviors of an element. Protons identify the element, neutrons add weight, and electrons handle all the chemical bonding.
Teacher's Tip: Remember that only the Neutron is neutral; the other two have opposite charges.
Exam Tip: List them in a standard order (Proton, Neutron, Electron) to keep your answer organized.

Question 5:
Answer:
1. (a) An atom - An atom is the smallest particle of an element which may or may not be capable of independent existence.
2. (b) An element - An element is usually defined as a pure substance that contains only one kind of particles. These particles may be atoms or molecules.
An atom is the basic unit of matter, like a single Lego brick. An element is a pure material made entirely of that one specific type of "brick."
Teacher's Tip: An atom is a single unit, while an element is the whole collection of that same type of atom.
Exam Tip: Mention "independent existence" when defining an atom to show a complete scientific understanding.

Question 6:
Answer: Atomic number - The number of protons present in the nucleus of an atom is called the atomic number of the atom. It is denoted by ‘Z’.
The atomic number is like an atom's Social Security number or fingerprint; it never changes for a specific element. If you change the number of protons, you create a completely different element.
Teacher's Tip: Use the letter 'Z' as a shortcut for Atomic Number in your notes.
Exam Tip: Remember that in a neutral atom, Z = {number of protons} = {number of electrons}.

Question 7:
Answer: The protons and neutrons are collectively called as nucleons.
Because they both live inside the nucleus, scientists gave them this special group name. Together, they account for almost 100% of the atom's total mass.
Teacher's Tip: Think of "Nucleons" as "residents of the Nucleus."
Exam Tip: If a question asks for the mass number, it is simply the sum of the nucleons.

Question 8:
Answer: Isotopes are atoms of the same element, having the same atomic number, same chemical properties but different mass number, i.e., the atoms differ in the number of neutrons. The three isotopes of hydrogen atoms are-
1. Protium
2. Deuterium
3. Tritium
Isotopes are like siblings; they are from the same family (element) but have different weights because they have different numbers of neutrons. Hydrogen is famous because its isotopes even have their own special names.
Teacher's Tip: Protium has 0 neutrons, Deuterium has 1, and Tritium has 2.
Exam Tip: Always emphasize that isotopes have the "same atomic number" but "different mass number."

Question 9:
Answer: Electrons take part in a chemical reaction. Therefore, the chemical properties of an element depend upon the electronic configuration. Since, isotopes of an element have the same atomic number and hence same electronic configuration. So, they exhibit the same chemical properties.

Chemical properties are all about how electrons are arranged on the outside of an atom. Since isotopes have the same number of protons and electrons, they react with other chemicals in the exact same way.
Teacher's Tip: Neutrons stay hidden in the middle and don't affect how an atom "talks" or bonds with others.
Exam Tip: Link "atomic number" to "electronic configuration" to explain why properties remain the same.

Question 10:
Answer:
S.N. 1. Dalton’s atomic theory: According to Dalton’s atomic theory, all matter is composed of extremely small particles called atoms.
Modern Atomic Theory: According to modern theory, an element is made up of extremely small particles called atoms.

S.N. 2. Dalton’s atomic theory: Atoms of a given element are identical, both in mass and properties.
Modern Atomic Theory: All atoms of an element may not be alike; they can have different masses (i.e. Isotopes).

S.N. 3. Dalton’s atomic theory: Atoms are indivisible and indestructible.
Modern Atomic Theory: Atoms are divisible and are no more indestructible. Atom consists of still smaller particles like proton, electron and neutron.

S.N. 4. Dalton’s atomic theory: Atom is the smallest particle of an element.
Modern Atomic Theory: Atoms take part in all chemical reactions, the relative numbers and kinds of atoms are constant in a given compound.

S.N. 5. Dalton’s atomic theory: Atoms take part in all chemical reactions, the relative no. and kinds of atoms are constant in a given compound.
Scientific ideas evolve as we get better tools to see nature. While Dalton gave us a great start, we now know that atoms are made of smaller parts and can come in slightly different weights.
Teacher's Tip: Dalton thought atoms were like solid billiard balls, but we know they are more like mini solar systems.
Exam Tip: When comparing theories, mention "Isotopes" to show why Dalton's second point was updated.

Question 11:
Answer: The outermost orbit of an element is called valence shell. The electrons present in the outermost orbit (valence shell) of an element are called valence electrons.
These electrons are the most important ones because they are on the very edge of the atom. They are the ones that actually touch other atoms and decide how a chemical reaction will happen.
Teacher's Tip: Think of valence electrons as the "hands" that an atom uses to hold onto other atoms.
Exam Tip: Always check the very last number in an electronic configuration to find the valence electrons.

Question 12:
Answer: Atoms may have incomplete octet. During the formation of a molecule, an atom of a particular element gains, loses or shares electrons until it acquires a stable configuration of eight electrons in its valence shell.
An "octet" simply means having eight electrons, which makes an atom feel very "happy" and stable. Most atoms will do almost anything—like giving away or stealing electrons—to reach this perfect number of eight.
Teacher's Tip: Think of the number 8 as a "full house" in an atom's outer shell.
Exam Tip: The noble gases naturally have an octet, which is why they don't react with other elements.

Question 13:
Answer:
1. (a) Atomic number = Number of protons = 20
2. (b) Mass number = Number of protons + Number of neutrons = 20 + 20 = 40
3. (c) Electronic configuration = 2, 8, 8, 2
4. (d) Valency = 2
These details describe a Calcium atom perfectly. It has two electrons in its outermost shell, which it wants to give away, giving it a valency of two.
Teacher's Tip: If the last number in the configuration is 1, 2, or 3, that number is the valency.
Exam Tip: Make sure the total sum of your electronic configuration (2+8+8+2) equals the atomic number (20).

Question 14:
Answer:
1. (a) A = 1
B = 3
C = 2
D = 1
E = 1
2. (b) E contains the greatest number of neutrons.
3. (c) A contains the least number of electrons.
4. (d) B contains equal number of electrons and neutrons.
5. (e) A is a metal.
6. (f) Fluorine is the most reactive of the non-metals.
7. (g) Both are electronegative and non-metals.
8. (h) D and E belongs to a particular family.
9. (i) A_{3}B will be the formula if atoms of A combines with atoms of B.
10. (j) ‘B’ will combine in a trivalent element while ‘C’ will combine as a bivalent element.
This exercise helps you identify different elements by looking at their subatomic parts. By comparing protons, neutrons, and electrons, we can predict everything from their names to how they form chemical formulas.
Teacher's Tip: Trivalent means a valency of 3, and bivalent means a valency of 2.
Exam Tip: When writing a formula like A₃B, always cross-multiply the valencies of the two elements.

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Question 15:
Answer: “During the formation of the molecule, an atom of a particular element gains, loses or shares electrons until it acquires a stable configuration of eight electrons in its valence shell” i.e. until it acquires octet.
This rule explains why atoms bother to bond with each other at all. They are searching for the stability that comes with a complete outer shell of eight electrons.
Teacher's Tip: This is essentially the "Rule of Eight" in the world of chemistry.
Exam Tip: Mention "gains, loses, or shares" to show you know the three ways atoms can reach an octet.

SQuestion 16:
Answer: K can accommodate maximum of 2 electrons.
L can accommodate maximum of 8 electrons.
M can accommodate maximum of 18 electrons.
Each shell or orbit has a specific capacity, just like rows in a theater. The first row (K) is small and only fits two, while the rows further out can hold much more.
Teacher's Tip: Use the formula 2n² to find the maximum capacity of any shell number 'n'.
Exam Tip: Even though M can hold 18, it usually stops at 8 for Class 6 level stability problems.

Question 17:
Answer: Cathode rays are formed at the negative electrode of the discharge tube experiment.
These rays are actually streams of electrons moving at very high speeds. This famous experiment was what first proved that atoms have tiny negative particles inside them.
Teacher's Tip: "Cathode" starts with 'C' and is the negative side in these specific experiments.
Exam Tip: Remember that cathode rays always travel in straight lines from the negative to the positive electrode.

Question 18:
Answer:
1. In K maximum number of 2 electrons can be accommodated.
2. In L maximum number of 8 electrons can be accommodated.
3. In M maximum number of 18 electrons can be accommodated.
4. In N maximum number of 32 electrons can be accommodated.
These limits are set by the laws of physics and ensure that electrons are organized in a specific pattern. Understanding these capacities is the first step to drawing correct atomic diagrams.
Teacher's Tip: For the K, L, M, N shells, the shell numbers are 1, 2, 3, 4.
Exam Tip: Always fill the shells in order, starting from K, before moving to the outer ones.

Question 19:
Answer: Suppose the percentage of ¹⁰₅B be A and the percentage of ¹¹₅B be (100-A). It implies that,
{A ×10 + (100 - A) ×11}/{100} = 10.8
11A - 10A = 1100 - 1080
A = 20
So, the percentage of¹⁰₅B = 20%
The percentage of ¹¹₅B = 100 - 20 = 80%
This math problem shows how we calculate the average weight of an element when it has multiple isotopes. Since most Boron is the heavier type (11{ amu}), the final average is much closer to 11 than it is to 10.
Teacher's Tip: The total percentage of all isotopes must always add up to 100%.
Exam Tip: Be careful with the cross-multiplication step to avoid simple arithmetic errors.

Question 20:
Answer: Electronic configuration of magnesium is 2, 8, 2. Since, it has 2 electrons in its valence shell, so its valency is 2.
Magnesium has 12 electrons in total, arranged in three shells. Because it has only two on the very outside, it prefers to lose them to become stable, which is why its valency is 2.
Teacher's Tip: Magnesium "donates" its 2 outer electrons to other atoms during reactions.
Exam Tip: When stating valency, always provide the electronic configuration to justify your answer.

Question 21:
Answer: Number of electrons in Sodium = 11
Number of protons in sodium = 11
Number of neutrons in sodium = 12
Number of nucleons in sodium = 23
Sodium is a neutral atom, so its positive protons and negative electrons are perfectly balanced at 11 each. The total mass or "nucleon count" is 23, which is why it is written as₁₁²³Na.
Teacher's Tip: Nucleons is just a fancy way of saying "Mass Number."
Exam Tip: Protons + Neutrons = Nucleons. Check your math one last time before finishing.

Question 22:
Answer: Inert elements are the elements which have completely filled valence shell. Since, they are already stable and do not need more electrons, they do not combine with other atoms. So, they exist as monoatoms in molecule.
Inert elements, like Helium and Neon, are the "loners" of the chemistry world because they are already perfect. They don't need to share or trade electrons with anyone else to be happy.
Teacher's Tip: Think of inert elements as atoms that have already won the "octet game."
Exam Tip: Use the term "Noble Gases" as a synonym for inert elements in your answers.

Question 23:
Answer:
1. (a) Hydrogen
2. (b) K shell.
3. (c) Magnesium
4. (d) Isotopes
5. (e) Helium has zero valency.
These one-word answers cover the basics of how different elements are structured. Each element has its own unique rule, like Helium having a valency of zero because its K-shell is already full.
Teacher's Tip: Hydrogen is the only element that usually doesn't have any neutrons.
Exam Tip: Remember that Helium is stable with only 2 electrons (duplet rule) instead of the usual 8 (octet rule).

Question 24:
Answer:
1. (a) The number of protons = 9
2. (b) The number of neutrons = 19 – 9 = 10
3. (c) The number of electrons = 9
These numbers describe the Fluorine atom, which has 9 protons and 9 electrons. By subtracting the atomic number from the mass number (19), we find that it has exactly 10 neutrons.
Teacher's Tip: The bottom number is always the number of protons and electrons.
Exam Tip: Always verify your proton and electron counts are equal for a neutral atom.

Question 25:
Answer: Atomic number is the number of protons of an atom which is unique to an atom but mass number is the total of number of protons and number of neutrons which may or may not be same to other atoms since there is probability of combination of number of protons and number of neutrons be same for two atoms.
The atomic number tells you the identity of the atom, while the mass number tells you how heavy it is. Two different elements can sometimes have the same weight, but they can never have the same number of protons.
Teacher's Tip: Atomic number = Name Tag. Mass number = Weight Scale.
Exam Tip: Define both terms clearly to show the specific difference between identity and mass.