Journey Inside the Atom introduces students to the basic structure of matter and explains that everything around us is made of tiny particles called atoms. The chapter describes the structure of an atom, including its three main particles—protons, neutrons, and electrons. Students learn about the arrangement of these particles, with protons and neutrons present in the nucleus and electrons moving around it in shells.
The chapter explains important concepts such as atomic number, mass number, isotopes, and isobars, helping students understand how atoms of different elements are identified. It also introduces the electronic configuration of atoms and shows how electrons are arranged in different energy levels. Students learn how atoms combine to form molecules and compounds through chemical bonding. The chapter highlights the importance of atoms in understanding the properties of elements and chemical reactions. Real-life examples and simple diagrams make these concepts easy to understand and provide a strong foundation for further study in chemistry.
Journey Inside the Atom carries steady weightage in Class 9th exams. Practising its MCQs and important questions is one of the fastest ways to secure marks from this chapter.
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The scientist was Ernest Rutherford. He performed the Alpha Particle Scattering Experiment, which showed that the nucleus of an atom is small, dense, and positively charged.
a) Atomic number: -The atomic number of an element is the number of protons present in the nucleus of an atom. It is denoted by Z. For a neutral atom, the number of protons equals the number of electrons.
b) Mass number: -The mass number of an atom is the sum of the number of protons and neutrons present in its nucleus. It is denoted by A.
Formula:
Mass number (A) = Number of protons + Number of neutrons
The complete symbol of an atom is written as:
A (mass number)
Z (atomic number)
X (symbol of element)
Example: For sodium, atomic number = 11 and mass number = 23, the symbol is ²³₁₁Na.
The number of protons in an atom is equal to its atomic number.
For ³⁵Cl₁₇, the atomic number is 17.
So, the atom of chlorine (³⁵Cl₁₇) has 17 protons.
The three isotopes of hydrogen are represented as:
A neutron is slightly heavier than a proton.
The mass of a proton is approximately 1.672 × 10⁻²⁷ kg, while the mass of a neutron is about 1.675 × 10⁻²⁷ kg.
Although their masses are almost the same, the neutron has a very small additional mass.
Both are found in the nucleus of an atom, but since the neutron has no charge and a slightly greater mass, it is considered heavier than the proton.
Charges on subatomic particles are -
Isobars are atoms of different elements that have thesame mass number but different atomic numbers.
This means they have the same total number of protons and neutrons combined, but the number of protons in each atom is different.
Example:
⁴⁰Ar (Argon) and ⁴⁰Ca (Calcium) are isobars because both have a mass number of 40, but their atomic numbers are 18 and 20 respectively.
Valence electrons are the electrons present in the outermost shell (energy level) of an atom. These electrons take part in chemical bonding and determine the valency of an element.
Example:
Nitrogen (N): Atomic number = 7 ---> Electronic configuration = 2, 5
Number of electrons in the valence shell = 5
Argon (Ar): Atomic number = 18 ---> Electronic configuration = 2, 8, 8
Number of electrons in the valence shell = 8
Helium has zero valency because its outermost shell is completely filled with 2 electrons. It does not need to gain, lose, or share any electrons to become stable.
That’s why helium is chemically inactive and has valency = 0.
Elements that have a tendency to lose electrons are called metals.
They usually have 1, 2, or 3 electrons in their outermost shell and lose them easily to form positive ions (cations).
Example:
Atomic number: Number of protons = 3
Mass number: Number of protons + Number of neutrons = 3 + 4 = 7
Valency: Atomic number 3 means the element is Lithium (Li). Its electronic configuration is 2, 1, so it has 1 valence electron -> Valency = 1.
A proton is about 1836 times heavier than an electron. The mass of a proton is much greater than that of an electron.
The protons and neutrons present in the nucleus determine the mass of an atom.
This is because electrons have a very small mass compared to protons and neutrons, so their contribution to the total mass is negligible.
The first shell of an atom can hold a maximum of 2 electrons.
According to Bohr’s model of the atom, the maximum number of electrons that can be present in any shell is given by the formula 2n², where n is the shell number.
For the first shell (K shell),
n = 1
So, maximum electrons = 2 × (1)² = 2 electrons.
Therefore, the K shell or first energy level can have only 2 electrons.
Achievement:
J.J. Thomson’s model explained that an atom is electrically neutral, as the positive and negative charges are equal in magnitude and balance each other.
Limitation:
His model could not explain the arrangement of charges inside the atom and failed to explain the results of Rutherford’s alpha particle scattering experiment.
Isotopes are atoms of the same element having the same atomic number but different mass numbers.
From the given atoms:
³³A₁₇, ⁴⁰B₂₀, ³⁷C₁₇, ³⁹D₁₉
The atoms A (³³A₁₇) and C (³⁷C₁₇) have the same atomic number (17) but different mass numbers (33 and 37).
Therefore, A and C are isotopes.
From the symbol ³²₁₆S:
i) Atomic number of sulphur = 16
ii) Mass number of sulphur = 32
iii) Electronic configuration of sulphur = 2, 8, 6
The nucleus of an atom has a positive charge.
This is because the nucleus contains protons, which are positively charged particles, and neutrons, which have no charge. The positive charge of the protons gives the nucleus its overall positive charge.
The three subatomic particles are:
The maximum number of electrons that each shell can hold is given by the formula 2n², where n is the shell number.
Thomson said that an atom looks like a sphere of positive charge with electrons stuck inside it, just like seeds in a watermelon or plums in a pudding. The total positive and negative charges balance each other, making the atom neutral.
The atomic number (number of protons) defines the identity of an element and determines its position in the periodic table (B). It also determines the number of electrons in a neutral atom, which in turn dictates the element's chemical properties (A). The mass number can vary due to isotopes (C), making it less suitable as a fundamental identifier for the element itself. Hence, all options contribute to why atomic number is more fundamental.
The Bohr model is useful for teaching because it successfully explains the stability of atoms, the existence of discrete energy levels, and the basic pattern of electronic configuration and valency for lighter elements. It provides a foundational understanding that is conceptually accessible for introductory chemistry.
The electronic configuration 2, 8, 8, 1 shows 1 electron in the outermost shell. It is easiest for the atom to lose this single electron to achieve a stable octet in the previous shell. Thus, its valency is 1, and it forms a cation with a +1 charge.
Some atoms of the same element have different mass numbers because they are isotopes. Isotopes of an element have the same number of protons (and thus the same atomic number) but differ in the number of neutrons in their nuclei, which leads to different mass numbers.
Atom A has 17 protons (Chlorine), and Atom B has 18 protons (Argon). Since they have different numbers of protons, they are unequivocally different elements.
Rutherford's alpha-particle scattering experiment showed that a small fraction of positively charged alpha particles were deflected at large angles or bounced back, indicating the presence of a small, dense, positively charged center within the atom, which he called the nucleus.
The neutron is a subatomic particle found in the nucleus that has a mass very close to that of a proton but carries no net electrical charge.
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