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ICSE • Class 7 • Science • Ch 11
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Atomic Structure

In ICSE Class 7 Science (Chemistry), "Atomic Structure" provides an authoritative, quantum-foundational master study guide analyzing Dalton's atomic postulates, discovery of subatomic particles, Thomson and Rutherford atomic models, Bohr's electronic orbital model, atomic number, mass number, and electronic configuration of the first 20 elements. This comprehensive chapter explores Dalton's Atomic Theory (Postulates: indivisibility of atoms, identical atoms of same element, conservation of atoms; Limitations discovered by modern physics), Subatomic Particles (1. Electrons: discovered by J.J. Thomson in cathode ray experiments; negative charge $-1.6 \times 10^{-19}\text{ C}$, relative mass $\frac{1}{1837}\text{ amu}$, 2. Protons: discovered by Goldstein / Rutherford in anode rays; positive charge $+1.6 \times 10^{-19}\text{ C}$, relative mass $1\text{ amu}$, 3. Neutrons: discovered by James Chadwick [1932]; neutral particle with zero charge, relative mass $1\text{ amu}$), Historical Atomic Models (Thomson's Plum Pudding Model; Rutherford's Gold Foil Alpha-Particle Scattering Experiment: discovery of the dense central positive Nucleus and empty atomic space), Bohr-Bury Atomic Model (Electrons revolve in discrete, stationary energy levels or shells: $K, L, M, N$; Maximum capacity rule: $2n^2$), Atomic Number ($Z = \text{Number of protons} = \text{Number of electrons in neutral atom}$), Mass Number ($A = \text{Number of protons} + \text{Number of neutrons} = Z + N$), Symbolic Notation ($_{Z}^{A}\text{X}$), Electronic Configuration of Elements from $Z = 1$ to $Z = 20$ (Hydrogen to Calcium: Octet rule, maximum capacity of valence shell $= 8$), and Isotopes aligned with the 2026–27 CISCE ICSE curriculum.

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How Did Shooting High-Speed Alpha Bullets at Thin Gold Foil Shatter the 2,000-Year-Old Belief in Solid Atoms?

For over two thousand years, from ancient Greek philosophers to John Dalton in 1808, humanity believed that the atom was a solid, unbreakable billiard ball. In 1911, New Zealand physicist Ernest Rutherford conducted an experiment so shocking it turned physics upside down: he fired high-speed alpha particles (positively charged helium bullets) at an ultra-thin sheet of gold leaf only 400 atoms thick. As expected, $99.99\%$ of the alpha bullets flew straight through the gold sheet as if nothing was there! But then, once in every 8,000 shots, an alpha particle slammed into something, bounced backwards, and ricocheted right back at the detector! Rutherford exclaimed: "It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch artillery shell at a piece of tissue paper and it came back and hit you!" Rutherford realized that the atom is NOT solid; it is $99.999999999999\%$ EMPTY SPACE! Almost all of its mass is compressed into a ridiculously tiny, dense, positively charged speck at the center called the NUCLEUS! What are electrons, protons, and neutrons? How do electrons orbit in $K, L, M$ shells using the $2n^2$ rule? Let's master atomic structure.

Why This Chapter Matters

Atomic structure is the foundational bedrock of all modern chemistry, nuclear physics, semiconductor microchips, lasers, quantum computing, and radiology in oncology medicine. Understanding atomic number ($Z$), mass number ($A$), and electronic configurations is essential for mastering chemical bonding, valency, and the Periodic Table in ICSE board exams.

Before You Begin (Prerequisites)

  • Concept of elements, compounds, and molecules from Chapter 10.
  • Basic positive and negative electrical charges from Chapter 7.
  • Basic arithmetic and squaring formulas ($2n^2$).

What You Will Learn (Core Objectives)

  • State Dalton's Atomic Theory and outline its modern scientific limitations.
  • Compare the mass, charge, and location of the three fundamental subatomic particles: electron, proton, and neutron.
  • Explain Rutherford's gold foil scattering experiment and its deduction of the atomic nucleus.
  • Calculate atomic number ($Z$), mass number ($A$), and number of neutrons ($N = A - Z$).
  • Apply the Bohr-Bury $2n^2$ rule and the Octet Rule to write electronic configurations for the first 20 elements.
  • Draw orbital shell diagrams ($K, L, M, N$) for elements from Hydrogen ($Z=1$) to Calcium ($Z=20$).

Chapter Roadmap & Progression

1 1. Dalton's Atomic Theory & Subatom...
2 2. Rutherford's Gold Foil Experimen...
3 3. Atomic Number ($Z$), Mass Number...
4 4. Bohr-Bury Rules & Electronic Con...

Complete Concept Guide (100% Curriculum Coverage)

1. Dalton's Atomic Theory & Subatomic Particles

Understand
A. Dalton's Atomic Postulates (1808):
  1. Matter consists of indivisible, indestructible particles called atoms.
  2. All atoms of a given element are identical in mass, size, and chemical properties.
  3. Atoms of different elements differ in mass and properties.
  4. Compounds are formed when atoms of different elements combine in simple whole-number ratios.

Modern Limitation: Atoms are divisible into subatomic particles (protons, neutrons, electrons), and atoms of the same element can have different masses (Isotopes!).

B. The 3 Subatomic Particles:
Particle Discoverer Absolute Charge (C) Relative Charge Relative Mass Location
Electron ($e^-$) J.J. Thomson (1897) $-1.6 \times 10^{-19}$ $-1$ $\frac{1}{1837}\text{ amu} \approx 0$ Extra-nuclear shells
Proton ($p^+$) E. Goldstein / Rutherford $+1.6 \times 10^{-19}$ $+1$ $1\text{ amu}$ Inside Nucleus
Neutron ($n^0$) James Chadwick (1932) $0$ (Neutral) $0$ $1\text{ amu}$ Inside Nucleus

2. Rutherford's Gold Foil Experiment & The Nucleus

Rutherford's Model
A. The Experiment:

Rutherford bombarded a microscopic gold foil ($~100\text{ nm}$ thick) with positively charged $\alpha$-particles ($He^{2+}$):

  • Observation 1: Most $\alpha$-particles ($~99.9\%$) passed straight through without deflection $\implies$ Most of the space inside an atom is empty!
  • Observation 2: A few $\alpha$-particles deflected at small and large angles $\implies$ Positive charge is concentrated in a tiny central region.
  • Observation 3: Extremely rare particles ($1$ in $20,000$) rebounded back at $180^\circ$ $\implies$ Almost all the mass and positive charge of the atom is concentrated in an unimaginably small, dense volume called the NUCLEUS!
B. Planetary Model:

The atom consists of a tiny positive nucleus at the center, with light electrons orbiting around it like planets around the Sun.

3. Atomic Number ($Z$), Mass Number ($A$) & Symbolic Notation

Atomic Parameters
A. Atomic Number ($Z$):

The total number of protons present inside the nucleus of an atom of that element. In an electrically neutral atom, it also equals the total number of electrons:

$$\mathbf{Z = \text{Number of Protons} = \text{Number of Electrons}}$$
B. Mass Number ($A$):

The total number of nucleons (protons $+$ neutrons) inside the nucleus of an atom:

$$\mathbf{A = Z + N = p + n}$$ $$\mathbf{\text{Number of Neutrons } (N) = A - Z}$$
C. Standard Symbolic Representation:
$$\mathbf{_{Z}^{A}\text{X}}$$

Example: $_{11}^{23}\text{Na}$ (Sodium): Atomic number $Z = 11$ (11 protons, 11 electrons), Mass number $A = 23$. Number of neutrons $N = 23 - 11 = \mathbf{12\text{ neutrons}}$.

4. Bohr-Bury Rules & Electronic Configuration ($Z=1$ to $20$)

Electronic Configuration
A. Bohr-Bury Scheme:
  1. Electrons revolve only in discrete circular orbits or energy shells designated as $K, L, M, N$ (where shell number $n = 1, 2, 3, 4$).
  2. Maximum Capacity Rule ($2n^2$):
    • $K\text{-shell } (n=1)$: $2(1)^2 = \mathbf{2\text{ electrons maximum}}$.
    • $L\text{-shell } (n=2)$: $2(2)^2 = \mathbf{8\text{ electrons maximum}}$.
    • $M\text{-shell } (n=3)$: $2(3)^2 = \mathbf{18\text{ electrons maximum}}$ (holds 8 initially).
    • $N\text{-shell } (n=4)$: $2(4)^2 = \mathbf{32\text{ electrons maximum}}$.
  3. The Octet Rule: The outermost valence shell cannot hold more than 8 electrons, regardless of its theoretical $2n^2$ capacity (except the $K$-shell, which saturates with a duplet of $2$).
B. Configurations of Key Elements:
  • Hydrogen ($Z=1$): $K=1$
  • Helium ($Z=2$): $K=2$ (Duplet complete, inert)
  • Carbon ($Z=6$): $K=2, L=4$
  • Oxygen ($Z=8$): $K=2, L=6$
  • Neon ($Z=10$): $K=2, L=8$ (Octet complete, inert noble gas)
  • Sodium ($Z=11$): $K=2, L=8, M=1$
  • Chlorine ($Z=17$): $K=2, L=8, M=7$
  • Argon ($Z=18$): $K=2, L=8, M=8$ (Octet complete)
  • Potassium ($Z=19$): $K=2, L=8, M=8, N=1$
  • Calcium ($Z=20$): $K=2, L=8, M=8, N=2$

Key Formulas, Reactions & Definitions

Mass Number Equation
$$A = Z + N \implies N = A - Z$$
Number of neutrons equals mass number minus atomic number.
Bohr-Bury Maximum Shell Capacity
$$2n^2 \quad (n=1 \implies 2, \, n=2 \implies 8, \, n=3 \implies 18)$$
Governs electron filling sequence in discrete shells.

Atomic Structure: Rutherford Nucleus & Bohr Shells

Chemistry: Atomic Structure & The Bohr-Bury Model SUBATOMIC PARTICLES & NUCLEUS • Proton (p+): +1 Charge • 1 amu Discovered by Goldstein / Rutherford • In Nucleus • Neutron (n0): 0 Charge • 1 amu Discovered by James Chadwick (1932) • In Nucleus • Electron (e-): -1 Charge • 1/1837 amu Discovered by J.J. Thomson • In Orbits / Shells Atomic Number Z = p = e Mass Number A = p + n ⇒ n = A - Z BOHR MODEL: SODIUM (2, 8, 1) 11p, 12n K: 2e L: 8e M: 1e (Valence) 2n2 Capacity: K=2, L=8, M=18 • Octet Max = 8 RUTHERFORD: NUCLEUS HOLDS ALL MASS • ATOM IS 99.99% EMPTY SPACE • 2n^2 RULE

Chapter Summary & 10 Key Takeaways

Takeaway 1
Dalton's atomic theory stated atoms are indivisible; modern physics revealed subatomic particles.
Takeaway 2
Protons have a +1 charge and 1 amu mass (nucleus); electrons have a -1 charge and negligible mass (shells).
Takeaway 3
Neutrons have 0 charge and 1 amu mass (nucleus), discovered by James Chadwick in 1932.
Takeaway 4
Rutherford's gold foil experiment proved atoms are mostly empty space with a dense positive nucleus.
Takeaway 5
Atomic Number (Z) is the number of protons in an atom (also electrons in a neutral atom).
Takeaway 6
Mass Number (A) is the total count of nucleons (protons + neutrons): A = Z + N.
Takeaway 7
Number of neutrons is given by N = A - Z.
Takeaway 8
Bohr-Bury rule: Shells K, L, M, N hold a maximum of 2n2 electrons (2, 8, 18, 32).
Takeaway 9
The Octet Rule mandates that the outermost valence shell cannot hold more than 8 electrons.
Takeaway 10
Isotopes are atoms of the same element having the same atomic number Z but different mass numbers A.

Check Your Understanding (Diagnostic Practice Questions)

Diagnostic questions testing core conceptual clarity. Answers are hidden initially — solve each problem first, then click to reveal the step-by-step verified solution.

1
An atom of an element is represented as $_{17}^{35}\text{Cl}$. Find: (a) Atomic number, (b) Mass number, (c) Number of protons, (d) Number of electrons, (e) Number of neutrons, (f) Electronic configuration.
Reveal Answer & Explanation
Answer:

Given symbol: $_{17}^{35}\text{Cl}$ (Chlorine).
• (a) Atomic Number ($Z$): $17$.
• (b) Mass Number ($A$): $35$.
• (c) Number of Protons: Equals $Z = \mathbf{17}$.
• (d) Number of Electrons: Equals $Z$ in neutral atom $= \mathbf{17}$.
• (e) Number of Neutrons ($N$): $A - Z = 35 - 17 = \mathbf{18\text{ neutrons}}$.
• (f) Electronic Configuration: Filling $K, L, M$ shells using $2n^2$ and octet rule:

$$K = 2, \quad L = 8, \quad M = 7 \quad \implies \mathbf{2, 8, 7}$$

.


$Z = 17, A = 35$. Protons $= 17$, Electrons $= 17$, Neutrons $= 35 - 17 = 18$. Configuration is $2, 8, 7$.
2
Explain the observations of Rutherford's $\alpha$-particle scattering experiment and the corresponding conclusions drawn about atomic structure.
Reveal Answer & Explanation
Answer:
  1. Observation: Over $99.9\%$ of $\alpha$-particles passed through the gold foil undeflected.
    • Conclusion: Most of the internal volume of an atom is completely empty space.
    2. Observation: A tiny fraction of $\alpha$-particles were deflected by large angles.
    • Conclusion: There is a concentrated, heavy positive electrical charge inside the atom that repels positive alpha particles.
    3. Observation: About 1 in 20,000 particles rebounded straight back along their path ($180^\circ$).
    • Conclusion: Almost the entire mass and positive charge of the atom is compressed into an unimaginably tiny, dense central core called the NUCLEUS.

Most passed straight $\to$ empty space; some deflected $\to$ positive core; rebound $\to$ dense central nucleus.
3
State the Bohr-Bury rules for writing the electronic configuration of an atom.
Reveal Answer & Explanation
Answer:
  1. $2n^2$ Maximum Rule: The maximum number of electrons that can be accommodated in a shell is given by $2n^2$, where $n$ is the shell orbit number ($K=1 \to 2$, $L=2 \to 8$, $M=3 \to 18$, $N=4 \to 32$).
    2. Octet Rule for Outermost Shell: The outermost (valence) shell of an atom cannot accommodate more than 8 electrons, even if its theoretical $2n^2$ capacity is higher.
    3. Stepwise Inward Filling: Electrons do not occupy a new shell until the inner shells are filled in a stepwise manner.

Max capacity is $2n^2$; outermost shell can hold at most 8 electrons; inner shells fill first.
4
Write the electronic configurations of: (a) Oxygen ($Z=8$), (b) Magnesium ($Z=12$), (c) Potassium ($Z=19$), (d) Calcium ($Z=20$).
Reveal Answer & Explanation
Answer:

• (a) Oxygen ($Z=8$): $K=2, L=6$ ($2, 6$).
• (b) Magnesium ($Z=12$): $K=2, L=8, M=2$ ($2, 8, 2$).
• (c) Potassium ($Z=19$): By the octet rule, the $M$-shell holds 8 electrons before the $N$-shell begins filling: $K=2, L=8, M=8, N=1$ ($2, 8, 8, 1$).
• (d) Calcium ($Z=20$): $K=2, L=8, M=8, N=2$ ($2, 8, 8, 2$).


Oxygen: 2, 6; Magnesium: 2, 8, 2; Potassium: 2, 8, 8, 1; Calcium: 2, 8, 8, 2.
5
Why is an atom electrically neutral as a whole, despite containing charged protons and electrons?
Reveal Answer & Explanation
Answer:

• In any neutral atom, the number of positively charged protons in the nucleus is strictly equal to the number of negatively charged electrons orbiting in the shells ($p^+ = e^-$).
• Furthermore, the magnitude of the positive charge on a single proton ($+1.6 \times 10^{-19}\text{ C}$) is strictly identical to the negative charge on an electron ($-1.6 \times 10^{-19}\text{ C}$).
• Consequently, the positive and negative electrical charges completely cancel each other out, leaving the atom with a net electric charge of zero (neutral).


Number of protons equals number of electrons, and their equal and opposite charges cancel each other out.
6
What are Isotopes? Give the names and symbols of the three isotopes of Hydrogen.
Reveal Answer & Explanation
Answer:

• Definition: Isotopes are atoms of the same element that have the same atomic number ($Z$) (same number of protons and electrons), but different mass numbers ($A$) due to different numbers of neutrons in their nuclei.
• Three Isotopes of Hydrogen:
1. Protium ($_{1}^{1}\text{H}$): 1 proton, 0 neutrons, 1 electron ($99.98\%$ abundant).
2. Deuterium / Heavy Hydrogen ($_{1}^{2}\text{H}$ or $\text{D}$): 1 proton, 1 neutron, 1 electron.
3. Tritium ($_{1}^{3}\text{H}$ or $\text{T}$): 1 proton, 2 neutrons, 1 electron (radioactive).


Same atomic number, different mass numbers. Hydrogen isotopes: Protium ($_{1}^{1}H$), Deuterium ($_{1}^{2}H$), Tritium ($_{1}^{3}H$).
7
The mass number of an element $X$ is $39$ and it contains $20$ neutrons. Determine its atomic number, write its electronic configuration, and state whether it is a metal or non-metal.
Reveal Answer & Explanation
Answer:

Given: Mass number $A = 39$, Neutrons $N = 20$.
• Atomic Number ($Z$):

$$Z = A - N = 39 - 20 = \mathbf{19}$$


• The element with $Z = 19$ is Potassium ($K$).
• Electronic Configuration: $2, 8, 8, 1$.
• Since it has $1$ valence electron in its outermost shell which it readily loses to achieve a stable octet, it is an active Metal (Alkali metal).


$Z = 39 - 20 = 19$. Element is Potassium ($K$). Configuration is $2, 8, 8, 1$. It has 1 valence electron, so it is a metal.
8
Compare the properties of an Electron, a Proton, and a Neutron regarding mass and electrical charge.
Reveal Answer & Explanation
Answer:
  1. Mass:
    • A Proton and a Neutron have virtually identical masses of approximately $1\text{ amu}$ ($1.67 \times 10^{-24}\text{ g}$).
    • An Electron is extremely light, having a relative mass of only $\frac{1}{1837}\text{ amu}$ ($9.11 \times 10^{-28}\text{ g}$), which is considered negligible in mass calculations.
    2. Electrical Charge:
    • Proton: $+1$ unit positive charge ($+1.602 \times 10^{-19}\text{ C}$).
    • Electron: $-1$ unit negative charge ($-1.602 \times 10^{-19}\text{ C}$).
    • Neutron: $0$ (Neutral, no electrical charge).

Proton: +1 charge, 1 amu; Neutron: 0 charge, 1 amu; Electron: -1 charge, 1/1837 amu (negligible mass).
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