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Chemistry · Class 11 Science

Ch 2Structure of Atom — Class 11 Chemistry, concept-first.

An atom consists of a tiny, dense, positively charged nucleus (containing protons and neutrons) surrounded by electrons occupying the much larger space around it.

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Bohr's Model

Niels Bohr's 1913 model for the hydrogen atom rests on four postulates, drawing on Planck's quantum theory and hydrogen's line emission spectrum to fix the instability that plagued Rutherford's model.

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Chapter contents

The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.

2.1

Introduction

An atom consists of a tiny, dense, positively charged nucleus (containing protons and neutrons) surrounded by electrons occupying the much larger space around it.

2.2

Bohr's Model of the Atom and Its Limitations

Niels Bohr (1913) built the first atomic model to correctly reproduce the observed hydrogen line spectrum, by grafting a single quantum assumption onto an otherwise classical picture of an electron or…

2.3

Concept of Shells and Subshells

In the fuller quantum mechanical picture, Bohr's orbits are reinterpreted as shells, and each shell is further divided into one or more subshells.

2.4

Dual Nature of Matter and Light: de Broglie's Relationship

By the early 1920s, the photoelectric effect had already forced physicists to accept that light -- long understood purely as a wave, on the strength of interference and diffraction experiments -- also…

2.5

Heisenberg's Uncertainty Principle

Once matter is accepted to have wave character, a serious conceptual problem follows: a wave, by its very nature, is spread out over some region of space, so it does not make sense to ask exactly wher…

2.6

The Schrödinger Wave Equation: An Elementary Idea

Since an electron in an atom has an associated wave (de Broglie, Section 2.4) and its exact path cannot even in principle be known (Heisenberg, Section 2.5), the correct way to describe an electron's…

2.7

Quantum Numbers

Solving the Schrödinger equation for an atom does not give a single wave function but a whole family of them, each one labelled by a specific combination of quantum numbers.

2.8

Shapes of s, p and d Orbitals

The azimuthal quantum number fixes a characteristic boundary-surface shape that is the same for every orbital of a given subshell type, in every atom -- only the size of the orbital changes with the p…

2.9

Filling of Orbitals: Aufbau, Pauli and Hund's Rule

Three rules, applied together, determine exactly how the electrons of a many-electron atom are distributed among its orbitals in the ground state.

2.10

Electronic Configuration of Atoms, Exchange Energy and Orbital Stability

Applying the Aufbau order, the Pauli exclusion principle and Hund's rule in sequence gives the ground-state electronic configuration of an atom: the number of electrons in each subshell, written as a…

Summary

This chapter replaced the simple, purely classical picture of an atom with the quantum mechanical model that chemistry actually uses.

Sample & Board Papers

Sample papers and previous-year board questions for this subject.

+Show 2 questions2 questions
  1. Q1Which of the following is the ground state electronic configuration of Cr? (Atomic number of Cr is 24) (a) 1s²2s²2p⁶3s²3p⁶3d⁴4s² (b) 1s²2s²2…Preview
  2. Q2(i) Write with an example the condition for two atoms to be considered as isobars. (ii) Why is Fe³⁺ more stable than Fe²⁺? [2 + 1] **OR** (i…Preview

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+Show 12 questions12 questions
  1. Example 1State the two postulates of Bohr's model of the hydrogen atom that (a) quantise the angular momentum of the electron and (b) fix the energy…Free
  2. Example 2Calculate the energy of an electron in the second Bohr orbit of a hydrogen atom. (Given: $E_n = -2.18 \times 10^{-18}\ \dfrac{Z^2}{n^2}\ \te…Free
  3. Example 3Calculate the radius of the third Bohr orbit of the hydrogen atom. (Given: $r_n = 52.9\, \dfrac{n^2}{Z}\ \text{pm}$.)Free
  4. Example 4Calculate the de Broglie wavelength associated with a ball of mass $0.1\ \text{kg}$ moving with a velocity of $10\ \text{m/s}$.Preview
  5. Example 5Calculate the de Broglie wavelength of an electron moving with a velocity of $1.0 \times 10^{6}\ \text{m/s}$ (mass of electron $= 9.11 \time…Preview
  6. Example 6The uncertainty in the position of an electron is $100\ \text{pm}$. Calculate the minimum uncertainty in its velocity.Preview
  7. Example 7The velocity of an electron is known to an accuracy of $1\%$ at $300\ \text{m/s}$. Calculate the minimum uncertainty in specifying its posit…Preview
  8. Example 8What are the possible values of the magnetic quantum number ($m_l$) for an electron with azimuthal quantum number $l = 2$?Preview
  9. Example 9Write the set of four quantum numbers for the last (differentiating) electron of a chlorine atom ($Z = 17$).Preview
  10. Example 10Using the $(n+l)$ rule, decide whether the $4s$ or the $3d$ subshell is filled first, and state which of the two has the lower energy in a m…Preview
  11. Example 11Write the ground-state electronic configuration of chromium ($Z = 24$) and explain why it deviates from the configuration predicted by the s…Preview
  12. Example 12Write the ground-state electronic configuration of copper ($Z = 29$) and explain the anomaly in its filling.Preview
+Show 16 questions16 questions
  1. Q13State two major limitations of Bohr's model of the atom.Free
  2. Q14Calculate the energy required to excite an electron in a hydrogen atom from $n = 1$ to $n = 2$. Express your answer in both joules and elect…Free
  3. Q15Calculate the wavelength of the photon emitted when an electron in a hydrogen atom falls from $n = 4$ to $n = 2$. (Rydberg constant $R_H = 1…Free
  4. Q16Calculate the de Broglie wavelength of an electron that has been accelerated from rest through a potential difference of $100\ \text{V}$.Preview
  5. Q17How many subshells and how many orbitals are there in the shell with principal quantum number $n = 4$?Preview
  6. Q18What is the maximum number of electrons that can be accommodated in the $M$ shell ($n = 3$) of an atom?Preview
  7. Q19Give the orbital notation and the general shape of the boundary surface for (a) $l = 0$ and (b) $l = 1$.Preview
  8. Q20Describe the shape of the $d_{z^2}$ orbital and explain how it differs from the shape of the other four $d$ orbitals.Preview
  9. Q21Using Hund's rule, draw the orbital (box) diagram for the $2p$ subshell of nitrogen ($Z = 7$, configuration $1s^2\,2s^2\,2p^3$).Preview
  10. Q22Using the idea of exchange energy, explain why the half-filled configuration $2p^3$ of nitrogen is more stable than a hypothetical arrangeme…Preview
  11. Q23Write the complete ground-state electronic configuration of iron ($Z = 26$) and find the number of unpaired electrons in it.Preview
  12. Q24Give the set of four quantum numbers for the $19^{\text{th}}$ (valence) electron of potassium ($Z = 19$).Preview
  13. Q25State the Pauli exclusion principle and give one direct consequence it has for the maximum number of electrons an orbital can hold.Preview
  14. Q26If the uncertainty in the position of an electron is taken to be $1 \times 10^{-15}\ \text{m}$ (of the order of a nuclear diameter), calcula…Preview
  15. Q27Using the $(n+l)$ rule, arrange the subshells $3d$, $4s$, $4p$ and $5s$ in increasing order of energy.Preview
  16. Q28Write the ground-state electronic configuration of molybdenum ($Z = 42$) and explain the anomaly, if any, in its filling.Preview