Class 11 Chemistry · Chapter 2 NotesStructure of Atom
Learn about the structure of atoms, subatomic particles, atomic models, quantum mechanical model, orbitals, electronic configurations, and periodic properties of…
This chapter explores the fundamental structure of atoms, the building blocks of matter. You will learn about the discovery of subatomic particles, atomic models from Dalton to Bohr, the quantum mechanical model, and the arrangement of electrons in atoms. Understanding atomic structure is crucial for grasping chemical behaviour and bonding. By the end of this chapter, you will be able to explain the properties of elements based on their electronic configurations and apply quantum mechanical principles to atomic theory.
What you'll learn
1Identify the subatomic particles (electrons, protons, neutrons) and their properties
2Compare the atomic models proposed by Dalton, Thomson, Rutherford, and Bohr
3Explain the quantum mechanical model of the atom and its key features
4Describe the shapes and energies of atomic orbitals using quantum numbers
5Apply the Aufbau principle, Pauli exclusion principle, and Hund's rule to determine electronic configurations
6Relate the periodic properties of elements to their atomic structure
Atoms consist of three fundamental subatomic particles: electrons, protons, and neutrons. Electrons, discovered by J.J. Thomson, are negatively charged particles with negligible mass. Protons, positively charged particles, were identified by Rutherford, while neutrons, uncharged particles, were discovered by Chadwick. The charge-to-mass ratio of the electron was determined by Thomson, and Millikan's oil drop experiment measured the electron's charge. Protons and neutrons, collectively called nucleons, make up the nucleus of an atom, with protons determining the atom's positive charge and neutrons contributing to its mass.
ElectronsNegative charge, negligible mass, discovered by J.J. Thomson
ProtonsPositive charge, mass ≈ 1.67 × 10^-27 kg, discovered by Rutherford
NeutronsNo charge, mass ≈ 1.67 × 10^-27 kg, discovered by Chadwick
02
Atomic Models: Dalton to Bohr
Early atomic models, such as Dalton's indivisible atom and Thomson's plum pudding model, were later refined by Rutherford's nuclear model, which proposed a small, dense nucleus containing protons and neutrons, with electrons orbiting around it. However, Rutherford's model could not explain the stability of atoms or the emission of discrete spectral lines. Bohr's model addressed these issues by introducing quantized electron orbits with specific energies. Electrons can jump between orbits, absorbing or emitting energy in the form of photons. Bohr's model successfully explained the hydrogen atom's spectrum but had limitations when applied to atoms with multiple electrons.
Dalton's modelAtoms are indivisible and indestructible
Thomson's modelElectrons embedded in a positively charged sphere
Rutherford's modelNucleus with electrons in orbit, explained alpha particle scattering
The quantum mechanical model, developed by Schrödinger, describes electrons as wave-particles with both particle-like and wave-like properties. This model uses wave functions (ψ) to describe the probability distribution of electrons in an atom. The Schrödinger equation provides solutions for the allowed energy levels and wave functions of electrons. Unlike Bohr's model, the quantum mechanical model does not specify exact paths for electrons, instead providing probabilities of finding electrons in certain regions. This model successfully explains the stability of atoms and their spectral properties.
Electrons exhibit wave-particle duality (de Broglie)
Wave functions (ψ) describe electron probability distribution
Schrödinger equation provides quantized energy levels
Heisenberg's uncertainty principleExact position and momentum cannot be simultaneously known
04
Orbitals and Quantum Numbers
Atomic orbitals are regions of space where electrons are likely to be found, described by wave functions. The shape and energy of an orbital are determined by quantum numbers: the principal quantum number (n), the azimuthal quantum number (l), and the magnetic quantum number (ml). The principal quantum number (n) defines the size and energy of the orbital, the azimuthal quantum number (l) defines the shape, and the magnetic quantum number (ml) defines the orientation. The electron spin quantum number (ms) describes the spin of the electron, which can be either +1/2 or -1/2. These quantum numbers help in understanding the arrangement of electrons in atoms and their chemical properties.
Principal quantum number (n)Determines size and energy of orbital
Azimuthal quantum number (l)Determines shape of orbital (s, p, d, f)
Magnetic quantum number (ml)Determines orientation of orbital
Electron spin quantum number (ms)Describes electron spin (+1/2 or -1/2)
05
Electronic Configuration and Aufbau Principle
The electronic configuration of an atom describes the arrangement of electrons in its orbitals. The Aufbau principle states that electrons fill orbitals in order of increasing energy. Pauli's exclusion principle states that no two electrons in an atom can have the same set of four quantum numbers. Hund's rule of maximum multiplicity states that electrons fill degenerate orbitals singly before pairing up. These principles help in predicting the electronic configurations of atoms, which in turn explain their chemical behaviour and periodic properties. The stability of half-filled and fully-filled subshells is due to symmetrical distribution of electrons and exchange energy.
Aufbau principleElectrons fill orbitals in order of increasing energy
Pauli exclusion principleNo two electrons can have the same set of four quantum numbers
Hund's ruleElectrons fill degenerate orbitals singly before pairing up
Stability of half-filled and fully-filled subshells due to symmetrical distribution and exchange energy
06
Periodicity and Properties of Elements
The periodic properties of elements, such as atomic radius, ionization energy, and electronegativity, can be explained by their electronic configurations. Elements in the same group have similar electronic configurations and exhibit similar chemical properties. The periodic table organizes elements based on their atomic numbers and electronic configurations, making it a powerful tool for predicting and understanding the behaviour of elements. The trends in the periodic table are due to the effective nuclear charge experienced by electrons, which increases with atomic number and affects the properties of elements.
Which of the following statements about electrons is correct?
AElectrons are positively charged particles found in the nucleus
BElectrons are negatively charged particles that orbit the nucleus
CElectrons have a mass equal to that of a proton
DElectrons are uncharged particles found in the nucleus
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Answer: (B) Electrons are negatively charged particles that orbit the nucleus
Electrons are negatively charged subatomic particles that move around the nucleus in electron shells or orbitals. They have negligible mass compared to protons and neutrons.
Question 02
According to Dalton's atomic theory, which of the following statements is correct?
AAtoms of different elements have different masses and properties
BAtoms can be created or destroyed in a chemical reaction
CAll atoms of an element are identical in mass but different in shape
DAtoms combine in non-integral ratios to form compounds
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Answer: (A) Atoms of different elements have different masses and properties
Dalton's theory states that atoms of the same element are identical in all respects but atoms of different elements differ in mass and chemical properties. This forms the basis of the law of definite proportions.
Question 03
Which scientist proposed the quantum mechanical model of the atom?
AErwin Schrödinger and Werner Heisenberg
BNiels Bohr and Ernest Rutherford
CJ.J. Thomson and Max Planck
DAlbert Einstein and Enrico Fermi
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Answer: (A) Erwin Schrödinger and Werner Heisenberg
Schrödinger and Heisenberg developed the quantum mechanical model in the late 1920s, which describes electrons as wave-particle entities in orbitals rather than fixed orbits.
Question 04
Which quantum number determines the size and energy level of an orbital?
APrincipal quantum number (n)
BAzimuthal quantum number (l)
CMagnetic quantum number (ml)
DSpin quantum number (ms)
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Answer: (A) Principal quantum number (n)
The principal quantum number n determines the size, energy, and distance of an orbital from the nucleus. As n increases, the orbital becomes larger and has higher energy.
Question 05
According to the Aufbau principle, which orbital is filled before the 4s orbital?
A3d orbital
B3p orbital
C4p orbital
D3s orbital
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Answer: (B) 3p orbital
The Aufbau principle states that electrons fill orbitals in order of increasing energy. The 3p orbital fills before 4s because it has lower energy, following the sequence 1s → 2s → 2p → 3s → 3p → 4s.
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Q1. What are the three fundamental subatomic particles found in an atom? Name them and state the location of each within an atom.
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Model answer
The three fundamental subatomic particles are: (1) Electrons – negatively charged particles found in the electron shells or orbitals surrounding the nucleus. (2) Protons – positively charged particles located in the nucleus at the centre of the atom. (3) Neutrons – electrically neutral particles also located in the nucleus alongside protons. Together, protons and neutrons form the dense nucleus, while electrons occupy the space around it.
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Q2. State Dalton's Atomic Theory and explain why it had to be modified. Which two discoveries led to modifications of Dalton's theory?
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Model answer
Dalton's Atomic Theory stated that all matter is made of indivisible atoms, atoms of the same element are identical, and atoms combine in simple whole number ratios to form compounds. However, this theory had to be modified because atoms were found to be divisible into smaller subatomic particles. Two key discoveries that led to modifications were: (1) J.J. Thomson's discovery of the electron in 1897, which showed atoms contain negatively charged particles; and (2) Ernest Rutherford's discovery of the nucleus in 1909 through his gold foil experiment, which revealed atoms have a dense, positively charged central core.
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Q3. What is the main difference between the Bohr model and the quantum mechanical model of the atom? Explain why the quantum mechanical model is considered more accurate.
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Model answer
The Bohr model proposed that electrons move in fixed circular orbits around the nucleus with definite energy levels, whereas the quantum mechanical model describes electrons as existing in orbitals or regions of probability around the nucleus. The quantum mechanical model is more accurate because it incorporates the wave-particle dual nature of electrons, as suggested by de Broglie, and explains the actual distribution of electron density rather than assuming fixed paths. Additionally, it successfully predicts the behavior of multi-electron atoms and accounts for all observed spectral lines.
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Q4. What is an orbital? How does it differ from the older concept of an electron orbit as proposed by Bohr?
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Model answer
An orbital is a three-dimensional region around the nucleus where there is a high probability of finding an electron. It differs from Bohr's orbit in that an orbit is a fixed, circular path with a definite radius and fixed energy, whereas an orbital has no fixed path, only a probability distribution. Orbitals are based on the wave nature of electrons and quantum mechanics, while Bohr's model treated electrons as particles moving in fixed paths.
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Q5. State the Aufbau principle and explain how it helps in writing the electronic configuration of atoms.
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Model answer
The Aufbau principle states that electrons fill orbitals in order of increasing energy levels, starting from the lowest energy orbital. According to this principle, electrons occupy orbitals with lower energy before moving to higher energy orbitals. This principle helps us write the electronic configuration of any atom systematically by following the energy sequence: 1s, 2s, 2p, 3s, 3p, 3d, 4s, and so on. Using this principle, we can predict the order in which electrons occupy different subshells and thus determine the correct electronic configuration of elements.
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What is the difference between an orbit and an orbital?
An orbit is a fixed path for an electron, as proposed by Bohr, while an orbital is a region of space where an electron is likely to be found, described by a wave function in the quantum mechanical model.
Why is the Bohr model limited to hydrogen-like atoms?
Bohr's model assumes a single electron orbiting the nucleus, which is only accurate for hydrogen-like atoms. It fails to explain the spectra of multi-electron atoms due to electron-electron interactions.
How does the quantum mechanical model explain the stability of atoms?
The quantum mechanical model describes electrons as wave-particles with probabilities of being in certain regions, eliminating the need for fixed orbits. The stability of atoms is explained by the quantized energy levels and wave functions.
What is the significance of the Pauli exclusion principle?
The Pauli exclusion principle states that no two electrons in an atom can have the same set of four quantum numbers, which explains the arrangement of electrons in orbitals and the electronic configurations of atoms.
How do the periodic properties of elements relate to their electronic configurations?
The periodic properties of elements, such as atomic radius, ionization energy, and electronegativity, are determined by their electronic configurations, which vary systematically across the periodic table.
Why do half-filled and fully-filled subshells have extra stability?
Half-filled and fully-filled subshells have extra stability due to symmetrical distribution of electrons and exchange energy, which results in lower energy and greater stability.