Structure of the Atom
Introduction
For a long time, scientists believed that atoms were indivisible. However, various experiments revealed that atoms are made up of even smaller particles called sub-atomic particles.
Discovery of Sub-Atomic Particles
1. Discovery of Electron
J.J. Thomson (1897)
Discovered the electron through cathode ray experiments.
Discovered the electron through cathode ray experiments.
Cathode Ray Experiment:
- Cathode rays are streams of negatively charged particles
- These particles were named electrons
- Electrons are negatively charged sub-atomic particles
- Mass of electron = 9.1 × 10⁻³¹ kg (approximately 1/2000 of hydrogen atom)
- Charge on electron = -1.6 × 10⁻¹⁹ coulombs
2. Discovery of Proton
E. Goldstein (1886)
Discovered canal rays which led to the discovery of protons.
Discovered canal rays which led to the discovery of protons.
Canal Ray Experiment:
- Canal rays (anode rays) are streams of positively charged particles
- Protons are positively charged sub-atomic particles
- Mass of proton = 1.67 × 10⁻²⁷ kg
- Charge on proton = +1.6 × 10⁻¹⁹ coulombs (equal and opposite to electron)
3. Discovery of Neutron
James Chadwick (1932)
Discovered the neutron, a neutral particle in the nucleus.
Discovered the neutron, a neutral particle in the nucleus.
Properties of Neutron:
- Neutrons are neutral sub-atomic particles (no charge)
- Mass of neutron = 1.67 × 10⁻²⁷ kg (approximately equal to proton)
- Present in the nucleus along with protons
Comparison of Sub-Atomic Particles
| Particle | Symbol | Charge | Mass (kg) | Location |
|---|---|---|---|---|
| Electron | e⁻ | -1 (negative) | 9.1 × 10⁻³¹ | Around nucleus |
| Proton | p⁺ | +1 (positive) | 1.67 × 10⁻²⁷ | In nucleus |
| Neutron | n | 0 (neutral) | 1.67 × 10⁻²⁷ | In nucleus |
Thomson's Model of Atom (1898)
Plum Pudding Model / Watermelon Model
According to Thomson, an atom is a sphere of positive charges with electrons embedded in it like seeds in a watermelon.
According to Thomson, an atom is a sphere of positive charges with electrons embedded in it like seeds in a watermelon.
Features:
- Atom is a positive sphere with electrons embedded in it
- Positive and negative charges are equal, making atom neutral
- Electrons are like plums in a pudding or seeds in a watermelon
Limitations:
- Could not explain the results of Rutherford's alpha particle scattering experiment
- Could not explain the stability of atoms
Rutherford's Model of Atom (1911)
Ernest Rutherford
Conducted the famous alpha particle scattering experiment (Gold foil experiment)
Conducted the famous alpha particle scattering experiment (Gold foil experiment)
Alpha Particle Scattering Experiment:
- Rutherford bombarded a very thin gold foil with alpha particles
- Alpha particles are positively charged particles (Helium nuclei)
- A fluorescent screen was placed to detect the scattered particles
Observations:
- Most alpha particles passed straight through the gold foil without deflection
- Some alpha particles were deflected by small angles
- Very few alpha particles (1 in 20,000) bounced back or were deflected by large angles
Conclusions:
- Most of the space in an atom is empty (as most particles passed through)
- All the positive charge and most of the mass is concentrated in a very small region called nucleus
- Electrons revolve around the nucleus in circular orbits
- The size of nucleus is very small compared to the size of atom
Rutherford's Nuclear Model:
• Atom consists of a tiny, dense, positively charged nucleus at the center
• Nucleus contains protons and neutrons
• Electrons revolve around the nucleus in circular paths called orbits
• Most of the atom is empty space
• Size of nucleus is about 10⁵ times smaller than the size of atom
• Atom consists of a tiny, dense, positively charged nucleus at the center
• Nucleus contains protons and neutrons
• Electrons revolve around the nucleus in circular paths called orbits
• Most of the atom is empty space
• Size of nucleus is about 10⁵ times smaller than the size of atom
Limitations of Rutherford's Model:
- According to electromagnetic theory, a charged particle moving in a circular path should emit energy continuously
- This would cause electrons to lose energy and spiral into the nucleus
- The atom would collapse, but atoms are stable
- Could not explain the stability of atoms
Bohr's Model of Atom (1913)
Niels Bohr
Modified Rutherford's model to explain the stability of atoms
Modified Rutherford's model to explain the stability of atoms
Postulates of Bohr's Model:
- Electrons revolve around the nucleus in fixed circular paths called orbits or shells
- Each orbit has a fixed energy level (K, L, M, N or 1, 2, 3, 4)
- Electrons do not emit energy while revolving in these fixed orbits
- Energy is absorbed or emitted only when an electron jumps from one orbit to another
- The orbits are also called stationary states or energy levels
Distribution of Electrons in Different Orbits
The distribution of electrons in different orbits follows certain rules:
Maximum number of electrons in a shell = 2n²
where n = shell number (K=1, L=2, M=3, N=4)
where n = shell number (K=1, L=2, M=3, N=4)
Maximum electrons in shells:
- K shell (n=1): Maximum 2 electrons (2×1² = 2)
- L shell (n=2): Maximum 8 electrons (2×2² = 8)
- M shell (n=3): Maximum 18 electrons (2×3² = 18)
- N shell (n=4): Maximum 32 electrons (2×4² = 32)
Rules for Electronic Distribution:
- Electrons are filled in shells starting from the innermost shell (K shell)
- Maximum electrons in outermost shell cannot exceed 8
- Maximum electrons in second last shell cannot exceed 18
- Shells are filled in order: K, L, M, N...
Valency
Valency: The combining capacity of an atom is called valency. It is determined by the number of electrons in the outermost shell.
Rules to determine valency:
- If outermost shell has 1-4 electrons, valency = number of electrons in outermost shell
- If outermost shell has 5-8 electrons, valency = 8 - number of electrons in outermost shell
- If outermost shell has 8 electrons (or 2 for K shell), valency = 0 (inert/noble gases)
Examples:
- Hydrogen (1 electron in K shell): Valency = 1
- Carbon (4 electrons in outermost shell): Valency = 4
- Oxygen (6 electrons in outermost shell): Valency = 8-6 = 2
- Chlorine (7 electrons in outermost shell): Valency = 8-7 = 1
- Neon (8 electrons in outermost shell): Valency = 0
Atomic Number and Mass Number
Atomic Number (Z): The number of protons in the nucleus of an atom is called atomic number. It is denoted by Z.
Mass Number (A): The sum of number of protons and neutrons in the nucleus is called mass number. It is denoted by A.
Mass Number (A) = Number of Protons (Z) + Number of Neutrons (N)
Number of Neutrons = Mass Number - Atomic Number
N = A - Z
Number of Neutrons = Mass Number - Atomic Number
N = A - Z
Important Points:
- Atomic number = Number of protons = Number of electrons (in neutral atom)
- Mass number is always a whole number
- Atomic number is unique for each element
- Electrons have negligible mass, so they don't contribute to mass number
Representation of an Atom
Symbolic Representation:
• X = Symbol of element
• A = Mass number (top left)
• Z = Atomic number (bottom left)
AZX
Where:• X = Symbol of element
• A = Mass number (top left)
• Z = Atomic number (bottom left)
Examples:
- ¹²₆C - Carbon with mass number 12 and atomic number 6
- ¹⁶₈O - Oxygen with mass number 16 and atomic number 8
- ²³₁₁Na - Sodium with mass number 23 and atomic number 11
- ³⁵₁₇Cl - Chlorine with mass number 35 and atomic number 17
Isotopes
Isotopes: Atoms of the same element having same atomic number but different mass numbers are called isotopes.
Characteristics of Isotopes:
- Same number of protons (same atomic number)
- Same number of electrons
- Different number of neutrons (different mass number)
- Same chemical properties (as electrons are same)
- Different physical properties (due to different mass)
Examples of Isotopes:
1. Isotopes of Hydrogen:
- Protium (¹₁H): 1 proton, 0 neutron
- Deuterium (²₁H): 1 proton, 1 neutron
- Tritium (³₁H): 1 proton, 2 neutrons
2. Isotopes of Carbon:
- Carbon-12 (¹²₆C): 6 protons, 6 neutrons
- Carbon-14 (¹⁴₆C): 6 protons, 8 neutrons (radioactive)
3. Isotopes of Chlorine:
- Chlorine-35 (³⁵₁₇Cl): 17 protons, 18 neutrons
- Chlorine-37 (³⁷₁₇Cl): 17 protons, 20 neutrons
Uses of Isotopes:
- Carbon-14: Used in radiocarbon dating to determine age of fossils
- Uranium-235: Used as fuel in nuclear reactors
- Cobalt-60: Used in treatment of cancer
- Iodine-131: Used in treatment of thyroid diseases
- Sodium-24: Used to detect blood clots
Isobars
Isobars: Atoms of different elements having same mass number but different atomic numbers are called isobars.
Characteristics of Isobars:
- Different number of protons (different atomic number)
- Different number of electrons
- Same mass number
- Different chemical properties
- Different physical properties
Examples of Isobars:
- ⁴⁰₁₈Ar (Argon) and ⁴⁰₂₀Ca (Calcium) - both have mass number 40
- ¹⁴₆C (Carbon) and ¹⁴₇N (Nitrogen) - both have mass number 14
Key Differences
| Property | Isotopes | Isobars |
|---|---|---|
| Atomic Number | Same | Different |
| Mass Number | Different | Same |
| Number of Protons | Same | Different |
| Number of Neutrons | Different | Different |
| Chemical Properties | Same | Different |
| Element | Same element | Different elements |
Important Points to Remember
- Atom is mostly empty space with a tiny, dense nucleus
- Nucleus contains protons and neutrons
- Electrons revolve around nucleus in fixed orbits
- Number of protons = Number of electrons (in neutral atom)
- Atomic number is characteristic of an element
- Isotopes have same chemical properties
- Maximum electrons in outermost shell = 8
- Valency is determined by outermost electrons
Multiple Choice Questions (MCQ)
Subjective Questions
Practice these questions to strengthen your understanding. Write your answers in the space provided.