CBSE Class 9 > Class 9 Notes > Science New NCERT 2026-27 (New Syllabus) > Cheat Sheet: Journey Inside The Atom
Cheat Sheet: Journey Inside The Atom
1. Introduction & Ancient Ideas
1.1 Introduction
- Matter: everything that can be seen, observed, or felt.
- Matter consists of tiny particles called atoms.
- Central question: Is an atom the smallest unit of matter or divisible?
1.2 Ancient ideas about matter
| Thinker | Idea |
|---|---|
| Acharya Kanada | Repeated division of matter reaches smallest indivisible particles called parmanus. |
| Parmanu combinations | Parmanus combine to form dyads and triads that create material universe. |
| Limitations of Kanada's idea | No specification of proportions for combinations to form different substances. |
| Leucippus and Democritus | Proposed indivisible particles called atomos (Greek for indivisible). |
| Origin of the atom concept | Originated as an imaginary idea rather than from experiments. |
2. Discoveries and Atomic Models
2.1 Discovery of the electron (J. J. Thomson, 1897)
| Fact | Detail |
|---|---|
| Experiment | Conduction of electric current through low-pressure gases using cathode ray tube. |
| Cathode rays | Rays moving from cathode to anode observed in electric and magnetic fields. |
| Universal nature | Cathode rays independent of cathode material and gas, indicating a fundamental component of all atoms. |
| Discovery year | 1897 |
| Scientist | J. J. Thomson |
| Nobel Prize | Thomson received Nobel Prize in Physics in 1906 |
2.2 Thomson's model of the atom
| Feature | Description |
|---|---|
| Atomic structure | Atom is a sphere of positive charge with electrons embedded throughout. |
| Metaphors | Plum pudding model; watermelon analogy. |
| Purpose | Explained overall electrical neutrality by distributed positive charge. |
2.3 Gold foil (alpha-scattering) experiment
| Aspect | Detail |
|---|---|
| Researchers | Geiger and Marsden under Ernest Rutherford (1911) |
| Setup | Narrow beam of alpha particles aimed at thin gold foil. |
| Alpha particle composition | Nucleus of helium atom: two protons and two neutrons. |
| Expected (Thomson) | Alpha particles pass straight through or deflect slightly. |
| Observed | Most passed undeflected; some deflected at large angles; a few bounced back. |
| Term | Deflection of alpha particles called scattering. |
| Conclusion | Thomson's model failed to explain large-angle deflections. |
2.4 Rutherford's nuclear model (1911)
| Feature | Detail |
|---|---|
| Central idea | Positive charge concentrated in a very small nucleus. |
| Atomic space | Most of atom is empty space. |
| Electron arrangement | Electrons revolve around nucleus (planetary model). |
| Important measurements | Atomic diameter ≈ 10⁻¹⁰ m. |
| Nucleus size | Nucleus diameter ≈ 10⁻¹⁵ m. |
| Relative size | Nucleus ≈ 10⁵ times smaller than atom. |
| Analogy | If atom ~100 m, nucleus ~a few mm at centre. |
| Scientist | Ernest Rutherford (1908 Nobel Prize in Chemistry) |
2.5 Limitations of Rutherford's model
- An accelerating charged electron should lose energy and spiral into nucleus.
- Rutherford's model could not explain atomic stability.
2.6 Discovery of the proton
| Fact | Detail |
|---|---|
| Positive charge location | Positive charge in nucleus arises from protons. |
| Neutrality rule | For neutrality, number of protons equals number of electrons. |
| Relative mass | Protons are much heavier than electrons. |
| Examples | Helium: 2 protons and 2 electrons; Sodium: 11 protons and 11 electrons. |
3. Bohr Model and Electron Distribution
3.1 Bohr's model (Niels Bohr, 1913)
| Postulate | Detail |
|---|---|
| Stationary states | Electrons move in fixed circular paths (orbits/shells) without losing energy. |
| Shell notation | Shells labeled K, L, M, N or n = 1, 2, 3, 4,... |
| Energy ordering | Energy increases with shell number; K (n=1) has least energy. |
| Transitions | Electrons change shells by absorbing or releasing energy equal to level differences. |
| Shell capacity | Each shell can hold a definite number of electrons. |
| Scientist | Niels Bohr (Nobel Prize 1922) |
3.2 Rules for electron distribution (Bohr-Bury)
| Rule | Expression |
|---|---|
| Maximum electrons per shell | 2n² where n is shell number. |
| K-shell (n=1) | 2 electrons maximum. |
| L-shell (n=2) | 8 electrons maximum. |
| M-shell (n=3) | 18 electrons maximum. |
| Outermost shell limit | Maximum electrons in outermost shell = 8. |
| Filling order | Shells filled starting from K outward (K, L, M, N,...). |
| Electronic configuration | Distribution of electrons among shells. |
3.3 Electronic configuration of first 18 elements - Element → Symbol
| Element | Symbol |
|---|---|
| Hydrogen | H |
| Helium | He |
| Lithium | Li |
| Beryllium | Be |
| Boron | B |
| Carbon | C |
| Nitrogen | N |
| Oxygen | O |
| Fluorine | F |
| Neon | Ne |
| Sodium | Na |
| Magnesium | Mg |
| Aluminium | Al |
| Silicon | Si |
| Phosphorus | P |
| Sulfur | S |
| Chlorine | Cl |
| Argon | Ar |
| Element | Atomic No. |
|---|---|
| Hydrogen | 1 |
| Helium | 2 |
| Lithium | 3 |
| Beryllium | 4 |
| Boron | 5 |
| Carbon | 6 |
| Nitrogen | 7 |
| Oxygen | 8 |
| Fluorine | 9 |
| Neon | 10 |
| Sodium | 11 |
| Magnesium | 12 |
| Aluminium | 13 |
| Silicon | 14 |
| Phosphorus | 15 |
| Sulfur | 16 |
| Chlorine | 17 |
| Argon | 18 |
| Element | Protons |
|---|---|
| Hydrogen | 1 |
| Helium | 2 |
| Lithium | 3 |
| Beryllium | 4 |
| Boron | 5 |
| Carbon | 6 |
| Nitrogen | 7 |
| Oxygen | 8 |
| Fluorine | 9 |
| Neon | 10 |
| Sodium | 11 |
| Magnesium | 12 |
| Aluminium | 13 |
| Silicon | 14 |
| Phosphorus | 15 |
| Sulfur | 16 |
| Chlorine | 17 |
| Argon | 18 |
| Element | Neutrons |
|---|---|
| Hydrogen | - |
| Helium | 2 |
| Lithium | 4 |
| Beryllium | 5 |
| Boron | 6 |
| Carbon | 6 |
| Nitrogen | 7 |
| Oxygen | 8 |
| Fluorine | 10 |
| Neon | 10 |
| Sodium | 12 |
| Magnesium | 12 |
| Aluminium | 14 |
| Silicon | 14 |
| Phosphorus | 16 |
| Sulfur | 16 |
| Chlorine | 18 |
| Argon | 22 |
| Element | Electrons |
|---|---|
| Hydrogen | 1 |
| Helium | 2 |
| Lithium | 3 |
| Beryllium | 4 |
| Boron | 5 |
| Carbon | 6 |
| Nitrogen | 7 |
| Oxygen | 8 |
| Fluorine | 9 |
| Neon | 10 |
| Sodium | 11 |
| Magnesium | 12 |
| Aluminium | 13 |
| Silicon | 14 |
| Phosphorus | 15 |
| Sulfur | 16 |
| Chlorine | 17 |
| Argon | 18 |
| Element | K-shell |
|---|---|
| Hydrogen | 1 |
| Helium | 2 |
| Lithium | 2 |
| Beryllium | 2 |
| Boron | 2 |
| Carbon | 2 |
| Nitrogen | 2 |
| Oxygen | 2 |
| Fluorine | 2 |
| Neon | 2 |
| Sodium | 2 |
| Magnesium | 2 |
| Aluminium | 2 |
| Silicon | 2 |
| Phosphorus | 2 |
| Sulfur | 2 |
| Chlorine | 2 |
| Argon | 2 |
| Element | L-shell |
|---|---|
| Hydrogen | - |
| Helium | - |
| Lithium | 1 |
| Beryllium | 2 |
| Boron | 3 |
| Carbon | 4 |
| Nitrogen | 5 |
| Oxygen | 6 |
| Fluorine | 7 |
| Neon | 8 |
| Sodium | 8 |
| Magnesium | 8 |
| Aluminium | 8 |
| Silicon | 8 |
| Phosphorus | 8 |
| Sulfur | 8 |
| Chlorine | 8 |
| Argon | 8 |
| Element | M-shell |
|---|---|
| Hydrogen | - |
| Helium | - |
| Lithium | - |
| Beryllium | - |
| Boron | - |
| Carbon | - |
| Nitrogen | - |
| Oxygen | - |
| Fluorine | - |
| Neon | - |
| Sodium | 1 |
| Magnesium | 2 |
| Aluminium | 3 |
| Silicon | 4 |
| Phosphorus | 5 |
| Sulfur | 6 |
| Chlorine | 7 |
| Argon | 8 |
4. Subatomic Particles, Atomic Number & Mass Number
4.1 Components contributing to atomic mass
- Most atomic mass concentrated in nucleus (protons + neutrons).
- Electron mass is negligible for atomic mass calculations.
- Observation: helium mass ≈ 4× hydrogen mass despite having two protons - implies additional nuclear mass from neutrons.
4.2 Discovery of the neutron (James Chadwick, 1932)
| Fact | Detail |
|---|---|
| Discovery year | 1932 |
| Discoverer | James Chadwick |
| Neutron properties | Neutral particle with mass nearly equal to proton; symbol n |
| Occurrence | Neutrons present in nuclei of all atoms except hydrogen. |
| Impact | Explains atomic masses greater than proton counts and enables nuclear reactions. |
| Nobel Prize | Chadwick received Nobel Prize in Physics in 1935 |
4.3 Subatomic particle symbols and relative charges
| Subatomic Particle | Symbol / Relative Charge |
|---|---|
| Electron | e⁻ / -1 |
| Proton | p⁺ / +1 |
| Neutron | n⁰ / 0 |
4.4 Atomic number (Z) and Mass number (A)
| Term | Definition / Expression |
|---|---|
| Atomic number (Z) | Number of protons in the nucleus; determines element identity; equal to number of electrons in neutral atom. |
| Mass number (A) | Total number of protons and neutrons (nucleons) in nucleus. |
| Mass number formula | A = Z + number of neutrons (n) |
| Standard atomic notation | Mass number on top-left, atomic number on bottom-left, followed by element symbol (example: ¹²₆C). |
5. Symbols of Elements & Valency
5.1 Rules for chemical symbols (IUPAC norms)
| Rule | Detail |
|---|---|
| Letter case | First letter uppercase; second letter lowercase if present. |
| Source of symbols | Often first one or two letters of element name or derived from Latin/Greek/German names. |
| Purpose | International standard to communicate element names and symbols. |
5.2 Names and symbols of some common elements
| Element | Symbol |
|---|---|
| Aluminium | Al |
| Argon | Ar |
| Barium | Ba |
| Boron | B |
| Bromine | Br |
| Calcium | Ca |
| Carbon | C |
| Chlorine | Cl |
| Cobalt | Co |
| Copper (Cuprum) | Cu |
| Fluorine | F |
| Gold (Aurum) | Au |
| Hydrogen | H |
| Iodine | I |
| Iron (Ferrum) | Fe |
| Lead (Plumbum) | Pb |
| Magnesium | Mg |
| Neon | Ne |
| Nitrogen | N |
| Oxygen | O |
| Potassium (Kalium) | K |
| Silicon | Si |
| Silver (Argentum) | Ag |
| Sodium (Natrium) | Na |
| Sulfur | S |
| Uranium | U |
| Zinc | Zn |
5.3 Valency, valence shell and octet
| Term | Definition |
|---|---|
| Valence shell | Outermost shell of an atom. |
| Valence electrons | Electrons present in valence shell. |
| Octet | Condition when outermost shell has 8 electrons (2 for helium). |
| Valency | Number of electrons gained, lost, or shared to complete octet. |
| Loss/gain rule | Elements with <4 valence="" electrons="" tend="" to="" lose="" electrons;="" with="">4 tend to gain electrons to complete octet. |
| Noble gases | Atoms with complete octet or 2 electrons (helium) have valency 0 and are largely unreactive. |
| Element | Valency / Electronic configuration |
|---|---|
| Sodium | Valency 1; configuration 2, 8, 1 |
| Oxygen | Valency 2; configuration 2, 6 |
| Carbon | Valency 4; configuration 2, 4 |
6. Isotopes, Isobars and Atomic Mass
6.1 Isotopes
| Definition | Atoms of same element with same atomic number but different mass numbers. |
|---|---|
| Protium ¹₁H | ~99.98% - 1 proton, 0 neutrons, 1 electron. |
| Deuterium ²₁H | ~0.015% - 1 proton, 1 neutron, 1 electron. |
| Tritium ³₁H | In traces - 1 proton, 2 neutrons, 1 electron. |
| Carbon isotopes | ¹²₆C, ¹³₆C, ¹⁴₆C - each has 6 protons and 6 electrons; differ in neutrons. |
| Chemical properties | Isotopes have similar chemical properties due to identical electronic configurations. |
| Applications | ²³⁵₉₂U fuel in reactors; ⁶⁰₂₇Co in cancer radiation treatment; ¹³¹₅₃I in thyroid treatment; ¹⁴₆C in archaeological dating. |
6.2 Isobars
| Definition | Atoms of different elements with same mass number but different atomic numbers. |
|---|---|
| Example | ⁴⁰₂₀Ca, ⁴⁰₁₉K, ⁴⁰₁₈Ar share mass number 40 but have different Z. |
6.3 Average and weighted average atomic mass
| Concept | Detail / Expression |
|---|---|
| Simple average (chlorine) | (35 + 37) / 2 = 36 u |
| Weighted average | Sum of (isotope mass × fractional abundance). |
| Chlorine weighted example | (35 × 75/100) + (37 × 25/100) = 35.5 u |
| Unit | Unified atomic mass unit (u) used to measure atomic mass. |
7. Development Summary & Key Terms
7.1 Development of atomic models - chronological
- Dalton: Atom as indivisible particle.
- Thomson: Plum pudding model (positive sphere with embedded electrons).
- Rutherford: Nuclear model with dense central nucleus.
- Bohr: Electrons in fixed energy levels (shells).
- Modern atomic model: Quantum mechanical model (electron clouds).
- Ongoing research
7.2 Key terms
| Term | Meaning |
|---|---|
| Atom | The smallest unit of an element that retains its chemical properties |
| Parmanu | Ancient Indian concept for the smallest indivisible particle of matter (by Acharya Kanada) |
| Atomos | Greek word meaning indivisible; the origin of the word 'atom' |
| Radioactivity | Emission of invisible energy and particles from certain elements |
| Electron | Negatively charged subatomic particle; charge = -1; negligible mass |
| Cathode rays | Streams of negatively charged particles (electrons) moving from cathode to anode |
| Proton | Positively charged subatomic particle; charge = +1; found in nucleus |
| Neutron | Neutral subatomic particle; no charge; mass nearly equal to proton; found in nucleus |
| Nucleus | Dense, positively charged central core of an atom containing protons and neutrons |
| Nucleons | Protons and neutrons collectively |
| Plum Pudding Model | Thomson's model - atom as sphere of positive charge with electrons embedded in it |
| Planetary Model | Rutherford's model - electrons revolve around a central nucleus like planets around the Sun |
| Alpha (α) particles | Tiny, positively charged particles emitted from radioactive elements; nucleus of helium atom |
| Scattering | Deflection of alpha particles from their straight path in the gold foil experiment |
| Shell / Orbit | Fixed circular paths followed by electrons around the nucleus (Bohr's model) |
| Energy levels | Different shells representing different definite energies for electrons |
| Stationary states | Fixed shells/orbits in which electrons revolve without losing energy (Bohr's concept) |
| Atomic number (Z) | Number of protons in the nucleus of an atom |
| Mass number (A) | Total number of protons and neutrons (nucleons) in the nucleus |
| Electronic configuration | Distribution of electrons among various shells of an atom |
| Valence shell | The outermost shell of an atom |
| Valence electrons | Electrons present in the valence (outermost) shell |
| Octet | A condition where an atom has 8 electrons in its outermost shell |
| Valency | The combining capacity of an atom - number of electrons gained, lost, or shared to complete the octet |
| Isotopes | Atoms of the same element with same atomic number but different mass numbers |
| Isobars | Atoms of different elements with same mass number but different atomic numbers |
| Weighted average atomic mass | Average mass of an element calculated by considering the relative abundances of its isotopes |
| IUPAC | International Union of Pure and Applied Chemistry - body that approves names and symbols of elements |
| Unified atomic mass unit (u) | Unit used to measure the mass of atoms |
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