[Revision] Chemistry Part-1: Matter, Mass, Isotopes, Isobars,X-Ray, Alpha-Ray, Gamma-Ray, Cathode-Ray & More[Mrunal]

  • This year’s UPSC CAPF paper contained significant number of questions science, particularly higher level chemistry (Above class7-10 NCERTs)
  • In recent years, UPSC has been shifting from Class 7 to 10 Science NCERTs towards 11-12 NCERTS) – This has been evident in all 3 exams conducted by UPSC viz. CAPF, CDS and CSE-prelims.
  • Therefore, with help of Venkat sir, I’m preparing selected “revision notes” out of NCERT Science Books (11, 12).  Most important: can’t guarantee this project will be finished before prelim comes.
  • First part of this article series deals with Class11 Chemistry NCERT Chapter 1 (Basic concepts of matter) and chapter 2 (Structure of Atom).

Matter classification


Properties of a Substance

physical properties chemical properties
  • they’re measured or observed without changing the identity of material- e.g. colour,  odour, melting point, boiling point, density
  • we can observe them only after chemical change occurs.
  • e.g. acidity or basicity, combustibility

SI system of measurement

  • Through Metre convention in Paris (1875), this International system of units was setup.
  • accordingly, each country has an institute to maintain standards of measurement
  • e.g. in India it is done by National physical laboratory @Delhi.

7 Basic units in SI measurement


Definition of SI units

Difference between Mass and weight

Mass Weight
A substance’s mass will remain same everywhere Will change depending on gravity of the given place.
Measured using “Analytical balance” measured using weighing scale

Why Pt-IR cylinder for weight standardization?

  • The Mass standard is kilogram.
  • Kilogram is defined as the mass of platinum-iridium (Pt-Ir) cylinder that is stored in an airtight jar at International Bureau of Weights and Measures in Sevres, France.
  • Pt-Ir was chosen for this standard because it is highly resistant to chemical attack and its mass will not change for an extremely long time.
  • Assertion reasoning question possible from above factoids


°C (degree celsius)
  • calibrated from 0° to 100° where these two temperatures are the freezing point and the boiling point of water respective
  • Negative values possible in celcius
°F (degree fahrenheit)
  • fahrenheit scale is represented between 32° to 212°.
  • F = [9C/5]+32
K (kelvin)
  • K=C+273.1
  • Negative values not possible
Five Laws of Chemical combination

Law of Detail
1.Conservation of Mass
  • matter can neither be created nor destroyed – Antoine Lavoisier (1789)
2.Definite Proportions
  • a given compound always contains exactly the same proportion of elements – Joseph Proust
  • E.g. both naturally occurring and artificially synthesized Cupric carbonate will have same elements in same ratio.
3.Multiple Proportions
  • if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers – Dalton
  • example 2 gm Hydrogen + 16 gm Oxygen = 18 gm water
  • but 2 gm Hydrogen + 32 gm Oxygen = 34 gm Hydrogen Peroxide.
4.Gaseous Volumes
  • Gay Lussac (1808)
  • When gases combine or are produced in a chemical reaction they do so in a simple ratio by volume provided all gases are at same temperature and pressure.
  • If 100 ml hydrogen combine with 50 ml oxygen, we get 100 ml water vapour.
5.Avogadro Law
  • equal volumes of gases at the same temperature and pressure should contain equal number of molecules
  • He differentiated atoms from molecules.


  1. Democritus, a Greek Philosopher (460 — 370 BC) said that matter is composed of small indivisible particles called ‘a-tomio’ (meaning — indivisible). Indian philosophers also made similar statements. But they had no proofs.
  2. Finally, a British Teacher John Dalton published book published ‘A New System of Chemical Philosophy’ (1808) with following points
  3. Matter consists of indivisible atoms.
  4. All the atoms of a given element have identical properties and identical mass.
  5. Atoms of different elements differ in mass.
  6. Compounds are formed when atoms of different elements combine in a fixed ratio.
  7. Chemical reactions involve reorganisation of atoms. These are neither created nor destroyed in a chemical reaction.
Good Bad
his theory can explain following:

  • laws of chemical combination
  • law of conservation of mass
  • law of constant composition
  • law of multiple proportion
He couldn’t explain following

  • If glass or ebonite when rubbed with silk or fur, it’ll generate electricity.  Why?
  • Same way result of many experiments, he couldn’t explain.


Discovery of Was discovered by
1.Electron A cathode ray discharge tube – cathode ray particles – observed though fluorescent or phosphorescent – negatively charged particles, called electrons
2.Proton modified cathode ray tube – canal rays – positively charged particles
3.Neutron Chadwick (1932). he bombarded alpha-particles on beryllium thin sheet.

Cathode Ray Tube & Television

  • 1830: Michael Faraday showed that if electricity is passed through a solution of an electrolyte, matter will be liberated and deposited at the electrodes.  (1830)
  • 1850s: Faraday began to study electrical discharge in cathode ray tubes
  • A cathode ray tube is a sealed glass tube containing two thin metal pieces (electrodes).
  • Cathode rays start from cathode and move towards the anode.
  • Cathode ray will travel in straight line IF there is no electrical or magnetic field,
  • If there is electrical/magnetic field, cathode rays will behave like charged particles.
  • Characteristics of cathode rays (electrons) do not depend upon (1) the material of electrodes (2) nature of the gas present in the cathode ray tube.
  • Cathode rays themselves are not visible but their behaviour can be observed with the help of certain kind of materials (fluorescent or phosphorescent) which glow when hit by them.
  • We can observe electrical discharge through the gases only at very low pressures and at very high voltages.
  • Television picture tubes are cathode ray tubes
  • Television pictures result due to fluorescence on the television screen coated with certain fluorescent or phosphorescent materials.

X-Ray & Roentgen

  • Wilhalm Röentgen: strike electron to dense anode metal in Cathode ray tube => rays produced => these rays cause fluorescence in the fluorescent materials placed outside the cathode ray tubes. He called them X-Rays (1895)
  • Henri Becqueral coined the term radioactivity. Marie Curie, Piere Curie, Rutherford and Fredrick Soddy worked further in this field.
Detail penetrating
Alpha particles 1
  • Alpha particles are Positive charged.
  • Alpha particle + 2 electron = helium
  • Rutherford bombarded very thin gold foil with alpha–particles but most of the a– particles passed through the gold foil undeflected.
  • Thus he provide Most of the space in the atom is empty. And volume occupied by the nucleus is negligibly small as compared to the total volume of the atom.
Beta Rays 100 x alpha negatively charged particles similar to electrons
Gamma-Rays 1000 x alpha
  • neutral in nature
  • don’t have any particles.

Atomic Mass

  • Greek word ‘stoichiometry’ =stoicheion (meaning element) + metron (meaning measure).
  • Stoichiometry calculation of masses and volumes of reactants and products of a chemical reaction.
  • 19th Century scientists assigned mass of “1” to Hydrogen. (no units, only number). All the remaining elements were given mass number relative to Hydrogen.
  • 1961: Carbon – 12 isotop is assigned a mass of exactly 12 atomic mass unit (amu) and masses of all other atoms are given relative to this standard.
  • Thus, One atomic mass unit (AMU) = one- twelfth the mass of one carbon – 12 atom.
  • Today, ‘amu’ has been replaced by ‘u’ which is known as unified mass.
  • Today, we have sophisticated techniques e.g., mass spectrometry for determining the atomic masses fairly accurately.
What’s the difference between

Atomic number (Z) Mass Number (A)
  • total protons = Z
  • total electrons =Z
  • Proton + neutron = nucleons.
  • total nucleons = A
  • Z written in subscript below the element symbol, on left side.
  • A written in superscript above element symbol, on left side.
  • Isotopes= same “Z” but different “A”. means Isotopes have same number of protons but different number of neutrons.
Isobars= same “A” but different “Z”
Misc. Definitions

Average Atomic Mass
  • If an element has more than one isotope
  • Its Avg. atomic mass = weighted average of (atomic mass x its relative occurrence in percentages)
Formula Mass e.g. formula mass of sodium chloride = atomic mass of sodium + atomic mass of chlorine
Isotopes of Hydrogen

Isotop proton neutron rarity
protium 1 0 99.985% hydrogen is like this
deuterium 1 1 0.015%
tritium 1 2 trace amount in earth
  • Similarly Carbon has 3 isotopes, chlorine has 2 isotopes
  • An Element’s chemical property depends on no. of electrons, and not much on neutrons. Therefore, Isotopes show same chemical behaviour. (can be asked for assertion-reasoning)

Mole and Avogadro

  • One mole = Amount of a substance that contains as many particles as there are atoms in exactly 12 g (or 0.012 kg) of the Carbon 12 isotope.
  • 1 mol is also known as ‘Avogadro’ constant, in honour of Amedeo Avogadro. It equals to 6.022×1023 atoms

Atomic Models

  • also known as plum pudding, raisin pudding or watermelon model (1904)
  • he said atom possesses a spherical shape (radius approximately 10 –10 m) in which the positive charge is uniformly distributed.
  • The electrons are embedded into it in such a manner as to give the most stable electrostatic arrangement
  • He got Nobel Physics for discovering how gases conduct electricity. (1906)
  • Atom resembles the solar system, nucleus is the Sun, and electrons are revolving planets.
  • Electrons and the nucleus are held together by electrostatic forces of attraction
  • he failed to explain stability of an atom.
  • it says nothing about the electronic structure of atoms i.e., how the electrons are distributed around the nucleus and what are the energies of these electrons.
3.NEILS BOHR, Denmark
  • Orbit is a circulate path around nucleus. Electrons move only in Orbit.
  • He was the first to explain quantitatively the general features of hydrogen atom structure and its spectrum. Though the theory is not the modern quantum mechanics.
  • He could not explain how atoms form molecules by chemical bonds. Hence Max Plank, Schrödinger et al began towards Quantum theory.
  • James Maxwell: discussed electromagnetic radiation.
  • Post WW-1, Bohr worked for peaceful uses of atomic energy, got Nobel in 1922.
4.Wolfgang Pauli, Austria
  • No two electrons in an atom can have the same set of four quantum numbers.
  • Only two electrons may exist in the same orbital and these electrons must have opposite spin.
  • Oil drop experiment to measure charge of electrons.

Quantum Mechanic Model

  • Classical atomic models ignore dual behavior of particles.
  • Just like radiation, particles also have dual properties i.e. Wave like properties and particle like properties (French physicist, de Broglie in 1924).
  • This is known as “Quantum mechanics”- Erwin Schrödinger – Nobel winner Austrian physicist was the front runner of this theoretical science.
  • We can’t find the exact position and exact momentum (or velocity) of an electron at the same time, because electron and other similar particles don’t have definite paths or trajectories of electrons and other similar particles- This is Heisenberg’s Uncertainty Principle.
  • Heisenberg was a German who shared Nobel with Schrödinger in Physics. He researched atomic bomb for Germany during WW2
  • Electrons wave-like properties are utilized in electron microscope, it can give a magnification of about 15 million times.

Photoelectric Effect: Einstein & Planck

German physicist Max Planck observed that:
  • When Light strikes surface = electrons ejected without any time lag.
  • How many electrons ejected? Ans. Proportional to light’s brightness.
  • How much is the kinetic energy of these ejected electrons? Ans. NOT in proportion of light’s brightness.
  • If red light shined on potassium for hours but no photoelectrons are ejected.
  • But even if a very weak yellow light shines on the potassium metal, the photoelectric effect is observed.
  • German born American physicist Albert Einstein was able to explain this Photoelectric effect using Planck’s quantum theory of electromagnetic radiation.
  • Einstein said light shining = shooting photon particle beam =collision with electrons=electrons ejected.
  • Brighter light = more protons = more electrons ejected.
  • He Won Nobel Prize in Physics in 1921 for his explanation of the photoelectric effect.


  • Speed of light depends upon the nature of the medium through which it passes.
  • As a result, the beam of light is deviated or refracted from its original path as it passes from one medium to another.
  • The light of red colour which has longest wavelength is deviated the least while the violet light, which has shortest wavelength is deviated the most.
  • Examples of continuous spectrum:  (1) White light spectrum (2) rainbow. Because they have all colors from violet to red.
  • The study of emission or absorption spectra is referred to as spectroscopy


Electromagnetic spectrum

  • Water wave and sound wave need medium. They can’t move in vaccum
  • Electromagnetic waves do not require medium and can move in vacuum.
  • There are many types of electromagnetic radiation depending on their wavelength (or frequency).
  • Collectively, they’re called electromagnetic spectrum
What’s the difference?

  • Distance between two successive crests (or troughts)
  • SI unit is units of length is meter (m).
  • Wave number = Wavelengths per unit length. Unit is 1/m.
  • frequency is the number of waves that pass a given point in one second
  • SI unit is Hz, named after Heinrich Hertz

Black Bodies

Wave nature of Electromagnetic radiation can explain following:
Diffraction it is the bending of wave around an obstacle
Interference it is the combination of two waves of the same or different frequencies to give a wave
  • But above things can’t explain a black body radiation. Later Max Planck explained it in following manner:
  • When solids are heated they emit radiation over a wide range of wavelengths.
  • Heating Iron rod = dull red (low frequency) => more heating => bright red color (higher frequency)=>more heating=> white=>Blue…frequency keeps on increasing, wavelength keeps on decreasing.
  • Black body is the ideal body that emits and absorbs all frequencies.
  • The radiation emitted by such a body is called black body radiation.
  • Black Body’s radiation depends only on its temperature….At a given temperature, intensity of radiation emitted increases with decrease of wavelength, reaches a maximum value at a given wavelength and then starts decreasing with further decrease of wavelength (something like a bell curve).
  • Planck suggested that atoms and molecules could emit (or absorb) energy only in discrete quantities and not in a continuous manner. This smallest energy quantity is “quantum”.
Mug up following drugs because given in class11 Chemistry NCERT
Cisplatin, Taxol Cancer
AZT: Azidothymidine AIDS

Mock Questions

Following type of MCQs can be framed:
  1. Whatever “cause-consequence” or “x because of y” type of information is given in this note, it can be utilized for assertion reasoning questions e.g. (1) why Pt-IR used in weight std. (2) how cathode-ray works in TV
  2. Match the following (1) scientist vs. principle. (2) basic physical quantity vs name of the SI unit (3) range of electromagnetic spectrum vs. utility of those rays (4) alpha, beta, gamma and x-rays vs their properties (similar qs. asked in CAPF) (5) isotopes of hydrogen vs. no. of neutrons present in them
  3. You can be given a term, and asked to identify the “factor responsible or variable attached.” example (1) Black body radiation depends on which of the following factors? (2) Cathode ray’s travel path depends on which of the following factors?
  4. 2 or 3 statements about Biography-contribution of Einstein- then you’re asked to identify the right or the wrong ones.
  5. Which of the following statements are correct about (1) Atomic mass / number (2) isotope vs. isobar (3) mass vs. weight and so on.
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