Atomic Structure: States of Matter, Bohr Model, Isotopes and Electronic Configuration

Matter, Its Classification and Changes of State

What Is Matter: Elements, Compounds and Mixtures

Everything that occupies space and has mass is matter: the air we breathe, the food we eat, stones, clouds and stars. Early Indian philosophers grouped matter into five basic elements, the Panch Tatva of air, earth, fire, sky and water. Modern science classifies it instead by its physical properties and its chemical nature.

• Element: A basic form of matter that cannot be broken into simpler substances by chemical reactions, the definition Antoine Lavoisier established. Elements are divided into metals, non-metals and metalloids such as boron, silicon and germanium.
• Compound: Two or more elements chemically combined in a fixed proportion; its properties differ from those of the elements that form it.
• Mixture: Two or more substances mixed without forming a new compound; its composition can vary and it keeps the properties of its parts.
• A few numbers: Mercury is the only metal that is liquid at room temperature; mercury and bromine are the two liquid elements, and eleven elements are gases at room temperature.

The same idea separates a physical change from a chemical change. Mixing iron filings with sulphur and crushing them gives a mixture that a magnet can still separate: a physical change. Heating the same mixture until it glows gives iron sulphide, a new compound with new properties: a chemical change, or chemical reaction.

States of Matter: Solid, Liquid, Gas, Plasma and Bose-Einstein Condensate

Matter is made of tiny particles that have space between them, attract one another and are always moving. How strongly they attract and how fast they move decides the state. The forces of attraction are greatest in solids, intermediate in liquids and least in gases, while the space between particles and their kinetic energy rise in the same order.

• Solid: A definite shape, distinct boundaries and a fixed volume, with negligible compressibility.
• Liquid: No fixed shape but a fixed volume; liquids flow and take the shape of their container.
• Gas: Highly compressible and fills its container; the pressure of a gas is the force its particles exert per unit area on the walls. LPG, medical oxygen and CNG are compressed gases.
• Plasma: Super-energetic, ionised gas particles. It glows in neon signs and fluorescent tubes, and the Sun and the stars glow because they are made of plasma; about 99.9 per cent of ordinary matter in the universe is estimated to be plasma.
• Bose-Einstein condensate: Formed by cooling a gas of extremely low density, about one hundred-thousandth the density of air, to super-low temperatures, close to absolute zero.

The fifth state carries an Indian name. Satyendra Nath Bose did the calculations for it: NCERT dates them to 1920, while other accounts date his decisive paper to 1924. Bose derived Planck's radiation law by counting particles of light as identical and indistinguishable; Albert Einstein translated the paper, extended the idea to atoms and predicted the Bose-Einstein condensate.

• Bosons: Paul Dirac named particles that obey Bose’s statistics bosons. They have whole-number spin; the other class, fermions, have half-number spin.
• Made in a laboratory: Eric Cornell and Carl Wieman created the first condensate in 1995 with rubidium atoms, and Wolfgang Ketterle made one with sodium atoms; the three shared the 2001 Nobel Prize in Physics.
• Wider reach: The Higgs boson, recognised in the 2013 Nobel Prize, belongs to the same class, and Bose’s statistics underlie superfluidity and superconductivity.

Changes of State: Melting, Boiling, Sublimation and Latent Heat

Heating gives particles more kinetic energy until they overcome the forces holding them. The temperature at which a solid melts at atmospheric pressure is its melting point; ice melts at 273.16 K. The temperature at which a liquid boils is its boiling point; for water it is 373 K, or 100 °C.

• Latent heat: While ice melts, its temperature stays the same even though heat keeps flowing in. That hidden heat breaks the forces between particles. The latent heat of fusion changes 1 kg of solid to liquid at its melting point; the latent heat of vaporisation changes 1 kg of liquid to gas at its boiling point.
• Why steam burns more: Particles in steam at 100 °C carry extra energy, the latent heat of vaporisation, compared with water at the same temperature. Vaporising water takes far more heat than melting ice, which is why steam is also the more hazardous.
• Sublimation: Some substances pass straight from solid to gas, such as camphor and ammonium chloride. Solid carbon dioxide, dry ice, sublimes at −78.5 °C at normal pressure.
• Evaporation: A surface process at any temperature below boiling, faster with more surface area, heat and wind and slower in humid air; it causes cooling, which is why sweating cools the body.

Pressure matters as much as temperature. Applying pressure and cooling can liquefy a gas. A liquid boils when its vapour pressure equals the pressure outside, so at high altitudes, where air pressure is low, water boils below 100 °C; that is why a pressure cooker is needed in the hills.

Atomic Structure: Electron, Proton, Neutron and the Nucleus

Dalton's Atomic Theory and the Laws of Chemical Combination

Atomic structure begins with two laws. The law of conservation of mass says mass can neither be created nor destroyed in a chemical reaction. The law of constant proportions, stated by Proust, says a compound always has its elements in the same proportion by mass: hydrogen and oxygen in water are always in the ratio 1:8.

John Dalton explained both laws in 1808 with his atomic theory. Atoms are extremely small: their radii are measured in nanometres, and the atom of hydrogen has a radius of about 10⁻¹⁰ m.

1. Tiny particles: All matter is made of atoms.
2. Indivisible: Atoms cannot be created or destroyed in a chemical reaction.
3. Same element, same atoms: Atoms of an element are identical in mass and chemical properties; atoms of different elements differ.
4. Whole numbers: Atoms combine in small whole-number ratios, and a given compound always has the same relative number and kinds of atoms.

Discovery of the Electron, Proton and Neutron

The atom turned out to be made of smaller particles. In cathode ray tubes, a stream of particles moved from the negative electrode to the positive one and behaved like negative charges whatever the gas or the metal used, so these electrons had to be part of every atom. J. J. Thomson measured their charge-to-mass ratio in 1897, and R. A. Millikan's oil drop experiment found their charge.

Protons and neutrons sit together in the nucleus and are called nucleons. Neutrons are present in the nucleus of every atom except ordinary hydrogen, whose nucleus is a single proton.

Atomic Number, Mass Number and Nucleons

Two numbers describe every atom. The atomic number, Z, is the number of protons in the nucleus; in a neutral atom it equals the number of electrons. Elements are defined by it: every hydrogen atom has Z = 1 and every carbon atom Z = 6. The mass number, A, is the total of protons and neutrons.

• Carbon: 6 protons and 6 neutrons give a mass of 12 u.
• Aluminium: 13 protons and 14 neutrons give 27 u.
• Notation: Nitrogen is written with mass number 14 above and atomic number 7 below its symbol, N.
• Average mass: Chlorine occurs as 35 u and 37 u atoms in the ratio 3:1, so its average atomic mass is 35.5 u, though no single atom weighs that.

Atomic Models: From Thomson to the Bohr Model

Thomson Model and Rutherford Model of the Atom

The first picture came from J. J. Thomson, who compared the atom to a Christmas pudding: electrons studded in a sphere of positive charge, like seeds in a watermelon. It explained why an atom is neutral, but not what Ernest Rutherford saw when he fired fast alpha particles at a thin gold foil.

1. Most went straight through: So most of the atom is empty space.
2. A few were deflected: So the positive charge occupies very little space.
3. About 1 in 12,000 bounced back: So the positive charge and nearly all the mass sit in a tiny centre, the nucleus. The radius of an atom is about 10⁻¹⁰ m and that of its nucleus about 10⁻¹⁵ m.

Rutherford's nuclear model put a small positive nucleus at the centre with electrons revolving round it, like planets round the Sun. It had one fatal flaw: an electron moving in a circle is accelerating, and an accelerating charge should radiate energy and spiral into the nucleus, so atoms could not be stable.

Bohr Model of the Atom: Energy Levels and Line Spectra

Niels Bohr fixed the flaw in 1913. His Bohr model keeps Rutherford's nucleus but allows electrons only certain orbits, and an electron in one of these orbits does not radiate energy.

• Stationary orbits: Electrons move in orbits of fixed radius and energy, called energy levels or shells and labelled K, L, M, N or n = 1, 2, 3, 4.
• Jumps: An electron absorbs energy to move to a higher orbit and gives out energy when it falls to a lower one; the energy changes in steps, not continuously.
• Spectra: These jumps explain the line spectrum of hydrogen, the Lyman, Balmer and Paschen series.
• Shell capacity: The Bohr-Bury scheme puts at most 2n² electrons in a shell (2, 8, 18, 32) and at most 8 in the outermost shell.

Quantum Numbers and Orbitals: The Quantum Mechanical Model

Two ideas replaced fixed orbits. Louis de Broglie proposed in 1924 that matter, like light, behaves as both particle and wave, an idea used in the electron microscope. Werner Heisenberg's uncertainty principle of 1927 says the exact position and exact momentum of an electron cannot be known at the same time.

Quantum mechanics, developed by Heisenberg and Erwin Schrödinger in 1926, therefore describes an electron by an orbital: a region where it is likely to be found, not a fixed path. Each orbital is labelled by quantum numbers.

• Orbitals per subshell: One s, three p, five d and seven f orbitals.
• Orbitals per shell: The number is n², so the third shell has nine.

Isotopes, Isobars and Isotones

Isotopes of Hydrogen: Protium, Deuterium, Tritium and Heavy Water

Atoms of the same element with the same atomic number but different mass numbers are called isotopes; they carry different numbers of neutrons. Hydrogen shows all three kinds: protium, with no neutron, makes up 99.985 per cent of hydrogen; deuterium, with one neutron, 0.015 per cent; and tritium, with two, occurs only in trace amounts.

• Heavy water: Water in which the hydrogen atoms are deuterium. It is about 10.6 per cent denser than ordinary water.
• In reactors: Pressurised heavy water reactors use heavy water as coolant and as the moderator that slows neutrons down, which lets them run on natural uranium.
• Discovery: Harold Urey discovered deuterium in 1931 and later concentrated it in water.
• Tritium: Radioactive, with a half-life of 12.32 years; it is used in self-luminous lights and, with deuterium, as fuel in fusion research.

Difference Between Isotopes and Isobars

Isotopes share protons; isobars share a mass number instead. Argon, atomic number 18, and calcium, atomic number 20, both have atoms of mass number 40, so argon-40 and calcium-40 are isobars; carbon-14 and nitrogen-14 are another pair. A third term, isotones, means atoms with the same number of neutrons, such as boron-12 and carbon-13, which have seven each.

Isotopes behave alike in chemistry because chemical properties depend on electrons, and the number of electrons follows the number of protons; neutrons change the mass and the physical properties, such as density, but hardly the chemistry.

Uses of Isotopes in Medicine, Dating, Industry and Nuclear Power

Some isotopes are radioactive, and their radiation is useful. An isotope of uranium is the fuel of nuclear reactors, an isotope of cobalt treats cancer and an isotope of iodine treats goitre.

• Cobalt-60: A synthetic isotope with a half-life of about 5.27 years, made in reactors; its gamma rays are used for radiation therapy and, at the Vadodara plant, to make sewage sludge safe for farms.
• Iodine-131: Half-life about eight days; used to diagnose and treat thyroid disease.
• Carbon-14 dating: Living things take in carbon-14 until they die, after which it decays with a half-life of about 5,730 years; the method, developed by Willard Libby, dates organic material up to about 50,000 years old.
• Industry and water: Isotopes are used to test materials without damaging them and to trace water resources and industrial processes.

India makes most of its medical isotopes through the Board of Radiation and Isotope Technology (BRIT), a unit of the Department of Atomic Energy that supplies isotopes for healthcare, agriculture, research and industry. BRIT meets about 85 to 90 per cent of the country's demand for lutetium-177, used in cancer treatment, while imports still cover part of the need for iodine-131 and molybdenum-99.

Electronic Configuration of Atoms

Aufbau Principle, Pauli Exclusion Principle and Hund's Rule

Electronic configuration is the way an atom's electrons are distributed among its orbitals. It is written as a list of subshells with the number of electrons in each, so sodium is 1s² 2s² 2p⁶ 3s¹. Three rules decide it.

1. Aufbau principle: Orbitals fill in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 4f, 5d, 6p, 7s. Aufbau is German for building up.
2. Pauli exclusion principle: No two electrons in an atom can have the same four quantum numbers, so an orbital holds at most two electrons, with opposite spins.
3. Hund’s rule: Electrons enter each orbital of a subshell singly before any pairing, so pairing in p, d and f subshells begins with the 4th, 6th and 8th electron.

Valence Electrons and Valency: Why Configuration Decides Chemistry

The electrons in the outermost shell are the valence electrons, and they decide how an atom reacts. Atoms with a full outer shell, eight electrons or two in helium, show little chemical activity. Other atoms react to reach such an octet by losing, gaining or sharing electrons, and the number involved is their valency, or combining capacity.

This is why configuration is the key to the rest of chemistry: it explains why some elements are metals and others non-metals, why helium and argon hardly react and why the halogens are so reactive. Arranged by configuration, the elements fall into the modern periodic table.

Sources

• NCERT: Science (Class IX), Matter in Our Surroundings; Is Matter Around Us Pure; Atoms and Molecules; Structure of the Atom
• NCERT: Chemistry Part I (Class XI), Structure of Atom and States of Matter
• Bhabha Atomic Research Centre: isotope and radiation technology applications
• PIB: Parliament question on medical radioisotopes (Department of Atomic Energy, 6 August 2025)
• India Science, Technology and Innovation portal: Satyendra Nath Bose
• Wikipedia: Atom
• Wikipedia: Bohr model
• Wikipedia: Isotope
• Wikipedia: Bose-Einstein condensate
• Wikipedia: Satyendra Nath Bose
• Wikipedia: Heavy water
• Wikipedia: Radiocarbon dating
• Wikipedia: Dry ice
• UPSC: Civil Services Examination question papers