Atomic Structure, Carbon Compounds and Fuels Notes

Chemistry questions in General Science papers of SSC, railways, defence, police and state examinations are mostly based on the atom, the periodic table, carbon compounds and fuels. These are scoring topics because the facts are fixed: who discovered which particle, which allotrope of carbon has which property, which gas is in LPG and CNG, and which coal has the most carbon. This note explains atomic structure, carbon and its compounds, and fuels in simple language with tables and quick-revision points.

Part 1: Atomic Structure

An atom is the smallest particle of an element that takes part in a chemical reaction. It has a central nucleus containing protons (positive charge) and neutrons (no charge), and electrons (negative charge) move around the nucleus. Protons and neutrons have almost the same mass, and an electron is about 1/1836 of the mass of a proton. Because the number of protons equals the number of electrons, the atom is electrically neutral.

Discovery of Sub-atomic Particles and Atomic Models

Important Terms

• Atomic number (Z): The number of protons in the nucleus; it identifies the element.
• Mass number (A): The sum of protons and neutrons.
• Isotopes: Atoms of the same element with the same atomic number but different mass numbers, such as hydrogen (protium, deuterium and tritium) and carbon-12 and carbon-14.
• Isobars: Atoms of different elements with the same mass number, such as argon-40 and calcium-40.
• Isotones: Atoms with the same number of neutrons.
• Electronic configuration: The shells K, L, M and N can hold a maximum of 2, 8, 18 and 32 electrons (2n² rule). Sodium (11) is 2, 8, 1.
• Valency: The combining capacity of an element, decided by the electrons in the outermost shell.

Isotopes have practical uses: carbon-14 is used for dating fossils, iodine-131 for the treatment of thyroid disease, cobalt-60 for cancer treatment and uranium-235 as a fuel in nuclear reactors. The modern periodic table is based on the atomic number, which was shown by Moseley, and has 18 groups and 7 periods; Mendeleev had arranged the elements by atomic mass. The noble gases (helium, neon, argon, krypton, xenon and radon) have complete outer shells and hardly react. A chemical bond is formed by the transfer of electrons (ionic bond, as in NaCl), by the sharing of electrons (covalent bond, as in water and methane), or between metal atoms (metallic bond). One mole of a substance contains 6.022 × 10²³ particles, which is the Avogadro number.

Part 2: Carbon and Its Compounds

Carbon has atomic number 6 and valency 4, and so it is called tetravalent. It has two special properties: catenation, the ability to form long chains and rings by bonding with other carbon atoms, and the ability to form single, double and triple bonds. Because of these, carbon forms millions of compounds, and the study of carbon compounds is called organic chemistry.

Allotropes of Carbon

Hydrocarbons and Functional Groups

Hydrocarbons contain only carbon and hydrogen. Alkanes have single bonds (formula CnH2n+2), such as methane, ethane, propane and butane. Alkenes have a double bond (CnH2n), such as ethene. Alkynes have a triple bond (CnH2n-2), such as ethyne or acetylene, which is used for gas welding. Compounds containing a functional group give special properties: alcohol (-OH), aldehyde (-CHO), ketone (>C=O) and carboxylic acid (-COOH).

The important reactions of carbon compounds are combustion (burning in oxygen to give carbon dioxide, water, heat and light), oxidation, addition, substitution and esterification (an acid and an alcohol form an ester with a sweet smell). In the addition reaction (hydrogenation), vegetable oil reacts with hydrogen in the presence of nickel to make vanaspati ghee. Saponification is the reaction of an ester with a base to make soap. Soaps are the sodium or potassium salts of fatty acids. A soap molecule forms micelles around dirt and cleans it. Hard water, which has calcium and magnesium salts, does not form lather with soap and leaves a scum, but detergents work in hard water as well.

Part 3: Fuels

A fuel is a substance that burns in the presence of oxygen to give heat energy. A good fuel has a high calorific value (heat given by 1 kg), burns easily without smoke, is cheap and is easy to store. Hydrogen has the highest calorific value, and it is a clean fuel because it gives only water on burning. The lowest temperature at which a substance catches fire is the ignition temperature.

Burning of fossil fuels gives off carbon dioxide, which causes global warming, and sulphur and nitrogen oxides, which cause acid rain. Incomplete combustion gives poisonous carbon monoxide. To put out a fire, you remove one of three things: the fuel, the oxygen or the heat. Water is not used on electrical or oil fires; carbon dioxide extinguishers are used for them. India has moved to the Bharat Stage VI (BS-VI) emission norms from April 2020 to reduce pollution from vehicles.

Nuclear Energy, Radioactivity and Renewable Fuels

Radioactivity was discovered by Henri Becquerel in 1896, and Marie Curie and Pierre Curie discovered polonium and radium. Radioactive elements give out alpha rays (helium nuclei), beta rays (electrons) and gamma rays (electromagnetic waves, the most penetrating). In nuclear fission, a heavy nucleus such as uranium-235 splits into smaller nuclei and gives a large amount of energy; this is the principle of the atomic bomb and the nuclear reactor. In nuclear fusion, light nuclei such as hydrogen join together to form helium, and the sun gets its energy from this process. The father of the Indian nuclear programme is Homi Jehangir Bhabha, and India follows a three-stage programme to use its thorium reserves in the long term.

Because fossil fuels like coal and petroleum are limited and cause pollution, the world is moving towards cleaner sources. Solar energy, wind energy, biogas, hydrogen and ethanol are called renewable or cleaner fuels. Biogas plants use cow dung and farm waste, and the leftover slurry is a good manure. Green hydrogen, which is made by splitting water with electricity from renewable sources, is a future fuel. Remember that the gas in a gobar gas plant is methane, the gas in a CNG cylinder is also methane, and the gas in an LPG cylinder is a mixture of butane and propane.

Practice Questions With Answers

(1) Who discovered the nucleus? Rutherford. (2) Who discovered the neutron? Chadwick. (3) What is the maximum number of electrons in the M shell? 18. (4) Which allotrope of carbon is a good conductor of electricity? Graphite. (5) What is the general formula of alkanes? CnH2n+2. (6) What is added to LPG to detect a leak? Ethyl mercaptan. (7) Which coal has the highest carbon content? Anthracite. (8) Which fuel has the highest calorific value? Hydrogen.

Quick Revision

• Electron: Thomson; proton: Goldstein; nucleus: Rutherford; neutron: Chadwick.
• Atomic number is the number of protons; mass number is protons plus neutrons.
• Diamond is the hardest; graphite conducts electricity; carbon is tetravalent.
• LPG is butane and propane; CNG is methane; anthracite is the best coal.

Learn the tables by writing them from memory once, and you will find that chemistry questions are among the easiest marks in the General Science paper.

Related Notes to Read Next

Continue your preparation with these related notes from the same subject:

• SI Units and Measurement Notes for Competitive Exams
• Periodic Table and Periodic Classification Notes
• Chemical Reactions and Equations Notes
• All General Science notes

Frequently Asked Questions (FAQ)

Who discovered the electron, proton and neutron?

J. J. Thomson discovered the electron, E. Goldstein discovered the proton and James Chadwick discovered the neutron.

Who discovered the nucleus of the atom?

Ernest Rutherford discovered the nucleus in 1911 through his gold foil experiment.

What is the difference between atomic number and mass number?

Atomic number is the number of protons in an atom, and mass number is the total number of protons and neutrons.

What are isotopes?

Isotopes are atoms of the same element with the same atomic number but different mass numbers, such as carbon-12 and carbon-14.

What is the maximum number of electrons in a shell?

The maximum number of electrons in a shell is 2n squared, so the K, L, M and N shells hold 2, 8, 18 and 32 electrons.

Why is carbon called tetravalent?

Carbon is called tetravalent because it has four electrons in its outer shell and forms four bonds with other atoms.

What is catenation?

Catenation is the ability of carbon atoms to join with each other to form long chains and rings.

What is the difference between diamond and graphite?

Diamond is the hardest substance and a bad conductor of electricity, while graphite is soft and slippery and a good conductor of electricity.

What is the general formula of alkanes, alkenes and alkynes?

Alkanes have the formula CnH2n+2, alkenes have CnH2n and alkynes have CnH2n-2.

What is LPG made of and why is a smell added to it?

LPG is a mixture of butane and propane, and ethyl mercaptan is added to give a smell so that a gas leak can be detected.

Which type of coal is the best?

Anthracite is the best type of coal because it has the highest carbon content and burns with little smoke.

Which fuel has the highest calorific value?

Hydrogen has the highest calorific value and gives only water when it burns.