NCERT Science: Atoms, Molecules, the Periodic Table and Bonding

The one-line takeaway

All matter is built from atoms, which have a tiny dense nucleus of protons and neutrons surrounded by electrons. Atoms combine into molecules and compounds, and their properties recur in a pattern when elements are arranged by atomic number (the periodic table). This chapter also covers acids and bases, metals and non-metals, and the special chemistry of carbon.

Atomic structure

• An atom has a central nucleus containing protons (positive) and neutrons (neutral), surrounded by electrons (negative) in shells. The atom as a whole is electrically neutral, so protons equal electrons.
• Atomic number (Z) is the number of protons, which fixes the identity of the element. Mass number (A) is the number of protons plus neutrons. Almost all the mass is in the nucleus; the proton and neutron are about 1,836 times heavier than the electron.
• Isotopes are atoms of the same element (same atomic number) with different mass numbers (different neutron count), for example carbon-12 and carbon-14, or hydrogen's protium, deuterium and tritium. Isobars have the same mass number but different atomic numbers.

The atomic models in sequence

• Dalton (atoms are indivisible solid spheres), then J.J. Thomson (the "plum-pudding" model, electrons embedded in a positive sphere; he discovered the electron), then Rutherford (the gold-foil experiment revealed a small dense positive nucleus, with electrons around it), then Bohr (electrons revolve in fixed energy shells without radiating energy). The modern picture adds the neutron (Chadwick) and electron clouds.

Valency and how atoms combine

• Valency is the combining capacity of an element, set by the number of electrons it gains, loses or shares to achieve a stable (usually eight-electron, "octet") outer shell. Noble gases are stable and largely inert because their outer shell is already complete.

The periodic table

• Mendeleev arranged the known elements by increasing atomic mass and left gaps for undiscovered elements, predicting their properties (the periodic law by atomic mass).
• The modern periodic law (Moseley) arranges elements by increasing atomic number. The table has 18 vertical groups (elements in a group share valency and chemical properties) and 7 horizontal periods.
• Metals sit on the left and centre, non-metals on the upper right, and metalloids (semi-metals such as silicon and germanium) along the dividing line. Across a period, metallic character decreases; down a group, it increases.

Chemical bonding

• Ionic (electrovalent) bond: complete transfer of electrons from a metal to a non-metal, forming ions held by electrostatic attraction (sodium chloride). Ionic compounds are usually solids with high melting points and conduct electricity when molten or dissolved.
• Covalent bond: sharing of electron pairs between non-metals (water, methane, carbon dioxide, oxygen). Covalent compounds usually have low melting points and do not conduct electricity.

Acids, bases and salts

• An acid releases hydrogen ions (H+) in water, tastes sour, turns blue litmus red, and reacts with metals to release hydrogen (hydrochloric acid, sulphuric acid, nitric acid; citric acid in citrus fruit, acetic acid in vinegar).
• A base releases hydroxide ions (OH-) in water, tastes bitter, feels soapy, and turns red litmus blue. A water-soluble base is an alkali (sodium hydroxide, calcium hydroxide).
• The pH scale runs 0 to 14: below 7 is acidic, 7 is neutral, above 7 is basic. It measures hydrogen-ion concentration. pH matters in soil, blood, the stomach (gastric acid), tooth decay and acid rain.
• Neutralisation: acid plus base gives salt plus water. Everyday salts and bases: common salt (sodium chloride), baking soda (sodium hydrogen carbonate), washing soda (sodium carbonate), bleaching powder (calcium oxychloride), plaster of Paris (calcium sulphate hemihydrate, from gypsum).

Metals, non-metals and the reactivity series

• Metals are malleable, ductile, lustrous, sonorous and good conductors of heat and electricity (mercury is a liquid metal; gold and silver are least reactive). Non-metals generally lack these properties (carbon as graphite conducts; diamond is the hardest).
• The reactivity series ranks metals by reactivity; a more reactive metal displaces a less reactive one from its compound. Highly reactive metals (sodium, potassium) react with water vigorously.
• Corrosion: rusting of iron needs both air (oxygen) and moisture; prevention by galvanising (zinc coating), painting, oiling and alloying.
• Alloys (mixtures of metals, or of a metal with a non-metal) have improved properties: brass (copper and zinc), bronze (copper and tin), steel (iron and carbon), stainless steel (iron, chromium, nickel), solder (lead and tin).

Carbon and its compounds

• Carbon forms a vast number of compounds because of catenation (carbon-to-carbon bonding into chains and rings) and its four valencies (tetravalency).
• Allotropes of carbon: diamond (hardest natural substance, each carbon bonded to four others), graphite (soft, conducts electricity, a lubricant, layered structure), and fullerene (a hollow molecular form).
• Hydrocarbons: saturated (alkanes, only single bonds) versus unsaturated (alkenes and alkynes, with double or triple bonds). A homologous series is a family with the same general formula and a fixed difference between members.
• Everyday organic compounds: ethanol (the alcohol in drinks and as a fuel) and ethanoic (acetic) acid (vinegar). Soaps and detergents work by having one end that dissolves in water and one that dissolves in oil.

Key terms to fix

• Atomic number / mass number: protons; protons plus neutrons.
• Isotopes: same atomic number, different mass number.
• Ionic versus covalent bond: electron transfer versus electron sharing.
• pH scale (0 to 14): below 7 acidic, 7 neutral, above 7 basic.
• Allotropes of carbon: diamond, graphite, fullerene.

CAPF angle

Atomic structure and isotopes are the basis of nuclear technology, both power and weapons, a core strategic-security topic (uranium-235, plutonium-239, the fission chain). Carbon chemistry underlies explosives and propellants. The chemistry of acids, bases and corrosion is relevant to equipment maintenance and to the handling of hazardous chemicals (a CISF and disaster-response concern). Bleaching powder and chlorine are used in water disinfection in the field.

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