UPSC foundations · Note 285
Atoms, elements and the periodic table
Study context: GS III — Science and Technology — developments and their applications and effects in everyday life (foundations: physics, chemistry, biology).
All matter is made of atoms, tiny units that are themselves made of smaller particles. At the centre of an atom is a nucleus of protons, which carry positive charge, and neutrons, which carry no charge. Around it move electrons, which carry negative charge. The number of protons — the atomic number — defines the element: hydrogen has one proton, carbon has six, oxygen has eight, and gold has seventy-nine. Atoms of the same element with different numbers of neutrons are called isotopes; some isotopes are unstable and radioactive, decaying over time and releasing energy.
Electrons occupy regions called shells or orbitals, and the outermost electrons largely determine how an atom behaves chemically. Atoms with a full outer shell, such as the noble gases helium and neon, are reluctant to react. Atoms with one electron to lose or gain, such as sodium or chlorine, react eagerly. Chemical bonds form when atoms share or transfer electrons to reach more stable arrangements. This single idea — outer electrons drive chemistry — explains why elements in the same column of the periodic table behave alike.
The periodic table organises the elements by atomic number into rows and columns that reflect these patterns. Its first widely successful form was published by the Russian chemist Dmitri Mendeleev in 1869, who arranged elements by atomic weight and left gaps where he predicted undiscovered elements — predictions later confirmed. The modern table, ordered by atomic number following the work of Henry Moseley in the early twentieth century, now contains 118 confirmed elements, of which 94 occur naturally on Earth and the rest have been made in laboratories.
Elements combine into compounds in fixed proportions: water is always two hydrogen atoms to one oxygen atom, and table salt is one sodium to one chlorine. Mixtures, by contrast, can vary in proportion — air is a mixture of mostly nitrogen and oxygen. This distinction matters for everything from cooking to industry: a compound has properties different from its constituent elements (sodium explodes in water and chlorine is a toxic gas, yet together they make harmless salt), while a mixture keeps the properties of its parts.
Key takeaways
- An element is defined by its number of protons; outer electrons determine how it reacts chemically.
- Mendeleev's 1869 table predicted missing elements; the modern table is ordered by atomic number and holds 118 elements.
- Compounds combine in fixed proportions and differ from their elements; mixtures vary and keep their parts' properties.
Why it matters for civil studies
It is the foundation for chemistry questions on materials, medicines, pollution, and industrial processes.
Think it through
Sodium explodes in water and chlorine is poisonous, yet table salt is safe to eat. What does this tell us about compounds versus their elements?
From the Public Notes companion study collection. Editorial context and corrections.