Chemistry Glossary

Electron

  • Discovered by J.J. Thomson in 1897
  • Charge of electron (e⁻) = –1.602 × 10⁻¹⁹ C
  • Mass of electron = 9.109 × 10⁻³¹ kg
  • Bohr’s model explains electrons revolve in fixed orbits.
  • Electron’s charge-to-mass ratio (e/m) was first measured by J.J. Thomson
  • Wave nature of electron discovered by Louis de Broglie (1924)
  • Electron diffraction was experimentally confirmed by Davisson and Germer (1927)
  • Electrons participate in chemical bonding and determine an element’s chemical properties
  • Spin of electron is ½ (fermion)
  • Quantum numbers define the state of an electron in an atom

Proton

  • Discovered by Ernest Rutherford in 1917
  • It has a positive charge of + 1.602 × 10⁻¹⁹ C
  • Mass =  1.6726 × 10⁻²⁷ kg
  • It is about 1836 times heavier than an electron
  • atomic number = Number of protons in an atom
  • Hydrogen is the only element with one proton and no neutron
  • Protons and neutrons together are called nucleons
  • Strong nuclear force binds protons inside the nucleus
  • Changing the number of protons changes the element itself
  • Protons are stable particles and do not decay under normal conditions

Neutron

  • It has no electric charge (neutral)
  • Its mass is slightly greater than that of a proton (≈ 1.675 × 10⁻²⁷ kg)
  • Discovered by James Chadwick in 1932
  • Neutrons stabilize the nucleus by reducing repulsion between protons.
  • Free neutrons are unstable and decay into a proton, electron, and antineutrino (beta decay)
  • Average lifetime of a free neutron is about 15 minutes
  • Neutrons are used in nuclear reactors to sustain chain reactions
  • Isotopes of an element differ only in the number of neutrons
  • Neutron number = Mass number – Atomic number
  • Neutron was discovered using beryllium radiation experiments
  • Neutron is important in nuclear fission and fusion processes

Atom

  • Smallest unit of an element retaining its chemical properties
  • Coined from the Greek word “Atomos” meaning indivisible
  • Discovered by John Dalton in his Atomic Theory (1803)
  • Atom consists of electrons, protons, and neutrons
    • Nucleus is positively charged, contains protons and neutrons
    • Electrons revolve around nucleus in shells/orbits (Bohr’s model)
  • Atomic number = number of protons
  • Mass number = protons + neutrons
  • Isotopes: Atoms of same element with same atomic number but different mass numbers (e.g., Carbon-12, Carbon-14)

Element

  • Simplest form of matter which cannot be broken into simpler substances by chemical methods
  • Represented by symbols (H, O, Na, Fe etc.)
  • 118 known elements, 92 natural and rest synthetic
  • Example: Hydrogen is the lightest and most abundant element in the universe
  • Mercury and Bromine are the only liquid elements at room temperature
  • Oxygen is the most abundant element in Earth’s crust by mass

Compound

  • Substances formed when two or more elements combine chemically in fixed ratio
  • Represented by chemical formula (e.g., H₂O, CO₂, NaCl)
  • Properties of compounds are different from those of constituent elements
  • Water (H₂O) is called universal solvent
  • Carbon dioxide (CO₂) is essential for photosynthesis
  • Common salt (NaCl) is used in food preservation
  • Compounds can be broken down into elements by chemical reactions
Chemistry Glossary