Chemistry

• Atoms are the building blocks of everything in the universe, including ourselves
• An atom is the smallest unit of an element that can exist alone or in combination with atoms of the same or different elements
• Molecules are the combination of two or more atoms held together by chemical bonds
• Greek philosophers Leucippus and Democritus proposed the concept of the atom as the smallest particle of matter that is indivisible
• John Dalton proposed the atomic theory of matter in 1803, stating that matter is made up of small, invisible, and indivisible particles called atoms
• Dalton's atomic theory includes:

  • Atoms of the same element have the same properties
  • Atoms of different elements differ in properties
  • Atoms cannot be created nor destroyed in any chemical reaction
  • Atoms combine in whole number ratios to produce compounds
• The cathode ray experiments led to the discovery of the electron by Joseph John Thompson in 1897
• Ernest Rutherford discovered the nucleus of the atom in 1911 using his alpha scattering experiment
• James Chadwick discovered the neutron, a neutral particle with no net electrical charge inside the nucleus
• Niels Bohr proposed an atomic model where electrons revolve around the nucleus in specific energy levels
• Erwin Schrodinger proposed the quantum mechanical model of the atom, describing electron clouds as the probable position of electrons
• Atoms are composed of protons, neutrons, and electrons

  • Protons carry a positive charge and are located inside the nucleus
  • Neutrons have no net electrical charge and are located inside the nucleus
  • Electrons carry a negative charge and are located outside the nucleus
• The atomic number is the number of protons in an element, while the mass number is the sum of protons and neutrons
• Isotopes are atoms of the same element with different mass numbers but the same number of protons
• Ionization occurs when an atom loses or gains electrons, forming ions

  • A positive ion is called a cation
  • A negative ion is called an anion