Lesson 2: Atoms from the Eyes of Philosophers & Scientists

• Empedocles


  According to the fundamental Greek theory of matter originated by the philosopher Empedocles, there were only four elements (fire, air, water) that make up matter
• Empedocles


  Asserted that properties of materials are imparted by the different proportions of the "four elements" that comprise them
• Democritus


  As early as 400 BC, some Greek philosophers, including Democritus, supported the idea of matter based on particle theory. They believed that all matter are composed of very small particles called atoms
• Atom


  The word atom came from the Greek word atomos which means "indivisible" or uncuttable
• Aristotle


  Asserted that all matter was continuous, an idea that was accepted for almost 2000 years
• Both theories (of Democritus and Aristotle) were not supported by experimental studies and therefore remained as speculations until 1800
• Dalton's Atomic Theory


  In 1808, John Dalton came up with a theory that marked the beginning of the modern era of chemistry
• Dalton's Atomic Theory Postulates

  • Elements are made up of small indivisible particles called atoms
  • In any given pure element, the mass and the properties of all the atoms are the same. Atoms of different elements differ in mass and other properties
  • Compounds are composed of atoms of more than one element. The constituent atoms in a given compound are present in a consistent or constant whole number ratio
  • In a chemical reaction, atoms are neither created nor destroyed. They simply combine, separate, or rearrange
• Atom


  Today, an atom is defined as the tiniest particle of an element that retains the chemical properties of that element
• Atom


  • All atoms are made up of the nucleus and the electrons
  • At the center of the atom is the nucleus, which is composed of protons and neutrons. These two units are collectively called nucleons. The nucleus is orbited by the atom's electrons
• Joseph John Thomson


  In 1897, Joseph John Thomson presented the "plum pudding" model of an atom, which illustrated how the negatively charged electrons are mixed with smeared out positive charges
• Protons


  The positively charged particle in the nucleus
• Neutrons


  The neutral subatomic particle in the nucleus, proven by James Chadwick in 1932
• Electrons


  Negatively charged particles surrounding the nucleus, considered elementary particles and 1840 times lighter than a proton
• Niels Bohr


  Proved the stability of atoms with equal number of protons and electrons, in a model where electrons stay in particular orbits without losing or gaining energy
• To easily picture the size of an electron and nucleus relative to the size of an atom, think of the Araneta Coliseum as the atom, its nucleus as the size of a calamansi fruit, and the electron as smaller than a grain of iodized salt moving around the extreme seats