Atomic Theory

• Greek Philosophers: Democritus said that atoms were uniform, and that they were the smallest thing that everything was made up of.
• Atomos ---> indivisible
• Demi's theory states that atoms are indivisible, those of an element are identical, and compounds are combinations of different atoms.
• His theory was wrong about only one part: atoms aren't indivisible, they're composed subatomic particles.
• Atomic theory- atomic theory is the study of the nature of atoms and how they combine to form all types of matter.
• John Dalton, England (1766-1844) made the theory:

  1. All matter is made of small, indivisible particles called atoms
  2. Atoms of an element are of identical size and mass
  3. Atoms of different elements have different properties
• Dalton's theory was right about two parts: atoms do exist, and atoms of an element are similar in size and shape. But it was wrong because he thought that atoms couldn't be divided into smaller pieces, which we now know isn't true.
• J. J Thomson, England (1856 - 1940): discovered electrons by using cathode ray tubes. He found out that there were negatively charged particles inside atoms. This led him to believe that atoms weren't solid spheres but instead had empty space between positively charged particles. His model was called plum pudding model.
• Ernest Rutherford (1871-1937) - conducted gold foil experiment which showed that most of the atom’s mass is concentrated at its center. This led him to conclude that there must be a positively charged nucleus with neutral electrons orbiting around it.
• Each level corresponded to a specific amount of energy. When an electron jumped from one level to another, it absorbed or emitted radiation. (BOHR)
• Niels Bohr (1885-1962) proposed that electrons orbit around the nucleus in fixed paths or shells. Electrons can jump between these shells when energy is absorbed or released.
• Ernest Rutherford

  • Discovered the idea of a positively charged nucleus
  • Found that protons and neutrons have similar mass
  • These ideas were made from the infamous gold foil experiment
  • Began to form the atomic model that most people still use today
• There must be positive charge within the atom to counterbalance the negative charge from the electrons. (JJ)
• 3 ways protons are different from electrons:

  • Positively charged
  • Protons were localised in the nucleus of an atom
  • Protons don’t move, while electrons can
• What is different about the ancient and modern definitions of ‘elements’?

  In ancient times, element meant Earth, Fire, Water, and Wind, but modern definitions mean a category of the pure substance.
• What is similar about the ancient and modern definitions of ‘elements’?

   Both ancient and modern definitions state that elements are made up of one type of particle/atom. 
• In Ernest Rutherfords Gold Foil Experiment, he fired alpha particles at a thin sheet of gold. The majority passed straight through, indicating that the bulk of the atom is empty space. A few bounced back, suggesting that there is something dense in the centre of the atom. Some were deflected sideways, which suggested that the central part has a positive charge.
• Raisin bun (1904)

  
• still used today (1911)

  
• 1913 model

  
• 1926

  
• An atom has three main parts: These are called subatomic particles 

  1.  Proton

  2.  Neutron

  3.  Electron
• The following is a schematic drawing of the Bohr-Rutherford model of the atom: 

  
• PROTONS

  symbol: p+

  Charge: +1

  Location: Nucleus

  Relative mass (amu): 1
• amu : atomic mass unit
• neutron:

  symbol: n 0

  charge: 0

  location: nucleus

  relative mass (amu): 1
• electron

  symbol: e-

  charge: -1

  location: orbitals

  relative mass (amu): tiny (1/2000)
• Electrons have no rest mass, so they do not contribute to an element's relative atomic mass.
• Atomic number: # of protons and electrons
• atomic symbol:

  
• Atomic mass: number of protons + electrons
• number of neutrons: atomic mass - atomic number
• assuming atom has neutral charge, electrons: number of electrons = number of protons
• isotope: different atoms with same number of protons but different numbers of neutrons
• relative atomic mass (A): average mass of all isotopes of the same element
• Democritus' theory was wrong, since atoms aren't solid or the same size. Also, failed to explain what held them together
• An ion is a charged atom or molecule. It is charged because the number of electrons do not equal the number of protons in the atom or molecule
• The number of protons in an atom determines its elemental identity. NO TWO ELEMENTS HAVE THE SAME NUMBER OF PROTONS IN THEIR ATOMS. 
• Metals won't combine to form a compound because they're both giving away their electrons so there's no receiving or giving electrons between the two.
• Generally, metal elements donate electrons and non metal elements receive them. The opposite charges on the ions cause the ions to bond, or be held together, by electrostatic forces.