atomic history (chem)

  • discovery of the atom

  • Democritus

    • believed that atoms were indivisible and indestructible

    • Aristotle believed everything was composed of 4 elements— earth, fire, water, air

  • Dalton’s Atomic Theory (scientific evidence)

    1. all elements are composed of indivisible particles called atoms

    2. atoms of the same element are identical

    3. atoms of different elements combine in simple whole-number ratios to form chemical compounds

    4. atoms are combined, separated, or rearranged, but never changed into atoms of another element.

  • problems w/ Dalton’s theory

    1. Atoms are made of subatomic particles: protons, neutrons, and electrons. Protons and neutrons are also subdivided. He was correct that the atom is the smallest particle that still maintains the properties of the element

    2. ISOTOPES! Atoms of the same element can have different numbers of neutrons

    3. Nuclear chemistry- can be changed to a different element

  • discovery of the electron

    • 1897— JJ Thomson used a cathode ray tube to deduce the presence of a negatively charged particle, the electron

      • negative charges were sent out the cathode, and you could see the sparks

      • cathode ray tubes pass electricity through a gas that is contained at a very low pressure

    • thomson’s atomic model

      • plum pudding model - he thought the positives were evenly distributed like pudding and that the electrons were like plums embedded inside the pudding

      • gave us the mass/charge ratio of an electron

  • mass of the electron

    • 1916— Robert Millikan determines the mass of the electron

    • oil drop experiment

      • put a negatively charged electric plate, where he could suspend the oil droplets so they’d float

      • he figured out the charge of the electron by seeing how high they were able to levitate on that plate

      • mass = 9.11*10^-28 g

      • 1/1840 the mass of a hydrogen atom

  • conclusion from the study of the electron:

    • cathode rays have identical properties regardless of the element used to produce them

    • all elements must contain identically charged electrons

    • atoms are neutral so there must be positive particles in the atom to balance the negative charge of the electrons

    • electrons have so little mass that atoms must contain other particles that account for most of the mass

    • Eugen Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge and a relative mass of 1

  • Ernest Rutherford’s gold foil experiment - 1911

    • alpha particles are helium nuclei - the alpha particles were fired at a thin sheet of gold foil

    • he thought they would go right through, but some were deflected

      • positive charges always repel each other, so the positive nucleus and positive alpha particle do not go along w/ each other so they deflect

    • results:

      • most passed right through = atoms are mostly empty space

      • few particles were deflected = there is a positive mass that is small

        • the nucleus is small

        • the nucleus is positively charged

      • very few were greatly deflected = very dense

        • the nucleus is dense

  • Rutherford atomic model

    1. atom is mostly empty space

    2. all the positive charge + all the mass was in a small area in the center— the nucleus

    3. nucleus is composed of protons and neutrons

    4. electrons distributed around the nucleus and occupy most of the volume

    • this was the first nuclear model

  • Neils Bohr

    • electrons have to act similarly to how the planets orbit the sun, in circular paths around the nucleus

    • BOHR MODEL (sometimes considered the Rutherford-Bohr model)

  • Quantum model

    • energy is quantized— it comes in specific values

      • seen in the Bohr model

      • electrons can move from orbital to orbital by gaining /losing energy, but it can not land in between orbital

    • since the energy of an atom is never in between, there must be a quantum leap in energy

    • 1926- Erwin Schrodinger derived an equation that describes the energy and position of the electrons in an atom

      • wave equation

      • electron cloud model

        • electrons can be anywhere in attraction to the nucleus but they are most likely to be in the orbitals

  • Heisenberg uncertainty principle

    • you can know one or the other— where it is, or where it’s going

      • can’t simultaneously know position and momentum