Scientists

Democritus vs. Aristotle

• Democritus (460-370) believed that atoms were uniform, solid, hard, incompressible, and indestructible

  • They moved in infinite number through empty space until stopped 

  • Differences in atomic shape and size determined the various properties of matter 

  • He came before Aristotle, and he coined the term atom, which is derived from the greek word “atomos” - meaning indivisible

• Aristotle (384-322) argued that the four elements were not composed of atoms but were continuous forms of matter

  • He also denied that a void between atoms could exist 

John Dalton (1803) 

• 5 postulates 

  • Each element is composed of tiny indestructible particles called atoms

  • The atoms of a given element are identical, the atoms of different elements are different 

  • Compounds are formed what atoms of different elements are combined, a given compound always has the same relative numbers and types of atoms (combination will always be the same)

  • Chemical reactions involve the reorganization of atoms but the atoms themselves are not changed during the process  

  • Atoms cannot be divided further 

    • Some were proved incorrect

      • All atoms of a particular element have identical properties, including mass (#2)

      • Subatomic particles like protons, neutrons, and electrons, which means atoms can be divided into smaller parts (#5)

Johann Wolfgang Dobereiner (1829) 

• Law of Triads (which was disproved by mid 19th century)

  • Mean of the atomic masses of the first and third element in a triad

  • Approximately equal to the atomic mass of the second element in that triad 

  • Grouped together by similar properties 

• Only a total of 5 Dobereiner’s trials were identified 

• Discovery of new elements refuted the law

• Several known elements did not fit into any of the triads

• Law did not apply to elements with extremely low or extremely high atomic mass

John Newlands (1865)

•  Law of Octaves 

  • Elements are arranged in order of increasing atomic mass, and those with similar chemical and physical properties occur after every eighth element

    • It was flawed because it only worked for lighter elements (first 20)

    • Did not take into account the unique properties of transition metals

Dmitri Mendeleev “Father of Periodic Table” (1869)

• His first table ordered the elements by increasing atomic mass and left spaces for new elements to be discovered

• Accounted missing elements by identifying gaps in his periodic table where no known element fit

• Predicted missing elements densities, mass and chemical behaviors

JJ Thomson (1897) 

• Discovered the electron using a cathode ray experiment to reveal the negatively charged particles 

• He also created the Plum Pudding Model, showing that a positive charge was spread out among electrons, thus balancing them

  • Was proved incorrect because the positive charge was inaccurately represented 

Ernest Rutherford (1911)

• Discovered the nucleus of an atom through the gold foil experiment

• His model showed the electrons outside the nucleus, but it had issues because he couldn't explain the orbit of electrons, and each atom he showed was unstable, when in reality most atoms are stable

Robert Millikan (1913)

• He discovered the negative charge of electrons, as well as their magnitude through the oil drop experiment 

Henry Mosely (1913)

• Created modern periodic table by arranging elements by atomic number, not atomic mass

• He used an x-ray apparatus to study elements that he thought were misplaced on the table by analyzing their properties with x ray frequencies 

Neils Bohr (1913)

• Created the Planetary (Bohr) Model which shows the electrons circling the nucleus like planets in the solar system, corrected rutherford model 

• An electron can move to a higher energy level by absorbing energy from an external source, like light or heat, which excites it and causes it to "jump" to a higher energy level further away from the nucleus

  • Fails to explain the effect of a magnetic field on the spectra of atoms, and only works for hydrogen atoms, not more complex atoms 

Erwin Schrodinger (1926)

• Made the Quantum-Wave model of electrons 

• His model shows the behavior of electrons as waves that wiggle and spread out, and can only be determined through probabilities 

• The position was not fixed, it is impossible to know both the position and momentum of a particle, with perfect accuracy, at the same time 

James Chadwick (1932)

• Discovered the nucleus through bombarding beryllium with alpha particles and observing the resulting radiation