Atomic Structure Timeline

Democritus

400 BCE

Greek Philosopher, Democritus, was the first to propose that there’s a small particle that composes all matter. He named this particle “atomus” which means indivisible. This is important because it begins the conversation regarding "atomus" and contributes later to the development of the atomic theory.

John Dalton

1803

John Dalton is best known for his theory of atomism. He combined all chemical knowledge known at the time and created the atomic theory in 1803. He did experiments with gases and discovered that when two substances are combined, they do so proportionately. He concluded that all matter is made up of atoms that are indestructible and indivisible. He is also responsible for the idea that atoms are the smallest building blocks of matter. John Dalton's ideas led to our modern understanding of atoms.

Dmitri Mendeleev

1869

Mendeleev is given credit for being the father of the periodic table. He noticed patterns in the elements and their properties. He made cards with all the elements on them and arranged them according to their atomic weight. The table is arranged so that the most similar elements in terms of properties are close together. He even left empty spaces for undiscovered elements. Mendeleev created periodic law and even predicted undiscovered (now discovered) elements.

Henri Becquerel

1896

Becquerel was the first person to discover evidence of radiation/radioactivity. Essentially, he found that a photographic plate had become exposed when uranium ore was left on it. This ultimately led him to conclude that the uranium had some type of spontaneous emission. Eventually, his work leads to the discovery of the neutron.

J.J. Thomson

1897

Thomson is best known for his cathode ray experiments in which he discovered that atoms contain a negatively charged subatomic particle called an electron. He also proposed that the negative charges would repel so they have to be offset by some sort of positive charge. This atomic proposal is called the "Plum Pudding Model" which includes a positively charged sphere of matter with electrons scattered throughout. His discovery of the electron revolutionized the atomic structure.

Marie and Pierre Curie

1898

Marie and Pierre Curie conducted experiments and isolated radioactive radium salts from another substance. Through this experiment, they discover the existence of elements polonium and radium. They go on later to win a Nobel Prize. Their discovery of these elements is an important contribution to finding cancer treatments modern day.

Ernest Rutherford

1899

Rutherford discovered emissions from radioactive atoms that he named alpha, gamma, and beta rays. This led to his "Gold Foil Experiment". A beam of alpha particles were shot at a sheet of gold foil. A few of the particles deflected. He theorized that a dense center was causing the deflections.This experiment is important as it proved that atoms have a small, positively-charged nucleus and that the atom is mostly empty space. He contributed significantly to the theory of nuclear decay.

Henry Moseley

1913

Moseley is most known for using new x-rays to redefine the periodic table. He observed the patterns formed by reflected rays. He discovered a relationship between atomic number and wave-length and called this "Moseley's Law". He is also responsible for the idea that each element's identity is based upon the number of protons. Because he uncovered the true nature of the periodic table, he was able to predict four new elements (that were later discovered).