The understanding of the atom has changed and advanced a lot of times throughout history. From ancient times through to present day, our thoughts about what makes up an atom have evolved. I will discuss the theories in more recent times starting with John Dalton. Englishman John Dalton presented his theory on the atom in the early 1800s. He was swayed by the fact that some elements could combine with different ratios of other elements but always in multiples of small whole numbers. For instance, tin could combine with oxygen to form tin oxide or tin dioxide, depending on whether one or two oxygen atoms combined with one tin atom. This he called the law of multiple proportions.
Dalton believed elements were made of extremely small particles called atoms which could not
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This was called the “plum pudding model”.
In 1909, Ernest Rutherford largely disproved this theory when with the help of Hans Geiger and Ernest Marsden, Rutherford found that alpha particles (two neutrons and two protons bound together) were unexpectedly deflected at angles of greater than 90 degrees when fired at metal foil. This suggested their charge was in fact not evenly spread leading Rutherford to think that the positive charge of an atom is concentrated in its centre. This was called the Rutherford Gold Foil Experiement.
In 1913, Denmark-born Niels Bohr proposed that electrons orbited the nucleus of an atom in much the same way that planets orbit the sun. But unlike the solar system, Bohr proposed that these electrons could jump orbits when they absorbed energy from a photon. Rutherford and Antonius Van den Broek refined Bohr’s work to propose that an atom’s nucleus contains a number of nuclear charges equal to its atomic number on the periodic