2.4

DEMOCRITUS

Leucippus and Democritus were the first to propose, in the fifth century B.C., that all matter is made of tiny units called atoms. The two philosophers held that these were solid particles without internal structure, and came in a variety of shapes and sizes. Intangible qualities such as taste and color, according to this theory, were made of atoms.

DALTON

"Billard Ball Model"

John Dalton built upon them to develop the law of multiple proportions, which states that the ratios of masses of elements in a compound are small whole numbers. Dalton's law of multiple proportions drew from experimental data. He proposed that each chemical element consists of a single type of atom that could not be destroyed by any chemical means. He also proposed that while all atoms of one element are identical, they are totally different from those that make up other elements.

How was the model wrong?

Atoms aren´t indivisible, they are composed from subatomic particles

THOMPSON

"Plum Pudding Model"

J.J. Thomson had discovered the electron. He believed atoms could be divided. Because the electron carried a negative charge, he proposed a plum pudding model of the atom, in which electrons were embedded in a mass of positive charge to yield an electrically neutral atom.

Experiment he performed

Cathode rays are a type of radiation emitted by the negative terminal, the cathode, and were discovered by passing electricity through nearly-evacuated glass tubes. The radiation crosses the evacuated tube to the positive terminal, the anode. Cathode rays produced by the CRT are invisible and can only be detected by light emitted by the materials that they strike, called phosphors, painted at the end of the CRT to reveal the path of the cathode rays. These phosphors showed that cathode rays travel in straight lines and have properties independent of the cathode material (whether it is gold, silver, etc.). Another significant property of cathode rays is that they are deflected by magnetic and electric fields in a manner that is identical to negatively charged material. Due to these observations, J.J. Thompson (1856-1940) concluded that cathode rays are negatively charged particles that are located in all atoms. It was George Stoney who first gave the term electrons to the cathode rays.

How was it wrong?

He did not talk about the nucleus of the atom

RUTHERFORD

"Nuclear Atom"

Ernest Rutherford, one of Thomson's students, disproved the plum pudding model in 1909. .Rutherford devised a model called the nuclear atom. In this model, the positive charge is held in an extremely small area called the nucleus, located in the middle of the atom. Outside of the nucleus the atom is largely composed of empty space. This model states that there were positive particles within the nucleus.

Experiment he performed

Ernest Rutherford (1871-1937) performed a series of experiments studying the inner structure of atoms using alpha particles. Rutherford knew that alpha particles are significantly more massive than electrons and positively charged. Using the plum-pudding model for reference, Rutherford predicted that particles in an alpha beam would largely pass through matter unaffected, with a small number of particles slightly deflected. The particles would only be deflected if they happened to come into contact with electrons. According to the plum pudding model, this occurrence would be very unlikely.

Experiment he performed

In order to test his hypothesis, Rutherford shot a beam of alpha particles at a thin piece of gold foil. Around the gold foil Rutherford placed sheets of zinc sulfide. These sheets produced a flash of light when struck by an alpha particle. However, this experiment produced results that contradicted Rutherford's hypothesis. Rutherford observed that the majority of the alpha particles went through the foil; however, some particles were slightly deflected, a small number were greatly deflected, and another small number were thrown back in nearly the direction from which they had come.

How was it wrong?

He did not explain why electrons remain in orbit around the nucleus and also he did not define the positive particles of the nucleus.

BOHR

"Solar System Model"

Niels Bohr proposed a planetary model, in which electrons revolve about the nucleus just as the planets orbit the sun. While the electrons are in orbit, they have what Bohr termed "constant energy." When these particles absorb energy and transition into a higher orbit, Bohr's theory refers to them as "excited" electrons. When the electrons return to their original orbit, they give off this energy as electromagnetic radiation.

How was the model wrong?

Moving electrons should emit energy and collapse into the nucleus, this model did not work well for heavier atoms.

SCHRÖDINGER

Erwin Schrödinger, an Austrian physicist, took the Bohr atom model one step further. Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom. Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron. This model can be portrayed as a nucleus surrounded by an electron cloud. Where the cloud is most dense, the probability of finding the electron is greatest, and conversely, the electron is less likely to be in a less dense area of the cloud. Thus, this model introduced the concept of sub-energy levels.