History of the Atom

The Atom…a Working Model The history of the atom has shown that through the years, the model has needed to be modified to reflect the discoveries that scientists have made. It’s important to know who the main contributors are to this “working model” of the atom and what their shortcomings were that needed modifications.

John Dalton Ancient Greeks had ideas about particles and atoms as early as 400 B.C. It wasn't until the start of the nineteenth century that a theory of atoms became linked to strong experimental evidence. It was then that an English scientist named John Dalton put forward his ideas about atoms.

Daltons 5 Points From his experiments and observations, he suggested that atoms were like tiny, hard balls. His theory was based on 5 points: All matter is composed of extremely small particles called atoms. Atoms of a given element are identical in size, mass and other properties; atoms of different elements differ in size, mass and other properties. Atoms cannot be subdivided created or destroyed. Atoms of different elements combine in simple whole number ratios to form chemical compounds. In chemical reactions, atoms are combined, separated or rearranged.

Dalton’s 3 Fundamental Laws Law #1 Law of Conservation of Mass – mass is neither created nor destroyed during ordinary chemical reactions or physical changes Ex. 15g C+20g O 2  35g CO 2

Law #2 Law of Definite Proportions – a chemical compound contains the same elements in exactly the same proportions by mass, regardless of the size of the sample or source of the compound Ex.Table Salt (NaCl) always consists of 60.66% by mass of sodium (Na) and 39.34% chlorine (Cl) A g sample of NaCl would consist of 60.66g Na and 39.34g Cl. A g sample of NaCl would consist of g Na and 78.68g Cl.

Law #3 Law of Multiple Proportions – if 2 or more compounds can be made by the same 2 elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers. Ex.1.0g carbon reacting with 2.66g oxygen to form CO 2 (carbon dioxide) 1.0g carbon reacting with 1.33g oxygen to form CO (carbon monoxide) 2.66g/1.33g = 2:1 ratio (small whole numbers)

John Dalton’s Atom

Shortcomings of Dalton’s Model In many ways, Dalton's ideas are still useful today but some of the parts of his theory were later proved to be incorrect. Which of Dalton's ideas below do we no longer believe? A. Elements contain only one type of atom. B. Atoms rearrange in chemical reactions. C. Atoms are solid masses that can't be split into smaller particles. D. All atoms of the same element are identical in size and mass.

We no longer believe letters c & d. Atoms can be split into smaller particles (electrons, protons, neutrons). Atoms of the same element are not always the same size and mass (isotopes of an element have the same number of electrons and protons, but differ in the number of neutrons)

J.J. Thomson At the end of the nineteenth century, a scientist named J.J. Thomson discovered the electron. The electron is a tiny negatively charged particle that is much, much smaller than any atom. When he discovered the electron, Thomson was experimenting by applying high voltages to gases at low pressure in a cathode ray tube. He noticed an interesting effect.

Cathode Ray Tube Experiment

Thomson did experiments on the beams of particles in his tube. They moved from the negative end of the tube (cathode) to the positive end of the tube (anode). Since these particles were attracted to a positive charge, Thomson correctly concluded that they must be negatively charged themselves. Other experiments showed that it would take about 2000 electrons to weigh the same as the lightest atom, hydrogen. He called the tiny, negatively charged particles electrons.

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Electron Source? But where had these tiny particles come from? Since they were so small, Thomson suggested that they could only have come from inside atoms. As a result, Dalton's idea of the indestructible atom had to be revised. Thomson proposed a different model for the atom. He said that the tiny negatively charged electrons must be embedded in a cloud of positive charge (after all, atoms themselves carry no overall charge, so the charges must balance out). Thomson imagined the electrons as the bits of plum in a plum pudding.

Thomson’s Plum Pudding Atom

Which statement about Thomson's model of the atom is true? A. The charge on the electrons is far greater than the positive charge in the atom. B. The total charge carried by the electrons equals the positive charge in the atom.

Letter b is correct. All atoms are neutral. Their positive charge must equal their negative charge.

Shortcomings of Thomson’s Model The location on the protons and electrons in the plum pudding model was later found to be incorrect according to Ernest Rutherford.

Ernest Rutherford The next development of the atom came about 10 years later, following Thomson’s discovery of subatomic particles. Two of Ernest Rutherford's students, Hans Geiger and Ernest Marsden, were doing an experiment at Manchester University with radiation. They were using the dense, positively charged particles (called alpha particles) as 'bullets' to fire at a very thin piece of gold foil. They expected the particles to barge their way straight through the gold atoms unimpeded by the diffuse positive charge spread throughout the atom that Thomson's model described. However, they got a big surprise.

Rutherford’s Gold Foil Experiment

In 1911, Ernest Rutherford interpreted these results and suggested a new model for the atom. He said that Thomson's model could not be right. The positive charge must be concentrated in a tiny volume at the centre of the atom, otherwise the heavy alpha particles fired at the foil could never be repelled back towards their source. On this model, the electrons orbited around the dense nucleus (center of the atom). The nucleus was Rutherford’s contribution to the evolving atomic theory.

Rutherford’s Model of the Atom

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In which two ways did Rutherford change Thomson's model of the atom? A. He said that the electrons were concentrated in the center of the atom (nucleus). B. He said that the positive charge was concentrated in the centre of the atom (nucleus).

Shortcomings of Rutherford’s Atom Rutherford could not explain why the negatively charged electrons did not get sucked into the densely packed, positively charged nucleus since we all know that “opposites attract.”

Niels Bohr The next important development came in 1914 when Danish physicist Niels Bohr revised the model again. It had been known for some time that the light given out when atoms were heated always had specific amounts of energy, but no one had been able to explain this. He solved the problem with Rutherford’s model which failed to explain why the positive nucleus did not suck in the negative electrons.

Bohr suggested that the electrons must be orbiting the nucleus in certain fixed energy levels (or shells) called orbitals. The energy must be given out when 'excited' electrons fall from a high energy level to a low one. This energy is given off in the form of light called photons. If an atom wants to move to a higher energy level, it must gain a specific amount of energy (photon).

Bohr’s Apparatus

Bohr’s Model of the Hydrogen Atom

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How did Bohr change Rutherford's model of the atom? A. He said that the electrons were concentrated in the centre of the atom (nucleus). B. He said that electrons could only occupy specific energy levels as they orbit the nucleus. C. He said that the electrons could orbit the nucleus in a completely random way.

The answer is letter b. Electrons must possess a certain amount of energy to orbit the atom. This will prevent the atom from getting sucked into the nucleus.

Shortcomings to Bohr’s Model Unfortunately, Bohr was unable to expand his model to other atoms besides Hydrogen. He could explain the activity of the single electron in a hydrogen atom but not for atoms with more than one electron.

Summary of History Our ideas about the nature of atoms have progressed over the last two centuries (and continue to develop today). John Dalton introduced a new form of the ancient Greek idea of atoms at the beginning of the nineteenth century through his 5 points supported by 3 fundamental laws of chemistry. In 1897, J.J. Thomson discovered the electron and suggested the 'plum pudding' model of the atom. In 1911, Rutherford suggested that the protons are located in the center of the atom in a densely packed bundle called the nucleus. In 1914, Bohr modified Rutherford's model by introducing the idea of energy levels keeping the electrons from getting sucked into the nucleus. We can think of the atom as a positively charged nucleus with negatively charged electrons orbiting the nucleus in energy levels (or shells).

Additional Exercises Which of the following ideas of John Dalton are still believed to be correct? Atoms are solid. Atoms in an element are all the same type of atom. Atoms cannot be split into smaller particles. Compounds contain different types of atom.

Who proposed the 'Plum Pudding' model of the atom? A. Dalton B. Thomson C. Rutherford D. Bohr

What is the charge on an electron? A. Neutral B. Positive C. Negative

Who performed the famous alpha particle scattering experiment? A. Dalton B. Thomson C. Rutherford D. Bohr

In the experiment carried out by Rutherford's team, what type of radiation was fired at gold foil? A. Gamma radiation B. Beta radiation C. Alpha radiation

Which statement below is correct? A. Most of the volume of an atom is taken up by a large nucleus. B. Atoms are mostly space.

How did Rutherford revise ideas about the distribution of positive charge in an atom compared with the 'plum pudding' model? A. Rutherford suggested that the charge was evenly spread throughout the atom. B. Rutherford suggested there was no positive charge in the atom. C. Rutherford suggested that the positive charge was concentrated in the center of the atom.

Who first suggested that the electrons orbited the nucleus in fixed energy levels (shells)? A. Dalton B. Thomson C. Rutherford D. Bohr

Which was the last of these 3 sub-atomic particles to be discovered? A. Electrons B. Protons C. Neutrons