History of the Atom Chapter 4

The Atom… a Working Model The history of the atom has shown that through the years, the model had 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 were then modified.

John Dalton Ancient Greeks had ideas about particles and atoms as early as 400 B.C. It wasn’t until the 19 th century that a theory of atoms became linked to experimental evidence. English scientist John Dalton put forward his ideas about atoms.

Daltons Atomic Theory 1. All elements are made of atoms 2. Atoms of the same element are identical 3. Atoms of different elements can physically mix together or combine chemically to form compounds 4. Chemical reactions occur when atoms are separated, joined, or rearranged in a different combination.

Dalton’s Shortcomings…. We now know that atoms can be split into smaller subatomic particles. Electrons, protons, and neutrons

J.J. Thomson Discovered the electron in 1897 The electron is a tiny negatively charged particle that is much, much smaller than any atom. Thomson was experimenting by applying high voltages to gases at low pressure in a cathode ray tube. He noticed interesting effects.

Thomson’s Plum Pudding Atom

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

Ernest Rutherford Made his discovery 10 years after Thomson’s plum pudding model. Used positively charged alpha particles as ‘bullets’ and fired them at a very thin sheet of gold foil. Expected all of the particles to go straight through.

Ernest Rutherford’s Gold Foil Experiment

In his model, electrons orbit around the dense nucleus (center of the atom).

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

Neils Bohr In 1914 Neils 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 didn’t stick to the negative electrons

Bohr 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 electron wants to move to a higher energy level, it must gain a specific amount of energy

Bohr’s Model of the Hydrogen Atom

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

Summary of History Over the last two centuries our understanding of the atom has progressed Dalton introduced his four points of the atom. Thomson discovered electrons and suggested ‘plum pudding’ model. Rutherford said protons were located in the center of the atom in a nucleus Bohr introduced 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 level, or shells.

Questions Which of the following ideas of John Dalton’s are still believed to be correct? A. Atoms are solid B. Atoms in an element are all the same type of atom C. Atoms cannot be split into smaller particles D. Compounds contain different types of atoms

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. Alpha B. Beta C. Gamma

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 that 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? A. Dalton B. Thomson C. Rutherford D. Bohr