Understanding the Atom

4-2: Discovering Parts of the Atom

4-2 Vocabulary

spectral lineenergy levelelectron cloud

How were electrons discovered?

In 1897, English scientist J.J. Thomson discovered electrons while doing an experiment to see how electric currents affected cathode rays.

How were electrons discovered? (cont’d)

A cathode ray is a stream of particles that can be seen when an electric current passes through a vacuum tube.The ray travels

from a negatively

charged disk at one

end to a positively charged disk at the

other end.

How were electrons discovered? (cont’d)

Thomson discovered the cathode rays did not travel in a straight line, but bent towards the positively charged plate.

How were electrons discovered? (cont’d)

Since opposite charges attract, Thomson concluded the cathode ray must have a negative charge and named the particles electrons.Thompson also proposed, like Dalton, that an atom was a solid sphere, but he added that an atom had a positive charge spread evenly throughout.

How were electrons discovered? (cont’d)

Positive charge spread evenly throughout and electrons mixed in to balance the charge.

Rutherford- Discovering the Nucleus

When two students of Ernest Rutherford shot alpha particles through gold foil, most passed straight through the foil, but some were scattered or even bounced straight back.

Rutherford- Discovering the Nucleus (cont’d)

Rutherford predicted the path of the particles would bend only slightly because the particles would not come upon a charge large enough to strongly repel them.

Rutherford- Discovering the Nucleus (cont’d)

Most particles did pass straight through.Some particles were strongly bounced to the side.One in about 8000 bounced completely backwards.

Rutherford- Discovering the Nucleus (cont’d)

Rutherford interpreted the unexpected results to mean that the alpha particles were hitting something with a positive charge and relatively large mass.

Rutherford- Discovering the Nucleus (cont’d)

Based on this information and further experiments, Rutherford developed a revised model of the atom.

Rutherford- Discovering the Nucleus (cont’d)

The atom as mostly empty space with the nucleus in the middle. Rutherford discovered the proton.Rutherford predicted the existence of the neutron.His model did not accurately explain how electrons are arranged in the atom.

Bohr and the Hydrogen Atom

A Danish scientist, Niels Bohr, studied the hydrogen atom and then proposed a new model for the arrangement of electrons in an atom.

Bohr and the Hydrogen Atom (cont’d)

A spectral line is a single wavelength of light that can be seen when the light from an excited element is passed through a prism.When excited, hydrogen and neon give off unique narrow bands of light on the spectrum.

Bohr and the Hydrogen Atom (cont’d)

An energy level is a region in space corresponding to a certain energy through which an electron moves.

Bohr and the Hydrogen Atom (cont’d)

Bohr proposed that electrons move in circles around the nucleus.The electrons can move only in an orbit that is a set distance from the nucleus.

Each energy level can hold a certain number of electrons.

Bohr and the Hydrogen Atom (cont’d)

Electrons fill the energy levels in order.

The lowest level is filled first.

The second level has no electrons until the first level is full.

The first level holds 2 electrons, the second level holds 8 electrons.The last energy level may or may not be filled.

Bohr and the Hydrogen Atom (cont’d)

An element will react with other elements to try to receive a full outer energy level. (also known as “completing the octet”)

Bohr’s model of circular orbits for electrons did not explain the behavior of electrons in outer energy level.

Bohr’s Atomic Models

This diagram shows how electrons are placed in the elements with atomic numbers 1–10 (according to Bohr’s model).

Bohr’s Atomic Models (cont’d)

Different numbers of electrons in outer energy level results in different elements.Unreactive elements have a full outer energy level.

Elements with incomplete outer energy levels are likely to form compounds.

The Electron Cloud

Today scientists think of an electron in an atom as being in an electron cloud, which is a region surrounding an atomic nucleus where an electron is most likely to be found.

Electron Cloud Model

4-2 Lesson Review Questions

An excited hydrogen atom emits narrow bands of light called ____.

A energy lines

B wave lines

C spectral lines

D wavelengths

4-2 Lesson Review Questions

Elements that do not react with other elements must have ____.

A completely filled energy levels

B excited electrons

C empty energy levels

D the same number of protons and neutrons

4-2 Lesson Review Questions

In the gold foil experiment, why did some particles pass straight through the foil?

A because they were repelled by the protons in the foil

B because they were attracted by the protons in the foil

C because atoms have no effect on charged particles

D because atoms are mostly empty spaces

Homework

Answer the three multiple choice lesson review questionsWrite a “Step-Up-To-Writing” style paragraph the answer the following question: “Have you ever noticed, or needed to find evidence for, something that was not directly observable? Besides learning about the structure of atoms, can you think of other ways people try to collect clues about things they cannot see directly?”