Lesson 14: The Nuclear Atom

In an important experiment used to determine the nature of atoms, a gas like hydrogen, neon or mercury is excited by sparking it with electricity. The light emitted is separated out into individual colors using a prism or diffraction grating and a few lines of colored light are seen. The light observed in such a spectrum is emitted when an electron in an atom Select one:

"jumps" from one energy level down to another lower energy level.

Solar System Model

- the positive portion is concentrated into a tiny nucleus at the atomic center -the negative electrons orbit about the nucleus. The orbital radii define the atomic diameter instead of the positive pudding

Bohr Model

-each electron has a characteristic energy depending on orbit; smaller radius, less. energy. Just like gravitational potential energy -to move from one orbit to another an electron must either gain or lose the exact amount of energy between the two levels -electrons radiate (emit) only when they jump to an allowed orbit of lower energy electrons absorb energy only when they jump to a higher energy orbit.

J.J Thompson and plasma tubes

-start with a neutral gas, heat it with an electrical current, and it breaks into positve and negative fragments. -negative particles are identical; small mass, called electrons -positive particles differ depending on gas; large mass, called ions

Niels Bohr knew of Rutherford's new picture of the atom. He also knew of the two serious problems that existed in the model.

1. According to the model, the electrons in an atom should always emit electromagnetic radiation, but, in reality, electromagnetic radiation was observed only under very special conditions. 2. The electromagnetic radiation that was observed was of a discrete form rather than the continuously varying energies predicted by Rutherford's model.

Thompson model of the atom

Atoms consist of a thin positive fluid, which contains most of the mass, with embedded point-like negative electrons to balance the charge, -positive "pudding" -negative particles

Why did Rutherford think that all of the alpha particles would go straight through?

Because in Thompson's model of the atom there was no concentration of charge.

Bohr's model of the atom could explain discrete spectra, but Rutherford's model could not. Why?

Bohr's model only had a few possible energy states for electrons, while Rutherford's model allowed electrons to have any energy.

Thompson model:

He proposed that the positive charge was distributed uniformly, but diffusely like a cloud, throughout the atom and that the electrons were stuck in the positively-charged region like raisins dispersed throughout a plum pudding or chocolate chips in a cookie

Nuclear model, or the Solar System model

In this model most of the mass of the atoms was found in the small, positively-charged nucleus.

What are Rutherford's Observations and Justifications?

Nearly all alpha particles went straight through the gold foil. (Most of an atom is empty space with a very small nucleus). 1 in 8000 alpha particles were deflected through a wide range of angles. (The nucleus pushes the alpha particles away with an electric force. Therefore, the nucleus is positively charged because the alpha particles are positively charged, like charges repel.) Of the deflected alpha particles some of those were deflected directly back. (For the deflection directly back, the nucleus must be massive/very dense.)

What did Rutherford conclude about atoms from these experiments? Select one:

That all of the positive charge was concentrated in a very small volume. - nucleus

What is the Rutherford Model?

The atom is overal neutral. The atom is made up of a positive nucleus consisting of protons. There are spaces within atoms. Electrons orbit the positive nucleus at a distance.

Thompson Model: In the late 1800's scientists found that when a gas is subjected to a large difference in charge, the atoms or molecules of the gas can be torn apart

These experiments showed that positively-charged ions with relatively large masses and negative particles with very small masses were produced. -it was found that the negative particles were the same for all gases, but the positive particles differed in mass and charge from gas to gas.

Bohr developed a mathematical theory and an associated model for the atom that explained the spectral lines observed for some of the elements. In his model called the Bohr model

These orbits correspond to different energy states of the electron. An electron remains in a particular orbit unless it changes its energy state. The most revolutionary part of Bohr's model is that the electron could only jump between the allowed orbits. It couldn't be just anywhere orbiting the nucleus and the energy differences between these allowed orbits were fixed by the charge on the nucleus.

In an experiment done in the early 1900's, Rutherford used a radioactive source to bombard a thin sheet of gold foil with a narrow beam of alpha particles. He carefully measured where these alpha particles went after they hit the foil. What were the actual results of Rutherford's experiment? Select one:

Very few of the alpha particles bounced off of the foil.

absorption

jumping to a higher energy level

emission

jumping to a lower energy level

bohr model

model of the atom in which electrons move rapidly around the nucleus in paths called orbits

If matter was made up of neutral, indivisible particles, would there be a current through the discharge tube?

no

What was discovered as a direct result of Thompson's experiments with gas discharge tubes?

that all atoms are made of charged particles.

Rutherford's gold foil experiment showed:

that all of an atom's positive charge was concentrated into a small volume.