Module 7

1. Module 7

1.13. Page 2

Lesson 3: Page 2

Module 7—Investigating the Nature of the Atom

Explore

An illustration shows Thomson’s raisin-bun model of the atom with electrons embedded in a positive fluid.

When J. J. Thomson discovered the electron in 1897, it became evident that the atom was not the smallest unit of matter. Moreover, since electrons are negatively charged but atoms are neutral, atoms must also consist of a positively charged substance. Recall that Thomson’s model described the atom as electrons embedded throughout a positively charged substance. Thomson’s model is often called the raisin-bun model or the plum pudding model.

A diagram shows a planetary model of the atom has with negative electrons orbiting a dense, positively charged, nucleus.

Thomson’s model had a relatively short lifespan. As new investigations into the infinitely small continued and evidence was gathered, it became clear that within the atom there existed an extremely tiny, but very massive, positively charged core, giving rise to the Planetary Model of the Atom. (Note: When you follow this link, do not read the lesson the link brings you to. Instead, scroll down until you see the animated planetary model of the atom.) Evidence supporting this idea was collected by Ernest Rutherford and his assistants, Hans Geiger and Ernest Marsden, who observed the scattering of positively charged particles as they encountered a thin layer of gold atoms.

In their experiment, positively charged helium ions (called alpha particles) from a small sample of radioactive radium were used to bombard gold atoms. When the charged alpha particles encountered gold atoms, they were scattered at various angles. The scattered alpha particles could be observed when they encountered a zinc sulfide screen attached to a microscope.

Rutherford’s experimental design for measuring the scattering angle of alpha particles that collide with gold atoms.

Read

Read “Rutherford’s Scattering Experiment” on pages 767 and 768 of the textbook.

Watch and Listen

An illustration shows a dense, positively charged nucleus scattering alpha particles.

Open the Rutherford Scattering simulation to see how a large nucleus scatters smaller, charged alpha particles. (Note that you will need to save this simulation to your desktop before using it.)

Notice in the simulation that alpha particles are composed of two red protons and two grey neutrons without any electrons, producing the characteristic +2 ion charge. Also notice the small but very distant electron that orbits the large nucleus.

You will see it pass along its circular path in the corners of the viewing area, giving a sense of how small the nucleus and electron are relative to the majority of empty space in the planetary model of the atom.

Self-Check

SC 1. Adjust the number of protons on the atom using the simulation slider. Set it to 20 protons. Select “Show Traces” to see the path of each alpha particle. Use the term many, few, or rare to complete the following three statements:

_______ of the alpha particles pass by with little or no scattering, indicating the atom was mostly empty space.

______ of the alpha particles are scattered at large angles, indicating the presence of a small, dense nucleus.

On occasion, _____ alpha particles are scattered straight back toward the source, indicating the presence of a very dense, positively charged nucleus. Presumably, a large electrostatic force of repulsion would be required to reverse the alpha particles’ direction of motion.

Check your work.

Self-Check Answers

Contact your teacher if your answers vary significantly from the answers provided here.

SC 1.

Many of the alpha particles pass by with little or no scattering, indicating the atom was mostly empty space.
Few of the alpha particles are scattered at large angles, indicating the presence of a small, dense nucleus.
On occasion, rare alpha particles are scattered straight back toward the source, indicating the presence of a very dense, positively charged nucleus. Presumably, a large electrostatic force of repulsion would be required to reverse the alpha particles’ direction of motion.

An applet shows alpha particles being scattered by a dense, positively charged nucleus.

Lab

Module 7: Lesson 3 Assignment

Remember to submit your answers to LAB 1 and LAB 2 to your teacher as part of your Module 7: Lesson 3 Assignment.

Open the Rutherford Scattering simulation once again.

LAB 1. Adjust the number of protons to the maximum of 100. Describe what happens to the amount of scattering that occurs and the angles at which it occurs. How can you explain the relationship between the amount of scattering and the number of protons in the nucleus?

LAB 2. Select the “Plum Pudding Atom” from the upper menu on the simulation. Explain why the alpha particles are no longer scattered.

The simulation shows alpha particles going straight through the Raisin bun model of the atom without any deflection.

Failure of the Rutherford Model

According to Rutherford, the scattering alpha particles indicated that within the atom there existed a tiny, but very massive, positively charged core. Rutherford concluded that the atom was not filled with a positively charged substance (as Thomson had described); rather, all the positive charge of the atom was located in a nucleus at the centre of the atom. This nucleus was small but contained almost all the mass of the atom. Thus, Rutherford proposed a nuclear model. In this model the atom has a dense nucleus with relatively vast amounts of empty space through which the electrons can pass. The negative charge of the orbiting electrons was the opposite of the positive charge of the nucleus; so, overall, the atom is still electrically neutral, as Dalton determined.

Watch and Listen

Look at the animated Planetary Model of the Atom again. When you follow this link, do not read the lesson to which the link takes you. Instead, scroll down until you see the animated planetary model of the atom.

The planetary model of the atom shows electrons orbiting the nucleus of the atom.

There was a problem with the nuclear model of the atom. Recall that positive and negative charges attract. If the nucleus were positively charged, then what stopped the electrons from being sucked into the nucleus? To deal with this problem, Rutherford suggested that the electrons orbit the nucleus, much like the moon orbits Earth or like Earth orbits the sun. The force of attraction between the electrons and the nucleus provides the force necessary to keep the electrons in orbit. Hence, Rutherford proposed a planetary model of the atom.

However, the planetary model was also severely flawed. See if you can figure out what the flaw is by answering the following questions.

Try This

TR 1. According Maxwell’s theory of electromagnetism, from Module 5: Lesson 1, what happens when an electron is accelerated? What is given off?

TR 2. In Rutherford’s planetary model of the atom, are the electrons accelerated? If so, what force causes the acceleration?

TR 3. What would happen to an atom if electrons emitted radiation as they orbit the nucleus? Would atoms even exist if they constantly lost energy in the form of emitted radiation?

You should have discovered that, according to Rutherford’s model, atoms are not stable and will collapse in on themselves. According to Maxwell’s electromagnetic theory, when charged particles like electrons are accelerated, they emit electromagnetic radiation. Electrons orbiting a nucleus undergo inward acceleration; thus, they should continuously emit electromagnetic radiation. And if the electrons emit electromagnetic radiation, they should be losing energy. And if the electrons lose energy, they will eventually spiral into the nucleus. What was going on? By the end of the 19th century an adequate model of the atom had not yet surfaced.

Read

Read “The Bohr Model of the Atom” on pages 771 of your physics textbook.

The Role of Atomic Spectra

In developing a model of the atom, scientists also had to contend with atomic spectra. By the early 1800s, scientists knew that every element emits unique line spectra. For example, when an evacuated bulb is filled with neon gas and a voltage is applied to the electrodes, a characteristic red glow is emitted.

Separating this light by wavelength reveals that neon gas is actually emitting a small collection of unique wavelengths that fall in the red to yellow region of the visible light spectrum. You can see these unique wavelengths identified on the line spectrum below the bulbs. If the bulb were filled with a different gas, for example argon, a blue colour would be produced, with a different and unique line spectrum. The line spectrum is like a chemical fingerprint with each element having its own distinct pattern.

Why did each element have unique spectra, and why was it a line spectra? Somehow, these phenomena must be tied into the mystery of atomic structure.

It is important to understand what a line spectrum means. There are three types of spectra: continuous, bright line (emission), and dark line (absorption). A glowing solid or liquid or high-pressure gas will produce a continuous spectrum but only low-pressure gases will produce line spectra.

Continuous Spectra

All wavelengths (and frequencies) of light are present. In the visible range (400 nm to 700 nm), all the colours are visible.

An illustration shows the continuous spectrum of EMR in the visible range. It is a band of all the colours.

Bright Line Spectra

An excited low-pressure gas produces a bright line spectrum. The spectrum only consists of lines of particular wavelength. A bright line spectrum is also called an emission spectrum because the gas emits certain wavelengths (frequencies).

emission spectrum: a pattern of bright lines produced by a hot gas at low pressure

An illustration shows the emission spectrum for hydrogen gas, in the visible range. It is a dark band marked with coloured vertical lines.

Here is a diagram of the experimental set-up to produce a bright line spectrum.

$A diagram shows emission spectrum equipment for hydrogen gas using a prism to separate the emission lines. The equipment comprises a high-voltage power source and an atomic gas tube. A prism or diffraction grating diffracts the hydrogen gas.$

Dark Line Spectra

A dark line spectrum is created when light from a glowing solid or liquid is passed through an unexcited (cool) gas. The typically continuous spectrum (from the glowing solid or liquid) is now missing certain wavelengths—these missing wavelengths appear as dark lines. A dark line spectrum is called an absorption spectrum because the gas absorbs certain wavelengths (frequencies).

absorption spectrum: a pattern of dark lines produced when light passes through a gas at low pressure

An illustration shows the absorption spectrum for hydrogen, in the visible range. It is a band of colours marked by dark lines that match the locations on the emission spectrum.

An illustration shows the absorption spectrum equipment for hydrogen gas using a prism to separate the spectrum

Self-Check

SC 2. The emission and absorption spectra of hydrogen are displayed above. Compare the position of these lines. What do you notice? Does this mean that a gas can only absorb and emit a limited number of unique EMR wavelengths?

Check your work.

Self-Check Answers

Contact your teacher if your answers vary significantly from the answers provided here.

SC 2. Both the emission and absorption spectral lines occur in the same place on the spectrum. This means that a gas can only absorb and emit the same, specific wavelengths of EMR.