Sample Course
Physical Science - An Atomic Whodunnit and Modern Theory
Learning Goal
- Show an understanding of atomic models by comparing the properties of atomic models.
Introduction
In 1910, the novelty we know today as neon lights was invented. You have almost certainly observed neon lights. Your observations of neon lights might lead to these questions: Why aren't all neon lights the same color? How can neon lights be designed to be a certain color or colors? How do the colors define the atoms from which the colored light comes?
Gravel
Credit: USGS
Atom Model
Credit: NASA
Galaxy
Credit: NASA
When you view neon lights, you are viewing the emission of light from different elements. Inside the glass tube there is a gas at low pressure like neon, argon, mercury vapor, xenon. When high voltage is applied to the electrodes attached to the tube, the gas ionizes and electrons flow. The electrons excite the gas atoms, causing them to emit light, forming a line emission spectrum.
The neon light manufacturers use different gases in their products to display different colors of light. Mixing gases and elements added to a neon light creates different hues.
| Gas | Emission color |
|---|---|
| Neon (Ne) | Red |
| Krypton (Kr) | Green or Gray |
| Mercury Vapor (Hg) | Light Blue |
| Xenon (Xe) | Blue or Gray |
| Helium (He) | Orange |
Emission Line Spectra
In 1912, following Ernest Rutherford's famous paper on the scattering of alpha particles, Danish physicist Niels Bohr was moving into Rutherford's science lab to study with the famous Rutherford. Recall from the previous lesson that Rutherford described the atom as having a positively-charged nucleus, surrounded by orbiting electrons.
In Rutherford's model, the orbiting electrons can be anywhere outside the atom's nucleus (central area). Bohr realized that there was a problem with this model – it could not explain the emission line spectra for each type of atom. When energy is passed through an element, light is emitted. When viewed through a prism, these elements reveal unique bands of light and dark. This set of bands is known as an emission line spectrum, and it is unique to each element, kind of like an atomic fingerprint. Could Bohr's observations of neon lights, invented only two years earlier, have led him to ponder the atomic nature of the gases that produce such colorful displays?
Sodium (Na)
Carbon (C)
Calcium (Ca)
Energy Levels
Remember that a model is useful if it explains observations. If electrons can be anywhere outside the atom's nucleus, what causes atoms to emit light when heated? Bohr explained that electrons exist in specific layers, or energy levels. An atom emits light when it is heated because the electrons jump to a higher energy level, then emit colored light as they drop back down to a lower energy level. The emitted colored light corresponds to a color in the emission spectrum. The color depends on where the electrons are located in the atom's energy levels. Each energy level allows the electrons a different amount of energy, which is released in the form of colored light.
Bohr also knew that electrons orbiting the atom's nucleus would eventually lose energy and begin a rapid inward spiral, leading to the destruction of the atom. It was time for a new model.
09. What prompted Bohr to modify Rutherford's model?
Teacher Comments:
Bohr felt that a loss of energy would cause the electrons to spiral into the nucleus causing destruction of the atom.
Do your notes reflect the teacher's comments? If so click 'Save'
Bohr's Model
In 1913, Niels Bohr was ready to modify Rutherford's model. Bohr proposed that the negatively charged electrons orbit the nucleus in circular patterns at fixed distances, depending on energy levels. The movements, he proposed, are like planets orbiting the sun (giving his model the name, planetary model). This distinguished his model from Rutherford's. He suggested that electron orbits, or shells, at greater distances from the nucleus could contain more electrons than inner shells. At any moment, an electron may have one of several energy levels, but it may not have an amount of energy between these energy levels. Bohr determined the energy values for each level. The lowest level, called the ground state, is where electrons would normally be found. Here, an atom will be completely stable, because the electrons can't jump to lower energy levels.
In Rutherford's model, a loss of energy would cause the electron to spiral into the nucleus, causing destruction of the atom in an instant.
Bohr's model places the electrons in fixed orbits (shells), with the electrons having a discrete amount of energy.
When the atom absorbs energy, the electrons can shift to a higher energy level.
Teacher Comments:
Bohr proposed that the negatively charged electrons orbit the nucleus in circular patterns at fixed distances, like planets orbiting the sun. The outer electron orbits, or shells, could contain more electrons than the inner shells.
Do your notes reflect the teacher's comments? If so click 'Save'
Electrons can also move from outer levels to inner levels. As previously explained, Bohr proposed that when electrons jump from an outer orbit to an inner orbit, the atom emits radiation, such as light.
As you know, models should be able to not only explain observations but accurately predict the outcome of an experiment. The Bohr model could accurately explain the emission line spectrum of the element hydrogen (H), but hydrogen is the simplest atom with only one electron. Bohr's model could not predict the emission line spectrum of any other element! As with other models of the atom to date, the Bohr model was replaced in the 1920s.
Electron Cloud Model
The Bohr model described electron movement around an atomic nucleus as similar to a planet orbiting the sun. This idea was discarded with the new model, called the quantum model or electron cloud model.
Today, scientists know that the electrons do not actually orbit the nucleus as described in Bohr's model. They move within distinct energy levels, but the exact position of the electrons is not known with certainty. This model is referred to as the electron cloud model. In the electron cloud model, electrons can be expected to be located more often in certain areas around the atom's nucleus. However, the exact location of an electron at a certain time can only be predicted. The locations where the electrons are more likely to be found are called atomic orbitals, each having a characteristic shape and energy. The "electron cloud" can shift between these orbitals.
Teacher Comments:
Electrons can be expected to be in certain areas more often than other areas around the atom's nucleus. However, the exact location of an electron at a certain time is only predicted. The likely locations are called atomic orbitals, each having a characteristic shape and energy. The 'electron cloud' shifts between these orbitals.
Do your notes reflect the teacher's comments? If so click 'Save'
Description of the Atom
An atom can be simply described as having a dense, positively charged nucleus made of protons and neutrons, and the nucleus is surrounded by negatively charged electrons. Electrons are in constant motion around the nucleus, existing in shells, or clouds. The position of the electron in the shells is determined by its energy level. Electrons farther from the nucleus have greater energy.
Because atoms are so tiny, it would be impractical to weigh them with typical units. For example, a proton has an approximate mass of 1.67 x 10-24 grams (0.0000000000000000000000167 grams). Scientists have created a new value to simply describe the mass of atomic components: the atomic mass unit, or AMU.
Protons have a positive charge and have a mass of approximately 1 atomic mass unit. Neutrons have no charge and have a mass of approximately 1 AMU. Electrons have a mass of 1/1836 AMU and have a negative charge.
Atoms have a small diameter to go along with their small mass. The nucleus of an atom is about 0.0000000000001 cm in diameter, with the diameter of the atom about 0.00000001 cm in diameter. If you drew an atom to scale, with the protons and neutrons one centimeter in diameter, the electrons would be 100,000 times distant! An atom is mostly empty space!
Remember that like charges repel and unlike charges attract.
Electron Cloud Model of the Atom
Credit: Yzmo CC BY-SA 3.0
Electrons repel each other because of their like charges, which keeps them spread out as they surround the atom's nucleus. Protons also repel each other, but nuclear forces in the atom's nucleus holds the protons together.
Since electrons and protons have different charges, they are attracted to each other. This electromagnetic force is the attraction that keeps the electrons around the nucleus.
Scientists have discovered that protons and neutrons are probably composed of even smaller particles called quarks. This is a new addition to the modern atomic model. Picture an atom. Electrons are in constant motion around the nucleus, while protons and neutrons are in the nucleus, slightly shifting position in a rapid movement.
Is this the final model of the atom? Certainly not. There are observations that are still unexplained by this model. Consider the atomic model a useful tool with which to explain observations, but not to be considered a final representation.
