An atom is the smallest possible unit of an element that has all the properties of the element. The word atom comes from the Greek word atomos, which means indivisible. The theory of atomic structure has gone through many changes since the idea of an atom was first proposed by the Greek philosopher, Democritus (460–370 bc). Today, the atom is known to be made up of a nucleus, containing positively charged protons and non-charged neutrons, with negatively charged electrons surrounding the nucleus at various energy levels.
With this atomic model, explore the energy levels of electrons in the first three shells and observe electron pairing. Construct a mobile for any of the first 18 elements.
- Electron shells
- Subatomic particles (electrons and protons and neutrons)
The atomic number of an element is indicative of the number of protons contained in the nucleus of that element. Protons carry a positive charge. The nucleus of an atom contains protons, as well as subatomic particles called neutrons, which have no charge. The mass of a neutron is similar to the mass of a proton. The mass number of the element is derived from the mass of the neutrons plus the mass of the protons.
The electron is another subatomic particle that is not contained within the nucleus of the atom. The electron has a negative charge, and although this charge equal is in magnitude to the change of a proton, the electron’s mass is negligible in relation to that of the proton and neutron.
The history of the atomic model begins with the Greek philosopher, Democritus who proposed that matter could not be divided into smaller and smaller pieces ad infinitum. One would eventually reach the smallest piece of matter that could be attained and this piece would be indivisible. He called this piece of matter the atom.
Many scientists—from the English chemist John Dalton (1766–1844), who performed experiments resulting in an atomic theory, to Erwin Schrödinger (1887–1961), who developed the electron cloud model—have refined and built upon the foundation laid by Democritus. One of the most important contributions was from Joseph John Thompson (1856–1940) who, in 1897, refined the atomic model by identifying the first of many subatomic particles which he called corpuscles.
These corpuscles were later renamed electrons and were determined to have a negative charge. Since atoms were known to be neutral, Thompson hypothesized that these negative electrons must be suspended in a positive material. This model was called the “plum pudding” model. In 1913, the Danish scientist Niels Bohr (1885–1962) refined the model even further by announcing that the electrons existed in specific energy levels. Bohr believed that the electrons orbit the nucleus of the atom in the same fashion that the planets orbit the Sun. This Atomic Mobile Kit closely resembles the Bohr model, in that the nucleus is central and the electron shells are in definite “planetary” orbits around it.
The current model is that the electrons are not orbiting in a “planetary” orbit; rather they exist in an electron cloud. This model, often called the electron cloud model or the quantum mechanical model, was developed by Erwin Schrödinger.
Ceiling hooks, 2*
Dry erase marker
Fishing line, 5'*
Metal ring, 7"*
Metal rings, 9½", 2*
Metal rings, 12", 2*
Nucleus tag, plastic, 2*
Periodic table of the elements*
Rubber spheres, blue, 8*
Rubber spheres, red, 14*
Swivels with hooks, 5*
*Materials included in kit.
Although the materials in this kit are nonhazardous, please observe all normal laboratory safety guidelines.
Materials for the Atomic Mobile Kit may be reused many times and should be stored for future use.
- Attach four swivels together, hook end to hook end, to form a chain (see Figure 1).
- Lay the chain horizontally in front of you on a table.
- Beginning with the hook on the left, mentally number each hook 1 to 8.
- On hook 1, attach the nucleus tag.
- On hook 3, attach the small, 7" metal ring.
- On hook 5, attach the two medium-sized, 9½" rings. Note: The hook may not close completely with two rings inserted.
- On hook 7, attach the two large, 12" rings.
- Finally, on hook number eight, attach fishing line to hang the mobile from the ceiling (using the ceiling hooks provided), a stand or another object from which it can hang freely (see Figure 2).
- Using the Periodic Table of the Elements or Table 1, select an element to be displayed.
- Using a dry erase marker, write the number of protons and neutrons in an atom of this element on the plastic nucleus tag (see Figure 3). The most common isotope of each element is listed in the following chart.
- Change the number of neutrons, as appropriate, to model different naturally occuring isotopes of the element. For example, magnesium has three naturally occuring isotopes, Mg–24. Mg–25 and Mg–26. Mg–24 has 12 neutrons, Mg–25 has 13 neutrons, and Mg–26 has 14 neutrons.
- Count out the number of electrons (rubber spheres) needed to make the element electrically neutral—this will be the same as the number of protons. Use the blue spheres to represent s-orbital electrons and the red spheres to represent p-orbital electrons.
- Affix the spheres to the metal rings by sliding the ring through the slit in the sphere. A maximum of 2 blue (s) electrons may be placed on the small ring, 2 blue electrons on one of the medium-sized rings, 6 red (p) electrons on the other medium-sized ring, 2 blue electrons on one of the large rings and 6 red electrons on the other large ring. Figure 3 shows an electrically neutral Argon atom.
- To make ions, add or remove electrons from the metal rings. Note: It is never appropriate to change the number of protons to affect the charge.
- The atomic mobile illustrates the Bohr theory of the atom, with electrons orbiting the nucleus in fixed orbits or paths. This is the most popular picture of the atom that most people have. It is important to remember that this familiar picture is, in fact, wrong. Electrons do not orbit the nucleus in fixed paths. Instead, they are located in regions of space.
- There are enough materials in this kit to create any of the first eighteen elements. They can be quickly changed by adding or removing electrons and erasing the plastic nucleus tag to adjust the number of protons and neutrons, or the element’s symbol.
- Write on the plastic nucleus tag with a dry erase marker to denote the element symbol and mass, or the number of protons and neutrons.
- Allow students to research other types of subatomic particles such as quarks and leptons.
- As an extension, have students create a pamphlet that outlines the historical advancements in the model of the atom from Democritus to Schrödinger. Request Flinn Publication No. 10743, Dinner with Democritus for other activity ideas.
- Have students research individual elements and use the Atomic Mobile as their visual aid.
- Two “orbitals” (metal rings) are provided for the second and third electron shells to distinguish the s-orbital and p-orbital found at each “shell” (electron energy level). There is only an s-orbital at the first electron shell. It would be appropriate to group the p electrons in three groups of 2 electrons to symbolize 3 electron pairs.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Developing and using models
Obtaining, evaluation, and communicating information
Disciplinary Core Ideas
MS-PS1.A: Structure and Properties of Matter
HS-PS1.A: Structure and Properties of Matter
Cause and effect
Structure and function
Scale, proportion, and quantity
MS-PS1-1: Develop models to describe the atomic composition of simple molecules and extended structures.
HS-PS1-1: Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.