An atom is the smallest component of an element. The word atom comes from the Greek word atomos, which means indivisible. Our knowledge of the atom has gone through many changes since it was first proposed by the Greek philosopher, Democritus (460–370 BC). The modern theory of the atom incorporated the existence of subatomic particles within an atom and also how they are arranged.
Observe the relationship between, and relative position of, each subatomic particle. The neutrons and protons compose the nucleus and the electrons are in shells around the nucleus. The model can be used to show the first ten elements on the Periodic Table and their isotopes and ions.
- Electron shells
- Subatomic particles
The atomic number of an element is indicative of the number of protons contained in the nucleus of that element. Protons are positively charged subatomic particles, which have an atomic mass of 1 amu. The nucleus of an atom contains protons, as well as other subatomic particles referred to as neutrons. Neutrons also have an atomic mass of 1 amu; however, they carry no charge. The nucleus of an atom is held together tightly by the strong nuclear force. The strong nuclear force, in spite of the strong repulsion of the positive protons (like charges repel), keeps the nucleus from “flying” apart. Therefore, the protons and neutrons are “packed” into the nucleus just as the pom-poms will be packed into the plastic spheres in this model.
The mass number of an element is equal to the mass of neutrons plus the mass of the protons. In other words, to find the number of neutrons in the nucleus of an atom, subtract the number of protons (or the atomic number) from the mass number. For example, hydrogen, which has an atomic number 1 and a mass number of 1 would have one proton and no neutrons (1 – 1 = 0), however, if the mass number of an isotope of hydrogen (deuterium) were 2, there would be one neutron (2 – 1 = 1). The nucleus of hydrogen usually does not contain any neutrons. Isotopes are atoms of the same element that have different numbers of neutrons.
There is a third subatomic particle, the electron, that is not contained within the nucleus of the atom. The electron has a negative charge and although it has a charge equal in magnitude to a proton, the 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 decided that matter could not be divided into smaller and smaller pieces indefinitely. One would eventually reach the smallest piece of matter attainable and this piece would be indivisible. He called this piece of matter the atom.
It was not until around 1803 (more than 2,200 years after Democritus!) that the English chemist John Dalton (1766–1844) performed experiments resulting in an atomic theory that finally led to the acceptance of the concept of the atom. Dalton said that elements are composed of atoms that are exactly alike, and these atoms are unlike atoms of other elements. He also stated that joining atoms of two or more elements would form compounds, which are defined by the number, type, and proportion of the constituent atoms.
In 1897, Joseph John Thompson (1856–1940) refined the model by identifying the first of many subatomic particles which he called corpuscles. These corpuscles were later renamed electrons and were determined to possess a negative charge. Since atoms were known to be neutral, he hypothesized that these negative electrons must be suspended in a positive material.
The British physicist, Ernest Rutherford (1871–1937), in an attempt to test Thompson’s model, discovered even more about the atom. In 1908, he directed a beam of positively charged particles at a piece of gold foil. If the particles passed through, Thompson’s model would be supported. However, this did not occur—some of the particles passed through, some were deflected, and some bounced back. Rutherford concluded that the atoms must have a nucleus which is small, dense, and positively charged. He concluded that atoms were almost entirely empty space with a dense, positive center and negative charges scattered on the outside.
Niels Bohr (1885–1962), a Danish scientist, refined the model even further with his 1913 proposal that the electrons existed in specific energy levels. The electrons were thought to orbit the nucleus of the atom in the same fashion that planets orbit the Sun. This Elementary Atomic Model Set closely resembles the Bohr model, in that the nucleus is central and the electron shells are in definite orbits around it.
The nucleus, although infinitesimally small when compared to the size of the entire atom, contains almost all the mass of the atom. The English physicist, James Chadwick (1891–1974) devised an experiment in 1932 that led to the discovery of the neutron. The discovery of this neutral particle, with nearly the same mass as the proton, helped explain the discrepancy in atomic mass, which could not be accounted for solely by the protons.
The current teaching is that the electrons are not orbiting in a planetary orbit. Rather, they exist as an electron cloud, and for each energy level there is a certain probability of finding an electron at a certain location around the nucleus. This model, often called the electron cloud model or the quantum mechanical model, was developed by Erwin Shrödinger (1887–1961).
Brass cup hooks, 2*
Curved wooden arm*
Metal ring, large*
Metal ring, small*
Periodic Table of the elements*
Plastic spheres, large, 2*
Plastic spheres, small, 2*
Pom-poms, black, approximately 50*
Pom-poms, red, approximately 50*
Rubber spheres, blue, 12*
Square wooden base*
Swivels with hooks, 4*
Wood screws, 2*
*Materials included in kit.
Although the materials in this kit are nonhazardous, please observe all normal laboratory safety guidelines.
Materials for the Elementary Atomic Model Set may be reused many times and should be stored for future use.
Model Stand Assembly
- Using a Phillips screwdriver, secure the curved wooden arm to the square wooden base with the two wood screws as shown in Figure 1.
- Hand-tighten one brass cup hook in each of the predrilled holes on the front of the curved wooden arm as shown in Figure 2.
- Unwrap each sphere and connect them together by inserting the swivel hook through the plastic loop on each half.
- Snap the halves together as shown in Figure 3.
- Using the Periodic Table of the elements or the table that follows, determine which element will be displayed.
- Obtain one of the clear plastic spheres and fill with the appropriate number of pom-poms to represent the element. Note: The black pom-poms represent protons and the red pom-poms represent neutrons. The small sphere may be more appropriate for a hydrogen, helium, lithium, beryllium or boron nucleus. The large sphere may be more appropriate for a carbon, nitrogen, oxygen, fluorine or neon nucleus. The most common isotope of each of the first ten elements is summarized in the following table.
- Close the sphere and hang it from the lower hook using the swivel loop.
- Obtain the small metal ring and hang it from the lower hook.
- Obtain the large metal ring and hang it from the upper hook.
- Count out the number of electrons (blue rubber spheres) needed to make the atom electrically neutral.
- Affix the rubber spheres to the metal rings. A maximum of two electrons may be placed on the small ring and up to eight electrons may be placed on the large ring. Figure 4 shows an electrically neutral neon atom. The rings represent the first and second principal energy levels (shells), respectively.
- Add or remove neutrons, as appropriate, to make different isotopes of the same element.
- To make ions, add or remove electrons from the metal rings as needed. Note: Ions may only be made by adding or subtracting electrons—never protons!
- There are enough pom-poms and spheres in this kit to create four nuclei that can quickly be interchanged.
- The plastic spheres may break if dropped, so use care in handling them.
- Additional spheres and pom-poms may be purchased from Flinn Scientific, Inc. (Catalog No. AP6875) to allow for more nuclei to be created at one time. The Atomic Model Nuclei Set contains four spheres, 50 red pom-poms, and 50 black pom-poms.
- Write on the plastic sphere with a dry erase or wet erase marker to denote the element or the number of protons and neutrons contained.
- This model only shows the three primary subatomic particles. Continue the discussion about the history of the atomic model by allowing students to research the fundamental particles that make up protons and neutrons, such as quarks and muons.
- As an extension, have students create a pamphlet that outlines the historical advancements in the model of the atom.
- Have students research individual elements and use the Elementary Atomic Model Set as their visual aid.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Developing and using models
Disciplinary Core Ideas
MS-PS1.A: Structure and Properties of Matter
HS-PS1.A: Structure and Properties of Matter
Scale, proportion, and quantity
MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended structures.