NOVA Online | Teachers | Viewing Ideas | NOVA scienceNOW: Island of Stability

Use a concept map to review atom-related terms. Concept maps are a way to visually show how the parts of a system relate to one another. In a concept map, nouns are used to describe the components of the system (i.e., the vocabulary term). The relationship between the different components is shown by arrows, which connect the parts. Each noun is put in a box, and the arrows are labeled with a verb describing the relationship between components. Have student pairs find the definitions for the following terms in their textbook (or other resource). As a class, discuss each term. Then, have students create a concept map that shows the relationships among the terms.

atom: The smallest unit of an element that retains the chemical properties of that element.

electrons: Negatively charged particles that orbit the nucleus of an atom.

element: A substance that cannot be broken down into smaller components by chemical reactions. There are 92 naturally occurring elements.

isotope: An atom that has the same atomic number as another atom but that has a different atomic mass.

nuclear forces: The binding forces in the nucleus of atoms that act over short distances and help overcome the protons' repelling forces.

nucleons: Particles that make up the nucleus of an atom. Protons and neutrons are nucleons.

nucleus: The positively charged core of an atom that contains most of the atom's mass and all of its protons and neutrons.

neutrons: Particles in the atomic nucleus without an electrical charge. Protons and neutrons have nearly identical masses.

protons: Particles in the atomic nucleus with a positive charge. The number of protons determines the identity of the element.

Discuss the Periodic Table of the Elements. Have students refer to a copy of the periodic table and answer the following questions:

• How are the elements' atomic numbers used to arrange them on the periodic table? Elements are arranged by a successive increase in atomic number (i.e., the number of protons in the nucleus) as one moves across each row from left to right. This number uniquely identifies each element.

• What is the general relationship between the atomic number and the atomic mass weight? For most elements, the atomic mass is double the atomic number. For elements such as tin, the atomic mass is slightly more than double the atomic number. This is because atomic weight is an average weight of all the isotopes of an element, and an element's isotopes have different numbers of neutrons.

• How might isotopes of an element differ from each other? An element's isotopes have different numbers of neutrons. In addition, some isotopes are more stable than others.

Model the size of an atom. Divide the class into pairs or teams, and provide each one with a coin—a penny, dime, nickel, or quarter. Different teams can have different coins. Tell students that the diameter of the nucleus is about 1/10,000 the diameter of an atom. (Most atoms range in diameter from 1 x 10-10 to 5 x 10-10 meters.) The diameter of an average nucleus is 1 x 10-14 meters, 10,000 times smaller than the diameter of an atom.) Have teams measure the diameter of their coin and calculate the diameter of an atom having a nucleus that size. Give them maps, and have them identify the location of a place this same distance away from school.

Establish that atoms are primarily empty space. Although an atomic nucleus is tiny relative to the size of an atom, it contains almost all the atom's mass—an atom is primarily empty space. To make this point, have students calculate how much a familiar object would weigh if it had no empty space and were made entirely of atomic nuclei. Begin by having students select a common object (e.g., a book, piece of chalk, ball) and calculate its volume in cubic centimeters. (You may have to review how to calculate the volume of basic three-dimensional shapes, such as a cube, sphere, or cylinder.) Have students estimate the object's mass. Remind them that even solid objects are mostly empty space. Then, calculate how much the object would weigh if it were made entirely of hydrogen nuclei (i.e., no empty space). Multiply the volume by 1 x 1015 grams/centimeter3, the density of an average hydrogen nucleus. How much would the object weigh if it were as dense as a hydrogen nucleus? (An object with the volume of a penny would weigh more than 30 million tons. This is about a quadrillion times denser than an object with such volume would normally be.) Students should realize that, for atoms to weigh as little as they do, they must not be consistently dense and must instead be mostly empty space.

Demonstrate that a concentrated mass can occupy a small space. Give students a ball of clay one to two inches in diameter and a small box, such as a shoebox. Have them calculate the density of the empty box, the density of the clay ball, and the density of the box with the ball of clay in it. (Density equals mass divided by volume.) Students will see that the mass of the box and clay is nearly identical to the mass of the clay alone, drawing a strong parallel to the relationship between an atom and its nucleus. Tell students that prior to 1911, scientists believed that the mass of an atom was evenly distributed rather than concentrated at the center. Physicist Ernst Rutherford (1871-1937) passed a beam of radioactive alpha particles (which are extremely tiny atomic particles) through thin gold foil and studied how the foil scattered the particles. He observed that some ricocheted off at an angle and some bounced straight back, like balls hitting a wall. But most passed straight through with little or no deflection. The pattern of the scattered particles suggested that each atom making up the gold foil (and matter in general) is largely empty space with a relatively massive nucleus at its center. Relate the box-clay model to the structure of an atom. (Protons and neutrons make up more than 99% of the mass of an atom. Protons and neutrons are found in the nucleus; electrons, which are considerably smaller and less massive than protons and neutrons, are found far outside the nucleus. So, just like the box, atoms contain large regions of empty space and have their mass—like the clay—concentrated in one place.)