Lesson 1 Background Notes

The Chemical Elements, Isotopes, and the Periodic Table

A chemical element is made up of atoms having the same “atomic number” (i.e., number of protons in the nucleus of each atom) and hence having the same chemical properties. An atom is the smallest unit of a chemical element that has all the chemical properties of that particular element.

All the atoms of a particular chemical element react with atoms of another chemical element in the same way. That is, all the isotopes of an element have exactly the same chemical properties as any other isotope of that element. Some examples of chemical elements are hydrogen, oxygen, and carbon.

Two hydrogen atoms react chemically with one oxygen atom to form a molecule of water (H₂O). Similarly, one carbon atom reacts chemically with two oxygen atoms to form a molecule of carbon dioxide (CO₂).

There are 92 naturally occurring chemical elements. (See the alphabetical list “The Chemical Elements and Their Symbols.”)

A chart of the chemical elements called “The Periodic Table” is also included. The Periodic Table is a chart that was originally devised by Dmitri Mendeleev in 1869. The Periodic Table does more than just list the elements. It groups the elements to help scientists understand the different relationships that elements have to one another. Each square in the periodic table gives information about a separate element.

As noted in the footnotes to the Periodic Table and earlier in the text of this unit, the atomic number is equal to the number of protons in the nucleus of an atom of a chemical element. The number of protons in the nucleus of an atom determines the chemical properties of the atom.

Atoms of a chemical element always have the same number of protons in their nuclei. (If they don’t, they’re no longer atoms of the same element.) However, atoms of the same element often have different atomic weights because, although the atomic nuclei have the same number of protons, the number of neutrons may vary. Atoms of an element having the same number of protons but different numbers of neutrons are called isotopes of that element. Any particular isotope may also be called a nuclide, as explained in the reading lesson.

For example, uranium has several isotopes, which all react in the same way chemically, but their nuclear reactions are different. Uranium-238 does not fission readily. (To fission is to split an atom’s nucleus.) For this reason, uranium-235, which fissions more readily, is used for reactor fuel.

Activity Assignment

Study both the alphabetical chart “The Chemical Elements and Their Symbols” and the Periodic Table. They are both provided.Then use them to do the exercises that follow.