Which Definition of Element is Correct?

According to Roundy (1989), chemistry teachers confuse students by using outdated definitions of element. Below are three examples taken from textbooks:

An element is a substance that cannot be broken down into anything simpler (Heyworth, 1999).
An element is a substance which cannot be broken down into anything simpler by chemical methods (Chung, 1997).
An element is a pure substance which cannot be broken down into anything simpler by chemical methods (Cheng & Chow, 1999).

Most elements consist of a mixture of isotopes and so are not pure substances. Isotopes of an element can be separated by physical methods such as diffusion and mass spectrometry. Also, the isotopes of some elements can be separated by chemical methods such as electrolysis. Each isotope of an element is a pure substance.

What is the modern definition of an element? An element consists of atoms, all of which have the same number of protons in their nuclei. It is important to note that an element is identified by its number of protons (i.e., atomic number), whether it exists as an atom or a compound. For example, all the following substances contain the element oxygen:

MgO, H2O, O2, O3, CH3OH

The symbol "O" is used to represent the element oxygen in each of the above formulae because there are eight protons in the nucleus. We do not have to bother about the number of electrons around the nucleus of O2- or O3.

To facilitate student learning of the modern definition of element, teachers may need to modify the sequence of topics presented in most textbooks. Do not start with the topic "elements". One possible teaching sequence is shown below:


Cheng, E. & Chow, J. (1999). Chemistry: A modern view (4th ed.). Wilson Welsh.
Chung, L. H. M. (1997). Integrated chemistry today (2nd ed.). Jing Kung.
Heyworth, R. (1999). Chem 2000 (2nd ed.). Macmillan.
Roundy, W. H. (1989). What is an element? Journal of Chemical Education, vol. 66(9), pp. 729-730.