4.2: Avogadro's Number: Introduction and Indicator Words

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Learning Objectives

State the value of Avogadro's number.
Identify the indicator words associated with Avogadro's number.

On a macro, or large, scale, objects are most often quantified by counting them, because each individual object is large enough to view with the human eye. Eggs are typically sold in sets of twelve. A package of printer paper contains 500 pieces of paper. Furthermore, when an object is present in standardized amounts, additional words, such as "dozen" or "ream," are used to refer to a larger collection of that item. Unfortunately, individual chemical "objects," or particles, are too small to see and, therefore, cannot be easily counted. As a result, chemists are unable to measure single particles and instead must quantify a large number of particles using a single measurement.

Avogadro's Number

As stated in Chapter 2, an atom is defined as the smallest unit of an element that still has the properties of that element. Therefore, an atom is a fundamental chemical particle. In Chapter 3, covalent molecules were generated by pairing the unpaired valence electrons of neutral atoms. Alternatively, atoms could gain or lose valence electrons in order to form ions, which then interacted in specific ratios to form ionic compounds. As a result, compounds, which will now be singularly-categorized with molecules, and ions are also classified as fundamental chemical particles.

As stated above, counting individual atoms, ions, or molecules that are present within a substance is not practical, as these particles cannot be observed with the human eye. Furthermore, in order to be accurately measured, a very large number of these very small particles must be present. Specifically, chemists determined that 6.02214076 × 10²³ particles should be contained in a typical chemical measurement. This quantity is named after Lorenzo Romano Amedeo Carlo Avogadro, the 19^th-century Italian scientist who first proposed its value. Due to the extremely large magnitude of this quantity, representing Avogadro's number in standard decimal format is highly impractical. Instead, this value is expressed using proper scientific notation, which is a format that is specifically intended to eliminate the tedium of writing numbers of very large, or very small, magnitudes. Finally, the coefficient of Avogadro's number is defined to nine significant figures. However, because the tools that are available to most scientists are unable to take measurements to such a high degree of accuracy, a rounded value of 6.02 × 10²³ is more commonly used. Finally, the highly-accurate nature of Avogadro's number makes its value a chemically-significant quantity and, therefore, a molar standard.

Avogadro's Number Indicator Words

Avogadro's number is defined based on the number of particles that should be present in a typical chemical measurement. Therefore, the terms for the specific types of fundamental chemical particles, namely "atoms," "ions," and "molecules," are the indicator words that are associated with applying this numerical value in a problem-solving context.

For example, consider a problem that would require a calculation of how many ions of Ca⁺² are present in 9.74 moles of Ca⁺². The word "ions" indicates that an Avogadro's number equality should be developed and applied to solve this problem.