Chapter 4.3 Distinguishing Among Atoms

Atomic Number Different number of protons in the nucleus is what defines different atoms. Atomic Number—the number of protons in the nucleus of an atom of that element. For atoms (which are electrically neutral), the number of protons is equal to the number of electrons.

Mass Number The total number of protons and neutrons in the nucleus of an atom is called the mass number. The number of neutrons in an atom can be calculated if you know the atomic number and the mass number. Number of neutrons = mass number minus atomic number.

Shorthand notation uses: Superscript—mass number Subscript—atomic number Both to the left of the chemical symbol. Also may be written as the chemical name followed by the mass number. Example: Gold-197

Isotopes Atoms of an element with different numbers of neutrons are known as isotopes. Because they have different number of neutrons, they also have different mass numbers.

Atomic Mass Using grams to indicate masses of individual atoms results in numbers that are inconveniently small. Instead, the carbon-12 isotope is used as a reference standard. An atomic mass unit (amu) is defined as one twelfth the mass of a carbon-12 atom.

The masses of the electrons are insignificant when calculating atomic mass. The atomic mass of an element is usually not a whole number. Because of an element’s isotopes, the element’s atomic mass is a weighted average of the relative abundance of the isotopes as they occur in nature.

To calculate atomic mass, multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.