PHS 100 - Self Assessment

Competency #6

Answer and Explanation

In general, the mass of atoms increases as you go to the right in a period (row) or down a group (column).

As you go across a row to the right or down a group, the mass number increases. This is because both the number of protons and the number of neutrons is increasing. For example, the most common isotope of sodium (Na) contains 11 protons and 12 neutrons, while the most common isotope of potassium (K) contains 19 protons and 20 neutrons and the most common isotope of calcium (Ca) contains 20 protons and 20 neutrons.