What do the numbers mean on the periodic table?

Question

Accepted Answer

The numbers on the periodic table mean different things depending on where they sit. The whole number in each tile, usually at the top, is the atomic number: the count of protons in the nucleus, which defines the element and sets its order. The decimal number is the atomic mass (relative atomic mass), the weighted average mass of the element's isotopes in atomic mass units; for radioactive elements without a stable isotope it is shown in brackets as the mass number of the longest-lived isotope, like [98] for technetium. Numbers 1 to 18 across the top are group numbers, and numbers 1 to 7 down the side are period numbers. Some tables add small numbers for electron configuration, oxidation states or electronegativity. Rounding the atomic mass to a whole number gives the mass number, and subtracting the atomic number from that gives the neutron count. Some numbers need extra care. The atomic number is always a whole number and never changes for an element, while the atomic mass changes slightly between data sources because isotope abundances vary and measurements improve. Two numbers on the same tile can be confused: for carbon, 6 is the atomic number and 12.011 is the atomic mass, so carbon-12 has 6 protons and 6 neutrons while carbon-14 has 6 protons and 8 neutrons. The atomic number also gives the number of electrons in a neutral atom, which is how you start drawing a Bohr model or writing an electron configuration. Small numbers in a corner of some tiles give oxidation states, such as +1 for sodium or −2 for oxygen, and numbers in an electron configuration are the shell, subshell and electron count, as in 1s 2 2s 2 2p 2 for carbon. Whenever a table has a legend, check it first, because tables differ in which numbers they show. Use the atomic number to find an element, and read the group number and colors to understand its family.