Final answer:
To describe electron configurations using noble-gas notation, use the symbol of the preceding noble gas from the periodic table followed by the remaining electron configuration. For sodium, it is [Ne]3s¹, and for phosphorus, it is [Ne]3s²3p³.
Step-by-step explanation:
To describe the electron configurations for the atoms of any element using noble-gas notation, we use the symbol of the last noble gas that appears before the element on the periodic table, followed by the configuration of the remaining electrons.
For example, sodium (Na), with an atomic number 11, has the full electron configuration of 1s²2s²2p⁶3s¹. In noble-gas notation, we use neon (Ne), which is the noble gas before sodium in the periodic table with a configuration of 1s²2s²2p⁶. Therefore, the noble-gas abbreviated configuration for sodium is [Ne]3s¹.
Another example is phosphorus (P), which has 15 electrons. The full configuration is 1s²2s²2p⁶3s²3p³. Phosphorus's noble-gas notation will be [Ne]3s²3p³, where [Ne] represents the first 10 electrons that match the configuration of neon.