Reviewing Noble Gas Configurations

When writing electron configurations, sometimes it is beneficial to have a shorthand method. Some electron configurations can get quite long. The outermost electrons, also known as the valence electrons, are the ones that are the most important to chemists, since they are the electrons involved in chemical reactions. A condensed electron configuration shows the outermost electrons. Here is how it works:

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Take the element lithium, Li. Lithium has 3 electrons. The electron configuration is: 1s²2s¹

To write the condensed configuration, find the noble gas that occurs on the periodic table just before the atom. In this case, helium is the noble gas that is just before lithium. In brackets, place the noble gas symbol: [He]. This noble gas in brackets represents all of the electrons that are not in the outer shell, the core electrons. In the case of lithium, [He] represents the 1s² electrons. After the noble gas, continue on with the valence electrons. The condensed electron configuration, also known as the noble gas configuration, for lithium is: [He]2s¹

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Let’s try another one.

Silicon (Si) has 14 electrons. Using the diagonal rule, the electron configuration for silicon looks like this: 1s²2s²2p⁶3s²3p²

To write the noble gas configuration, find the noble gas that is just before silicon on the periodic table. This would be neon. Notice that argon is closer to silicon than neon. But, we must go back to find the closest noble gas. The noble gas must have few electrons than the element we are working with. Write neon in brackets: [Ne]. This takes care of the first 10 electrons (the atomic number of neon). The remaining 4 electrons must be shown as normal.

The noble gas configuration for silicon is: [Ne]3s²3p²

You try one. What is the noble gas configuration for strontium (Sr)?  (Highlight the blank areas to reveal the answers)

electron configuration: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²

noble gas configuration: [Kr]5s²

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