Valence Electrons | Summary and Q&A

August 26, 2009
Khan Academy
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Valence Electrons


Understanding electron configurations helps determine groupings on the periodic table and how elements react with each other.

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Questions & Answers

Q: How do you determine an element's electron configuration?

An element's electron configuration can be determined by following the periodic table and adding electrons based on the atomic number. Each shell is filled with electrons, starting from the innermost shell and moving outward.

Q: Why does the d-block subshell backfill the previous shell?

As atoms get larger, there are more spaces between the orbitals of the previous shells. Backfilling the d-block allows electrons to occupy these spaces before moving to the next shell, creating a lower energy state.

Q: Why do atoms want to have eight electrons in the outermost shell?

Having eight electrons in the outermost shell, or achieving an electron configuration similar to a noble gas, is the most stable configuration for atoms. This number likely arises from the resonance and lack of repulsion between the electrons.

Q: How do alkali metals and halogens react with each other?

Alkali metals, such as potassium, tend to give away electrons, while halogens, like chlorine, want to gain electrons. When alkali metals and halogens come into contact, the alkali metals readily give away their outermost electron to the halogens, resulting in the formation of ionic compounds.

Summary & Key Takeaways

  • Electron configurations determine the arrangement of electrons in an atom, with each shell and subshell containing a specific number of electrons.

  • Elements can be grouped based on their electron configurations, which can be determined by comparing them to noble gases or by backfilling previous shells in the d-block.

  • The number of electrons in the outermost shell, or valence electrons, determines an element's reactivity and its ability to form bonds with other elements.

  • The outermost shell generally wants to have eight electrons, except for hydrogen and helium, which are stable with just two electrons.

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