Valence electrons are the electrons in the outermost occupied shell of an atom; they determine the element's chemical properties and its group in the Periodic Table.
Valence electrons are the electrons in the outermost occupied shell of an atom; they determine the element’s chemical properties and its group in the Periodic Table. Malay: elektron valens · Chinese: 价电子.
The valence electrons are simply the last number in the electron arrangement. They matter because chemical reactions involve the outer electrons gaining, losing or sharing to reach a stable full outer shell, so two elements with the same number of valence electrons tend to react in similar ways.
Example. Sodium has the arrangement 2.8.1, so it has 1 valence electron and belongs to Group 1; it readily loses that electron to form Na⁺. Chlorine, 2.8.7, has 7 valence electrons and is in Group 17; it readily gains one electron to form Cl⁻. Neon, 2.8, already has a full outer shell of 8 valence electrons, which is why it is an unreactive noble gas in Group 18.
Confusion to avoid. Do not confuse valence electrons with the total number of electrons, and do not confuse them with the electron arrangement as a whole. Only the electrons in the outermost occupied shell are valence electrons; the inner electrons are not counted. Take care, too, not to mix the number of valence electrons (which gives the group) with the number of occupied shells (which gives the period).
Atoms are most stable when their outermost shell is full, usually eight valence electrons, or two for the first shell, and this drive towards a stable arrangement is what makes reactions happen. Elements with one, two or three valence electrons tend to lose them to form positive ions, those with five, six or seven tend to gain electrons to form negative ions, and the noble gases with full shells barely react at all.
For the first twenty elements, counting the valence electrons is how you place an element in its group, and it is the first step towards understanding why metals and non-metals bond as they do. In the Matter and the Atomic Structure chapter this is the idea that carries straight into the Periodic Table and Chemical Bonding, so it repays being learned securely.
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