The arrangement of electrons (electron configuration) determines the chemical properties of an atom. In particular, the valence electrons in the outermost shell control reactivity.
Basic Knowledge
Electrons are arranged in layers called electron shells around the nucleus. They are named the K shell, L shell, M shell, N shell, and so on from the inside outward. The maximum number of electrons each can hold is as follows:
・K shell: maximum 2 electrons
・L shell: maximum 8 electrons
・M shell: maximum 18 electrons (in basic chemistry, in principle, only up to 8 are considered)
・N shell: maximum 32 electrons (same as above)
Electrons fill the inner shells with lower energy first (K→L→M→N). The outermost shell is called the valence shell, and the electrons in it are called outer-shell electrons (valence electrons).
The number of valence electrons is important because it is directly involved in chemical reactions. Elements with the same number of valence electrons have similar chemical properties (elements in the same group). A key point is that noble gases are treated as having 0 valence electrons (their outermost shells are filled, so they are not very reactive).
Electron shell (a layer of electrons extending outward as K, L, M, N, and so on)
Electron configuration (the number of electrons in each shell; example: Na = K2 L8 M1)
Outer-shell electron (an electron in the outermost shell; determines reactivity)
Valence electron (an outer-shell electron involved in reactions; noble gases are treated as having 0)
Closed-shell structure (an electron shell is full; He (K=2) and Ne (K=2,L=8))
Octet rule (stability achieved with 8 electrons in the outermost shell, as in Ne)
Further Study (Background and Significance)
Remember the electron configurations of the main elements.
・H: K=1 (1 valence electron)
・He: K=2 (closed shell, 0 valence electrons)
・Li: K=2, L=1 (1 valence electron)
・C: K=2, L=4 (4 valence electrons)
・N: K=2, L=5 (5 valence electrons)
・O: K=2, L=6 (6 valence electrons)
・Ne: K=2, L=8 (closed shell, 0 valence electrons)
・Na: K=2, L=8, M=1 (1 valence electron)
・Cl: K=2, L=8, M=7 (7 valence electrons)
・Ar: K=2, L=8, M=8 (treated as a closed shell, 0 valence electrons)
Atoms exchange electrons to achieve the same electron configuration as a noble gas (the octet rule). Na gives up its one electron in the M shell to achieve a Ne-type configuration, while Cl gains one more electron in its M shell, which already contains seven, to achieve an Ar-type configuration. This is the principle by which ions are formed.
- Electron shells = K (2), L (8), M (18), and N (32)
- Electrons fill shells from the inside outward
- Valence electrons = outer-shell electrons (0 for noble gases)
- Elements in the same group have the same number of valence electrons and similar properties
- Closed shell = the outermost shell is full and stable
- Octet rule = stability with 8 electrons in the outermost shell
- Atoms form ions in order to achieve a noble-gas-type configuration
Points to Note (Easily Confused)
① Outer-shell electrons and valence electrons are almost the same, but note that noble gases are treated as having 0 valence electrons (Ne has 8 outer-shell electrons but 0 valence electrons). ② In basic chemistry, there is no need to study in depth how transition elements fill the M shell with up to 18 electrons. ③ Closed shell (a full shell) and octet (8 outermost-shell electrons) are similar concepts, but He has a closed shell with only 2 electrons. ④ Elements with many valence electrons tend to become anions (F, Cl), while elements with few valence electrons tend to become cations (Na, K).
Practice
- Express the electron configuration of Na (atomic number 11) using K, L, and M.
- What are the chemical properties of elements with the same number of valence electrons like?
- How many valence electrons are noble gases treated as having?