How many inner, outer, and valence electrons does an atom of each of the following elements have? (a) Br (b) Cs (c) Cr (d) Sr (e) F
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Answer: (a) Bromine (Br): Bromine has an atomic number of 35, which means it has 35 electrons. These electrons are distributed in the following shell structure, 2-8-18-7. Out of these, the first three shells (2+8+18) are considered inner electrons, amounting to 28 inner electrons. The outermost shell has 7 electrons, and these are also the valence electrons.
(b) Cesium (Cs): Cesium has an atomic number of 55. Its electron configuration ends in the 6s orbital, with electrons arranged as 2-8-18-18-8-1. The inner electrons are all the electrons in completed energy levels, so that's 2+8+18+18+8 = 54. The outer electrons are the ones in the outermost shell; for cesium, that's the 6s orbital with 1 electron. This is also the number of valence electrons, which is 1.
(c) Chromium (Cr): Chromium has an atomic number of 24. The electron configuration of chromium is a bit unusual due to the stability provided by half-filled or completely filled subshells. The electron configuration is 3d^5 4s^1 instead of the expected 3d^4 4s^2, with electrons arranged as 2-8-13-1. Thus, it has 2+8 = 10 inner electrons. The outermost shell has 14 electrons (13 in the 3d and 1 in the 4s), but for counting valence electrons, we usually look at the highest principal quantum number, which here is n=4, so Cr technically has 1 valence electron.
(d) Strontium (Sr): Strontium has an atomic number of 38. The electron configuration is 2-8-18-8-2. Hence, the inner electrons are 2+8+18+8 = 36. The outermost shell has 2 electrons, and these are also considered the valence electrons.
(e) Fluorine (F): Fluorine has an atomic number of 9. Its electron configuration is 2-7. Therefore, it has 2 inner electrons (in the first shell), and the outermost shell also has 7 electrons, which are the valence electrons.