The valence electrons describes the electrons in the outermost shell of an atom, hence, outer electrons is the same as the valence electrons. The distribution of atoms in the elements given are:
Recall :
1.) Oxygen, O :
Atomic number = 8
Inner electrons = 2
Outer and valence electrons = 8 - 2 = 6
2.) Strontium, Sn :
Atomic number = 38
Inner electrons = 2
Outer and valence electrons = 38 - 2 = 36
3.) Calcium, Ca :
Atomic number = 20
Inner electrons = 18
Outer and valence electrons = 20 - 18 = 2
4.) Iron, Fe :
Atomic number = 26
Inner electrons = 26 - 8
Outer and valence electrons = 8 - 2 = 6
5.) Selenium, Se :
Atomic number = 34
Inner electrons = 34 - 6 = 28
Outer and valence electrons = 34 - 6 = 28
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