Erbium is 68 on the periodic table and you find it in the lanthanide series. For electron configurations you have to work from the first period(row) down to your element. You start with period 1 which would be 1s2. Then down to period 2 you would do 2s2 2p6. The first number represents the period, the letter represents the type of orbital and the second number represents the number of electrons in that orbital. With this logic we can do period 3 as 3s2 3p6. When you get to a transition metal the first number gets subtracted by one and the letter would become d. so for period 4 you would do 4s2 3d10 4p6. period 5 would be 5s2 4d10 5p6. Period 6 would start as usual with 6s2 but then you reach the lanthanides for which you have to subtract the first number by 2 and the letter becomes f. Because Erbium is in this period it would be 6s2 4f12.
The full electron configuration of Erbium is: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f12
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