Question

Given the following three electron configurations:

1s^2 2s^2 2p^5

How many core electrons does this element have?

A. 4
B. 5
C. 2
D. 9

Answer 1

@JFraser B right?

Answer 2

Core electrons are all electrons of an atom except valance electrons

Answer 3

for example if you take Aluminium
the electronic configuration \[1S ^{2}~~~2S ^{2}2P ^{6}~~~3S ^{2}3P ^{1}\]
\(1S ^{2} \) - 1 means its period 1; \(S ^{2} \) means it passed group 1 and 2 of periodic table
\(2S ^{2}2P ^{6}\), 2 before S and P means period 2; 2 on S means group 1 and 2 are passed, 6 on P means that group 3-8 are passed
\(3S ^{2}3P ^{1}\), 3 means we are on period 3; 2 on S means again group 1 and 2 are passed, 1 on P means that THIS element/atom in group 3
so the element is in period 3 and group 3
which is Aluminium

when you add up all the numbers that are shown as powers of S and P
you will get total number of electrons
but well easier way is : number of electrons = number of protons = atomic number
so if atomic number of Al is 13
which means it has in total 13 electrons
now of this 13 electrons only 3 are in Al's last shell (last shell of Al is 3rd thats why Al is in period 3) - and these 3 are valance electrons / electrons that can participate in reactions/bonds formation
so 13-3 = 10
these 10 are inside, in other words these electrons are in shells starting from nucleus of an atom till last shell - all shells between these (last shell is not included) - have electrons which are called 'core electrons'