sublevel, with 5 orbitals (ml = –2, –1, 0, 1, 2), is capable of holding a maximum of 5 × 2 or 10
electrons.6 . C
An element with an atomic number of 26 will have 6 electrons in its 3d subshell. This can be
determined by writing the electron configuration for the element, 1s^2 2 s^2 2 p^6 3 s^2 3 p^6 3 d^6 4 s^2.
The number of electrons must equal 26; recall that the 4s subshell must be filled before the 3d
because it has the lower energy. Thus, 3d will carry 6 electrons.
7 . A
The difference between the first and second electron configurations is that in the second
configuration, one electron has moved from the 3p subshell to the 4s subshell. Although the
3 p and 4s subshells have the same (n + l) value, the 3p subshell fills first because it is slightly
lower in energy. In order for an electron to move from the 3p subshell to the 4s subshell, it
must absorb energy.
8 . A
In order to determine which subshell has the lowest energy, the (n + l) rule must be used. The
values of the first and second quantum numbers are added together, and the subshell with the
lowest (n + l) value has the lowest energy. The sums of the five choices are (2 + 1) = 3, (3 + 0) = 3,
(3 + 2) = 5, (4 + 0) = 4, (3 + 1) = 4. Choices A and B have the same (n + l) value, so the subshell
with the lower principal quantum number has the lower energy. This is the 2p subshell, choice
A.
9 . C
Scandium has 21 electrons. When it is in its excited state, one or more of the electrons will be
present in a subshell with a higher energy than the one in which it is usually located. The
number of electrons and ordering of subshells will not vary from the ground state electron
configuration of scandium. Choice C has one of the 4s electrons present in the 3d orbital. This
represents an excited state because energy is required to cause an electron to jump from 4s to
3 d. Note that choice B is not physically possible because the 2s orbital cannot contain three
electrons.