This particular configuration denotes a particle with ten total electrons. The sodium atom, with eleven electrons, is the only one listed that could not have this configuration. Ionized sodium, however, symbolized as Na+, does apply. (Be careful to distinguish neutral atoms and ions).

Potassium (K) is orignially in the electron configuration of \[Ar\]4s1. To obtain a +1 charge it loses an electron, resulting in a configuration of \[Ar\].

When an element loses an electron it is generally taken away from the highest electron shell. The electron configuration of iron (Fe) is . The 4s orbital is farther away from the nucleus than the 3d orbital, therefore the electron configuration of Fe+ will be .

When determining electronic configuration, the answer is made much easier by starting with the next smallest noble gas in brackets. As a result, \[Ar\] is an appropriate way to incorporate every previous electron before argon.

After argon, vanadium has five other electrons to distribute, and because vanadium is a transitional element, it will fill its 3d subshells before filling the 4p subshells. The 4s subshell is filled first, and the last three electrons are placed into the 3d subshells.

\[Ar\]3d34s2

Nuclear orbitals will always fill from the innermost to the outermost subshells. Using the rule, we can approximate the order in which these orbitals will fill. Because electrons fill starting with the centermost orbitals, the electron that is closest to the nucleus will belong to the orbital that fills first.

corresponds to the principle quantum number, the first number in the given orbital location. is the azimuthal quantum number, and dictates the shape of the orbital.

5s and 3d produce the same number from the equation, but in the event of a tie we always pick the orbital with the lowest letter (s < p < d < f). The innermost orbital will be 5s.

Due to the phenomenon of half-orbital stability in the transition metals, electrons can easily move between 4s and 3d orbitals. The atom achieves greater stability from having only one atom in the 4s orbital, allowing a half-filled 3d orbital, as opposed to a full 4s orbital and four electrons in the 3d subshell.

For elements like chromium and copper, which could have valence shell configurations of 4s23d4 and _4s_23d9, respectively, an electron from the 4s orbital jumps down to the 3d orbital to harness added stability from the half-filled orbital. The given electron configuration is that of chromium.

Note that you can also solve this question by counting the electrons to determine the atomic number. In this case, the electrons add up to 24, indicating the twenty-fourth element: chromium.

Valence electrons will be housed in the outer shell (highest numbered orbital) of the electron configuration.

Calcium:

Magnesium:

Zinc:

Copper:

Arsenic:

Calcium, magnesium, and zinc all have two valence electrons in their highest energy orbital. Copper has only one valence electron. Arsenic has five total valence electrons in the fourth shell.

Were this question asking for the electronic structure of magnesium (Mg) in its ground state, would be the correct answer; however, the charge on means that the molecule shed two valence electrons to achieve a more stable orbital. Those electrons will be shed from the outermost valence shell, which in this case is the shell; therefore, is correct.

Note that the ground state of magnesium will have twelve electrons (the same as its atomic number), while the ion will have ten.

Cobalt is a transition metal; therefore, it is found in the D block of the periodic table. A ground state cobalt atom has an electron configuration of . The last orbitals that gain or lose electrons must be either the or orbitals, since these are the orbitals with highest energy and located farthest from the nucleus.

Recall that electrons are filled from orbitals of low energy to high energy. A orbital has a lower energy than a orbital. This means that when you are filling electrons, the last orbital you fill is the orbital. When you are assigning electrons to each orbital you assign two electrons to the orbital and then the remaining seven electrons to the five orbitals; therefore, orbitals are filled last when gaining electrons.

When an element loses electrons, the first orbital that loses electrons is the outermost orbital. This occurs because the attractive force of the nucleus on the electron will be weakest in the outermost orbital (because it is farthest away from nucleus); therefore, it will be easy to pull the electron away from the nucleus. In cobalt, the outermost orbital is the orbital (because it has the highest shell number). This means that electrons will be lost from the orbital before the orbital.

Notice that the orbital that gains electron last is not the same orbital that loses electrons first. Gaining electrons is dependent on the energy of the orbital, and losing electrons is dependent on the location of the orbital. The highest energy orbital will gain electron last and the outermost orbital will lose electron first.

A neutral atom of sodium would contain eleven electrons to balance out the charge of the eleven protons in the nucleus. We are asked, however, for the configuration of a sodium ion. Sodium is an alkali metal, meaning that it will ionize by losing only one electron, gaining a charge of . By losing one electron, sodium drops from having eleven electrons to ten. We will need to select the answer that shows an electron removed form the outermost shell.

Neutral sodium:

Sodium ion: