6.2. Electron Configuration and the Periodic Table http://www.ck12.org
the combined number of electrons in thesanddsublevels. Zirconium is in Period 5 and Group 4. Recall that there
are several deviations from the expected order of filling thedsublevel that cannot always be easily understood. The
element cobalt (Co) is in Period 4 and Group 9. It has the expected electron configuration of [Ar]3d^74 s^2. Directly
below cobalt is the element rhodium (Rh). However, its configuration is [Kr]4d^85 s^1 , meaning that one of its 5s
electrons has moved to the 4dsublevel. The total of nine electrons still allows you to predict that rhodium is a
member of Group 9.
Because electrons in thedsublevel do not belong to the outermost principal energy level, they are not valence
electrons. Mostd-block elements have two valence electrons, which are the two electrons from the outermosts
sublevel. Rhodium is an example of a transition metal with only one valence electron, because its configuration
deviates from the expected filling order.
The
The first of thefsublevels is the 4fsublevel. It fills after the 6ssublevel, meaning thatfsublevels are two principal
energy levels behind. The general electron configuration for elements in thef-block is (n- 2)f^1 −^14 ns^2. The seven
orbitals of thefsublevel can each accommodate two electrons, so thef-block is 14 elements in length. It is usually
shown pulled out of the main body of the periodic table and is placed at the very bottom. Because of that, the
elements of thef-block do not belong to any of the numbered groups; they are wedged in between Groups 3 and
- Thelanthanidesare the 14 elements from cerium (atomic number 58) to lutetium (atomic number 71). Most
lanthanides have a partially filled 4f sublevel. They are all metals and are similar in reactivity to the Group 2
alkaline earth metals.
Theactinidesare the 14 elements from thorium (atomic number 90) to lawrencium (atomic number 103). Most
actinides have a partially filled 5fsublevel. The actinides are all radioactive elements, and only the first four have
been found to occur naturally on Earth. All of the others have only been artificially made in the laboratory. The
lanthanides and actinides together are sometimes called theinner transition elements.
Sample Problem 6.1: Electron Configurations and the Periodic Table
The electron configurations for atoms of four different elements are shown below. Without consulting the periodic
table, name the period, group, and block in which each element is located. Determine the number of valence
electrons for each. Then, using a periodic table, name the element and identify it as a metal, nonmetal, or metalloid.
- [Kr]4d^105 s^25 p^3
- [Rn]5f^77 s^2
- [Ar]4s^2
- [Xe]4f^145 d^66 s^2
Step 1: Plan the problem.
The period is the highest occupied principal energy level. The group is the vertical column. The block depends on
which sublevel is in the process of being filled. The valence electrons are those in the outermost principal energy
level. For name and type of element, use a periodic table.
Step 2: Solutions.
- The highest occupied principal energy level is the fifth, so this element is in Period 5. By adding 10 + 2 +
3 from the given configuration, we can see that the element is in Group 15. Since thepsublevel is partially
filled, it is in thep-block. There are five electrons in the outermost energy level, so it has 5 valence electrons.
The element is antimony, a metalloid. - The element is in Period 7. Since thefsublevel is partially filled, it is part of thef-block, which means it does
not belong to a group. It has 2 valence electrons. The element is americium, a metal from the actinides. - The element is in Period 4 and Group 2. Even though the 4ssublevel is filled, the last electron went into that
sublevel, making it a member of thes-block. It has 2 valence electrons. The element is calcium, a metal.