The 1920s marked a profound shift in the way scientists understood the atom. Thanks to scientists such as Louis de Broglie, Erwin Schrodinger, and Wolfgang Pauli, the quantum mechanical model of the atom gained acceptance.

The quantum model states that electrons do not orbit randomly around the nucleus of an atom. Instead, electrons move in specific electron shells, which can be divided into subshells. Each subshell in turn consists of one or more regions of space called orbitals. The way electrons fill the shells, subshells, and orbitals is called the electron configuration of the element.

While this model may seem complex, it helped explain chemical properties that had been discovered many years before. The entire periodic table makes much more sense when viewed in the light of electron configurations.

Electron Configurations

In this activity, you will determine the electron configurations of elements in the first three rows of the periodic table.

  1. On the PERIODIC TABLE tab of the Gizmo™, click on the box for the element Lithium (Li).
    1. What is Lithium's atomic number? The atomic number identifies the element and indicates the number of protons in an atom of that element. For neutral atoms, there are equal number of positively-charged protons and negatively-charged electrons. How many protons and electrons are found in an atom of Lithium?
    2. Click on the ELECTRON CONFIGURATION tab, and select Show number of electrons. Find the label 1s at the top. The number 1 identifies the first electron shell, and the letter s identifies the subshell. The box represents an orbital, or the region within a shell where the electron is likely to be found. How many orbitals are found in each of the s subshells? How many orbitals are found in the p, d and f subshells?
    3. Click once in the 1s box. The arrow that appears represents an electron. The direction the arrow points indicates the "spin" of the electron (clockwise spins are represented by an "up" arrow, while counterclockwise spins are represented by a "down" arrow). What direction is the first electron's spin?
    4. Click again in the 1s box. What direction is the second electron's spin? Note that two electrons in the same orbital cannot have identical spins.
    5. Click again in the 1s box. Notice that the box clears. (No more electrons can be added to the orbital.) From this observation, what do you think is the maximum number of electrons that can fit in an orbital? You have just demonstrated the Pauli exclusion principle, which states that an orbital can hold no more than 2 electrons.
    6. The Aufbau principle states that electrons occupy the orbital with the lowest energy available. In general, lower-energy subshells are marked with the lower numbers. After 1s, the next subshell to be filled is 2s. Add a third electron to complete the electron configuration of Lithium (recall that neutral Lithium has 3 electrons). Then click the Check button. (If your configuration is not correct, keep trying!)
  2. Now that you know that each orbital (in the Gizmo, each small box) can hold up to 2 electrons, answer the questions below.
    1. What is the maximum number of electrons that can fit in an s subshell? A p subshell? A d subshell? An f subshell?
    2. What is the maximum number of electrons that can fit in the 2nd electron shell (2s and 2p combined)? In the 3rd electron shell (3s, 3p, and 3d combined)? In the 4th electron shell?
  3. Click Next element to advance from Lithium to Beryllium (Be), the next element the periodic table. Beryllium has one more electron (and proton) than Lithium. Complete the electron configuration by adding a new electron and click Check. Then click Next element and do the same for Boron (B). (Hint: after 2s, the next subshell to be filled is 2p.)
  4. Click Next element to show Carbon (C).
    1. Hund's rule states that electrons will occupy an empty orbital within an electron subshell if possible. So, adding an additional electron to the first 2p orbital is actually incorrect. The additional electron for Carbon must be placed in the second 2p orbital. Once all orbitals are half full, electrons can again be paired, filling the orbitals. When you have finished the configuration of Carbon, click Check.
    2. According to Hund's rule, where will the next electron be placed for Nitrogen, the next element in the period? Use the Gizmo to check your work.
    3. Complete the electron configurations for the remaining elements in the period (Oxygen, Fluorine, and Neon). Remember to check your work as you complete each element.
  5. Complete the electron configurations for the next row of the periodic table (Sodium through Argon). Click Check after each completed element to check your progress. How are these electron configurations similar to the Lithium-through-Neon configurations?

Atomic Radii

In this activity, you will explore trends in atomic radii throughout the periodic table. (Note: There are several valid ways to describe atomic radii. The values used in this Gizmo are calculated values and were obtained from WebElements.com. The calculated radii of some rare elements are not available.)

  1. Select the PERIODIC TABLE tab. As you move from left to right across a period, or row of the periodic table, each element has an additional proton and electron.
    1. Based on this fact, do you think atoms will get larger or smaller as you move across a period from left to right? Explain your reasoning.
    2. Click the ATOMIC RADIUS tab to test your hypothesis. The graph shows the atomic radii of the elements you have done so far. Within a period, does the radius of atoms increase or decrease as you move from left to right?
    3. As positively-charged protons are added to the nucleus, the nuclear charge increases. Thus, the negatively-charged electrons are pulled toward the nucleus by a greater force. How does this explain the trend in atomic radii you just observed?
    4. Based on this trend, which side of the periodic table contains elements with the largest radii? Smallest?
  2. Click Clear, and then select the PERIODIC TABLE tab. Select Hydrogen (H), return to the ELECTRON CONFIGURATION pane to complete its electron configuration, and click Check. Then, move down the first column in the periodic table and complete the electron configurations for Lithium (Li), Sodium (Na), and Potassium (K). (Hint: the outermost electron for Potassium goes in the 4s subshell, not 3d.) Remember to click Check after completing each element.
    1. Compare the electron configurations of the elements in this column. What do these electron configurations have in common? Can you guess why columns of elements are also called families? Discuss your answer with your classmates.
    2. When electrons are added to higher electron shells, electrons are added farther from the nucleus. The outermost electrons also experience a shielding effect from the electrons in the inner orbitals, reducing the effect of the increased nuclear charge. What result will these conditions have on the radius of the atom?
    3. Select the ATOMIC RADIUS tab. What is the radius trend in this family? Explain your results.
    4. Do elements on the top or the bottom of the periodic table have the largest radii?
  3. Based on what you have learned, which atom in the periodic table do you think has the largest radius? Why? If possible, discuss your answer with your classmates and teacher.

Extension Activity: Using the Diagonal Rule

So far, you have completed electron configurations for some of the simpler elements. In this activity, you will determine the electron configuration of any element.

  1. The subshells are filled in order of increasing energy. Beyond the 3p subshell, the order becomes somewhat counter-intuitive, as the 4s subshell is filled before the 3d subshell. The trick for remembering the order is known as the diagonal rule, shown below.

    Diagonal rule
    1. In the diagram, the order is indicated by arrows. For example, the 4s subshell is filled after 3p. What subshell is filled after 3d?
    2. When you get to the end of a set of arrows, proceed to the top of the next set to the right. For example, 3d is filled after 4s. What subshell is filled after 5s? 6s?
    3. Use the diagonal rule to find the electron configurations of several larger elements. As you proceed, you may notice that there are exceptions to this rule. (In these cases, the actual configuration is provided by the Gizmo when a "theoretically correct" configuration is submitted.)
  2. Click the PERIODIC TABLE tab and observe the unique shape of the table. Compare the areas of the table to the order of subshells filled using the diagonal rule.
    1. How many elements are there from Lithium to Beryllium (Li to Be)? How many electrons are needed to fill the s subshell?
    2. How many elements are there from Boron to Neon (B to Ne)? How many electrons are needed to fill the p subshell?
    3. What subshell is represented by the elements Scandium to Zinc (Sc to Zn)? How do you know?
    4. What subshell is represented by the elements Cerium to Lutetium (Ce to Lu)? Explain your reasoning.
    5. When Dmitri Mendeleev published the first periodic table in 1869, nothing was known of electrons. Mendeleev grouped the elements in columns based on their similar chemical properties. How is the shape of Mendeleev's periodic table related to the electron configurations predicted in the quantum model? If possible, discuss your answer with your classmates and teacher.