In this activity, you can use the Atomic Explorer to investigate the energy of electrons in different atoms and the order in which electrons fill available orbitals.

Electrons can occupy different orbitals in an atom, and the order of occupancy is based on the most stable arrangement. By investigating the order in which electrons occupy the available orbitals, you will understand the characteristics of electrons and orbitals that contribute to the formation of stable atoms.

In this activity, you will investigate the rules that determine electron occupancy in available orbitals. First, you will determine the order in which electrons fill available orbitals. Second, you will identify any patterns that exist in the order in which the orbitals are filled. Finally, you will determine how the electron structure of an atom determines the placement of each element on the periodic table.

• atomic structure
• valence electrons
• electron orbital
• electron density
• s orbital
• p orbital

1. Open the Atomic Explorer application, which is set to display the atomic orbital of hydrogen.
2. In the electron orbital diagram, click on the lowest energy orbital for hydrogen. Answer the following questions:
   1. How many valence electrons are in this atom?
   2. What is the energy of the ground state electron(s) in this atom?
3. From the Edit menu, select Set Atom and select helium from the menu of elements. Note the number valence electrons in helium and the ground state orbital(s) that they occupy. Answer the questions of Step 2 above for the helium atom.
4. Complete the following table for the valence electrons in atoms of first period elements:
   
   

   Element	H	He
   # electrons in s orbitals
   Energy of s orbital electrons

   
   
5. Examine the elements of the second period. For atoms with more than one occupied orbital, select one orbital at a time and note the energy of each individual orbital.
6. Complete the following table for the valence electrons in atoms of second period elements:
   
   

   Element	Li	Be	B	C	N	O	F	Ne
   # electrons in s orbitals
   # electrons in p orbitals
   Energy of s orbital electrons
   Energy of p orbital electrons

   
   
7. Examine the elements of the third period. For atoms with more than one occupied orbital, select one orbital at a time and note the energy of each individual orbital.
8. Complete the following table for the valence electrons in atoms of third period elements:
   
   

   Element	Na	Mg	Al	Si	P	S	Cl	Ar
   # electrons in s orbitals
   # electrons in p orbitals
   Energy of s orbital electrons
   Energy of p orbital electrons

   
   

1. Compare the energy of valence electrons of the same atom in the following pairs of orbitals:
   1. an s orbital and a p orbital
   2. two p orbitals
2. By comparing electron orbital occupancy for elements in the second period, describe the order in which electrons fill the available orbitals.
   1. What is the relationship between the energy of the orbitals and the order in which they are filled?
   2. Propose a rule for the order in which p orbitals in the same energy level are filled.
3. Compare the number of valence electrons for each element of the same group of the Periodic Table.
   1. Describe the trend in the number of electrons going down a group.
   2. What is the relationship between the number of valence electrons in an atom and the group number of that element?
4. Examine the number of available orbitals for a He atom and the number of its valence electrons. Explain why helium is place in Group VIIIA of the Periodic Table rather than Group IIA.
5. Based on your observations, write a summary of how the arrangement of atoms on the Periodic Table is associated with the valence electrons of the atoms of each element.