Electrons are involved in bonding, and therefore, their arrangement in an atom is very important.

Subatomic particle
Atoms are composed of protons, neutrons and electrons.  The atomic number is equal to the number of protons.  The charge of the atom or ion is equal to the number of protons – the number of electrons.  The number of electrons can be determined knowing the atomic number and charge.

Energy levels, subshells and orbitals
Electrons are placed in energy levels.  These energy levels are sub-divided into subshells (labeled s, p, d or f).  The s subshell is the lowest energy and begins in level 1.  The p subshell is higher energy and therefore doesn’t begin until level 2.  The d is higher energy and begins in level 3 and the f is even higher energy and begins in level 4.  The subshells are further sub-divided into orbitals (s has 1 orbital, p has 3 orbitals, d has 5 orbitals and f has 7 orbitals).  Each orbital can hold 2 electrons.

Rules for writing electron configurations
The Aufbau principle states that energy levels must be filled from the lowest to the highest and you may not move on to the next level unless the previous level is full.  Use the periodic table as a guide (read left to right):

Hund’s Rule says that when placing electrons in orbitals of equal energy, place one in each orbital before doubling up in order to arrive at the lowest energy configuration.  The Pauli Exclusion Principle states that when electrons do share an orbital, they must be of different “spin.”

Writing electron configurations
The Boxes and Arrows method uses boxes to show orbitals and arrows to signify electrons.  An up arrow and a down arrow have different “spins.”  The spectroscopic method uses superscripts to show the number of electrons in a subshell (specific orbitals are not shown).  The noble gas method uses a noble gas (the far right column) to represent the inner, or core, electrons and just shows the outer level of electrons using the same method as spectroscopic.

Exceptions to the rules
There are a few exceptions to the rules listed above when filling electron configurations.  A half-full “s” orbital and a “d” subshell with 5 or 10 is more stable than following the Aufbau Principle.  Cr, Mo, W: s1 d5 and Cu, Ag, Au: s1 d10

Quantum numbers
Quantum numbers are used to describe the location of an electron.  Quantum numbers are a set of 4 numbers.

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Atoms are composed of 3 subatomic particles: protons, neutrons and electrons.  This tutorial review the basic atomic structure information presented in an earlier tutorial and introduces the concept of placing electrons in energy levels, subshells and orbitals to show an electron configuration.

Specific Tutorial Features:

• The periodic table is used as visual tool to remember orbital orders
• How do these questions look on the AP Exam?

 Series Features:

• Concept map showing inter-connections of new concepts in this tutorial and those previously introduced.
• Definition slides introduce terms as they are needed.
• Visual representation of concepts
• Animated examples—worked out step by step
• A concise summary is given at the conclusion of the tutorial.

• Atomic structure
  • 3 subatomic particles
  • Determining number of protons and electrons in an atom or ion
• Electron configurations
  • Energy levels, subshells and orbitals
  • Rules for writing electron configurations
  • Boxes & Arrows configuration
  • Spectroscopic configuration
  • Noble gas configuration
  • Exceptions to the rules
• Quantum numbers
  • Assigning numbers to an electron
  • Identifying a non-possible quantum number