Orbital Diagrams and Electron Configuration - Basic Introduction - Chemistry Practice Problems

Main Ideas

• Orbital diagrams represent electrons as arrows in boxes or lines, with each box or line representing an orbital. Opposite arrow directions indicate opposite electron spins.
• Electrons fill orbitals in order of increasing energy, occupying lower-energy orbitals before higher-energy ones. An electron configuration can guide the diagram, or electrons can be counted directly into the orbitals.
• Orbital capacities and filling rules:
  • Each orbital can hold up to two electrons, which must have opposite spins.
  • The three orbitals in a p sublevel have the same energy; they are called degenerate orbitals. Place one electron in each orbital before pairing them. Unpaired electrons in these orbitals have parallel spins.
  • A d sublevel contains five orbitals.
• Magnetism: A substance with unpaired electrons is paramagnetic and weakly attracted to an external magnetic field. A substance with only paired electrons is diamagnetic and weakly repelled.

How to Draw an Orbital Diagram

1. Determine the number of electrons:

   • For a neutral atom, the number of electrons equals the atomic number.
   • For an ion, adjust the electron count for its charge. A negative charge means electrons have been added, while a positive charge means electrons have been removed.
   • In general: [ \text{electrons} = \text{atomic number} - \text{ionic charge} ] Treat a negative ionic charge as a negative number.

2. Fill orbitals in order of increasing energy, following the orbital capacities and the rules for electron spins and degenerate orbitals.

Examples

• Nitrogen (7 electrons): Its electron configuration is (1s^2\,2s^2\,2p^3). The three 2p electrons occupy separate orbitals with parallel spins. Nitrogen has three unpaired electrons and is paramagnetic.
• Magnesium (12 electrons): Its electron configuration is (1s^2\,2s^2\,2p^6\,3s^2). All electrons are paired, so magnesium is diamagnetic.
• Phosphorus (15 electrons): The diagram is filled to a total of 15 electrons. This is presented as a practice example.
• Sulfide ion, (S^{2-}) (18 electrons): Neutral sulfur has 16 electrons; the (2-) charge adds two. The resulting orbitals are filled, so the ion is diamagnetic.
• Aluminum ion, (Al^{3+}) (10 electrons): Neutral aluminum has 13 electrons; the (3+) charge means three are removed. The diagram is filled to 10 electrons.
• Cobalt ion, (Co^{2+}) (25 electrons): Transition-metal ions require special care. Neutral cobalt’s configuration ends in (4s^2\,3d^7). Remove two electrons from the (4s) level to obtain (4s^0\,3d^7), rather than (4s^2\,3d^5). Draw the orbital diagram after determining the ion’s configuration.

Speaker and Source

The Organic Chemistry Tutor is the instructor and narrator. He refers to a separate video about electron configurations; no other speaker is heard.