Summary
Highlights
The octet rule states that atoms tend to react to achieve eight valence electrons. However, there are exceptions. Hydrogen follows the duet rule, aiming for two valence electrons. Boron and aluminum can form stable molecules with only six valence electrons. Atoms in the third period and beyond can sometimes maintain more than eight valence electrons.
To draw the Lewis diagram for aluminum hydride (AlH3), first count the total valence electrons: aluminum has 3, and each of the three hydrogens has 1, totaling 6 valence electrons. Placing aluminum in the center and bonding it to three hydrogens uses up all 6 valence electrons (3 bonds * 2 electrons/bond). Each hydrogen satisfies its duet rule, but aluminum only has 6 valence electrons, demonstrating an exception to the octet rule.
For the Lewis diagram of xenon pentafluoride cation (XeF5+), calculate total valence electrons: xenon has 8, and each of the five fluorines has 7 (5 * 7 = 35). This sums to 43. Since it's a cation with a +1 charge, subtract one electron, leaving 42 valence electrons. Place xenon in the center and bond it to the five fluorines, using 10 electrons. Distribute the remaining 32 electrons to give each fluorine an octet (6 each for 5 fluorines = 30 electrons). The final 2 electrons are placed on xenon, resulting in xenon having 12 valence electrons, an expanded octet. This is viable for elements in the third period and beyond, potentially due to available empty d-orbitals.
The ability of atoms in the third period and beyond to exceed eight valence electrons is debated among chemists. One theory suggests they can place additional electrons in their empty valence d-orbitals.