Valence Electrons And Ionic Compounds

Sodium has one valence electron that is weakly held due to its low ionization energy, while chlorine has seven valence electrons and a high electron affinity. The valence electron of sodium experiences a strong attractive force toward the chlorine nucleus, resulting in electron transfer. The resulting Na⁺ and Cl⁻ ions are held together by coulombic attraction, which lowers the potential energy of the system compared to the separated atoms.

A bond forms when the attraction between one atom’s valence electrons and the other atom’s nucleus is strong enough to overcome electron-electron and nucleus-nucleus repulsions. This net attractive interaction lowers the potential energy of the system relative to the separated atoms, making the bonded state more stable.

Strontium is in Group 2 and has two valence electrons, so it forms Sr²⁺ ions. Fluorine is in Group 17 and has seven valence electrons, so it forms F⁻ ions. Two fluoride ions are needed to balance the charge of one strontium ion, so the formula is SrF₂. This is analogous to other Group 2 fluorides such as CaF₂ and BaF₂.

Nitrogen and phosphorus are both in Group 15 and therefore have the same number of valence electrons (five). Because bonding behavior is governed by valence electron count, both elements form analogous hydrides — NH₃ and PH₃ — each combining with three hydrogen atoms to satisfy their bonding requirements.

Gallium is in Group 13 with three valence electrons, so it forms Ga³⁺ ions, analogous to Al³⁺. Oxygen is in Group 16 with six valence electrons, so it forms O²⁻ ions. Balancing charges requires two Ga³⁺ ions and three O²⁻ ions, giving the formula Ga₂O₃, analogous to Al₂O₃.

Barium is in Group 2, so it has two valence electrons. It loses both valence electrons to form Ba²⁺. The 2+ charge matches the number of valence electrons removed, which equals the group number for main-group metals.

Aluminum is in Group 13 and forms Al³⁺ by losing three valence electrons. Sulfur is in Group 16 and forms S²⁻ by gaining two electrons. To achieve a net charge of zero, two Al³⁺ ions (total charge 6+) combine with three S²⁻ ions (total charge 6−), giving the formula Al₂S₃.

Both potassium and lithium are in Group 1 and have one valence electron. However, potassium’s valence electron is in a higher energy level (4s versus 2s), farther from the nucleus and more shielded by inner electron shells. This means potassium has a lower ionization energy, so its valence electron is removed more easily during the interaction with chlorine’s nucleus, making potassium more reactive.