An atom’s chemical behavior is determined by its outermost electrons, known as valence electrons. These electrons occupy the highest energy shell and participate in chemical bonding, fundamentally dictating how the atom interacts with others. An ion is an atom that has gained or lost electrons, resulting in a net electrical charge. The magnesium ion, written as \(\text{Mg}^{2+}\), is a cation, meaning it has lost electrons and carries a positive charge. The direct answer to how many valence electrons \(\text{Mg}^{2+}\) has is zero, a result of its transformation from a neutral atom into a stable ion.
The Electron Structure of Neutral Magnesium
To understand the \(\text{Mg}^{2+}\) ion, we first examine the neutral magnesium atom (\(\text{Mg}\)). Magnesium has an atomic number of 12, meaning it has 12 protons and 12 electrons orbiting the nucleus. These electrons are arranged in distinct energy shells. The first shell holds two electrons, and the second shell holds eight electrons.
The remaining two electrons occupy the third and outermost energy shell. These two electrons are the valence electrons for neutral magnesium, residing in the highest principal quantum number shell (\(n=3\)). The standard electron configuration is \(1s^2 2s^2 2p^6 3s^2\). The \(3s^2\) term represents the two valence electrons in the third shell. The presence of only two electrons in the outer shell makes the atom highly reactive, as atoms favor a full outermost shell, often referred to as the octet rule.
Why Magnesium Forms a \(+2\) Ion
Magnesium’s chemical drive is to achieve the stability of a noble gas, which has a full outer shell of eight electrons. Since neutral magnesium has two valence electrons, it has two paths: gaining six electrons or losing the two it possesses. Losing the two electrons from the outermost \(3s\) orbital requires significantly less energy than gaining six. This energetic preference dictates the formation of the magnesium ion.
When the neutral atom sheds its two valence electrons, the total number of electrons drops from 12 to 10. The 12 protons in the nucleus remain unchanged, creating an imbalance between positive and negative charges. The loss of two negative charges results in an ion with a net charge of \(+2\), written as \(\text{Mg}^{2+}\). This positive ion, or cation, now has 10 electrons and 12 protons. This process exemplifies how alkaline earth metals in Group 2 readily form \(+2\) cations to achieve stability.
Counting Valence Electrons in the \(\text{Mg}^{2+}\) Ion
After the neutral magnesium atom loses its two outermost electrons, the electron configuration of the resulting \(\text{Mg}^{2+}\) ion changes dramatically. The configuration is now \(1s^2 2s^2 2p^6\), meaning the ion possesses a total of 10 electrons. The third shell is now empty, and the outermost shell is the second shell.
This second shell is completely filled with eight electrons, a configuration identical to that of the stable noble gas Neon. Since valence electrons are defined as those in the outermost partially filled shell or those available for bonding, the \(\text{Mg}^{2+}\) ion is considered to have zero valence electrons. The eight electrons in the newly exposed second shell are now considered core electrons; they are tightly bound and unavailable for chemical reactions. Because the ion’s highest energy shell is full and it has no electrons beyond that full shell, \(\text{Mg}^{2+}\) achieves stability with zero available valence electrons.