The chemical formula for strontium nitride is \(\text{Sr}_3\text{N}_2\). This compound is classified as an inorganic ionic substance, forming through the chemical combination of the metal strontium (Sr) and the non-metal nitrogen (N). Like all ionic compounds, strontium nitride exists as a crystalline solid in which positively and negatively charged particles are held together by strong electrostatic forces. Understanding how this specific formula is determined requires a systematic look at the electron transfer that occurs between the two elements. The final formula represents the precise ratio of atoms needed to create a stable, electrically neutral compound.
Determining the Ionic Charges
The first step in determining the formula for any binary ionic compound, such as strontium nitride, involves identifying the stable ionic charge, or valency, for each element. Strontium (Sr) is an alkaline earth metal found in Group 2 of the periodic table. All elements in this group possess two valence electrons, which they readily lose to achieve the stable electron configuration of a noble gas. By losing these two electrons, strontium forms a cation with a positive two charge, represented as \(\text{Sr}^{2+}\).
Nitrogen (N), by contrast, is a non-metal located in Group 15 of the periodic table. Non-metals tend to gain electrons to complete their outer shell, and nitrogen needs three additional electrons to achieve a stable octet. Upon gaining these three electrons, nitrogen forms an anion known as the nitride ion, carrying a negative three charge, represented as \(\text{N}^{3-}\). The combination of a metal cation and a non-metal anion is the defining characteristic of an ionic bond.
Achieving Electrical Neutrality
A fundamental principle of chemical bonding is that a finished compound must possess an overall net charge of zero, meaning the total positive charge must exactly cancel out the total negative charge. When combining the \(\text{Sr}^{2+}\) cation and the \(\text{N}^{3-}\) anion, a simple one-to-one pairing would not achieve this balance, as the charges are unequal (+2 and -3). Therefore, the ions must combine in a specific ratio that allows the positive and negative charges to perfectly neutralize each other.
The easiest way to find this necessary ratio is to determine the lowest common multiple (LCM) between the magnitude of the two charges, which are 2 and 3. The lowest common multiple of 2 and 3 is 6. To reach a total positive charge of \(+6\), three strontium ions are required, since \(3 \times (+2) = +6\). Similarly, to reach a total negative charge of \(-6\), two nitride ions are required, because \(2 \times (-3) = -6\).
The \(6+\) and \(6-\) charges sum to a net charge of zero, satisfying the requirement for electrical neutrality. This process shows that the correct ratio for the ions is three strontium ions for every two nitride ions. The ratio of the charges becomes the subscript for the opposite element in the final formula.
The Strontium Nitride Formula
The precise ratio derived from the charge-balancing process is codified in the final chemical formula. Since three strontium ions (Sr) are needed and two nitride ions (N) are needed, the formula is written as \(\text{Sr}_3\text{N}_2\). The subscripts ‘3’ and ‘2’ indicate the minimum number of each ion required to form the stable compound.
These subscripts are fixed and represent the empirical formula, which describes the simplest whole-number ratio of atoms in the compound. The resulting compound is an alkaline earth metal nitride, which typically appears as a yellowish or grayish solid. Strontium nitride is a relatively stable compound, though it reacts readily with water to produce strontium hydroxide and ammonia gas.