An ion is an atom or molecule that carries a net electrical charge, formed by gaining or losing electrons. This imbalance between protons and electrons creates a charged particle. Calculating ion properties, such as charge or concentration, is fundamental in chemistry for understanding reactions and material properties.
Predicting the Charge of Simple Ions
The simplest way to predict the charge of a monatomic ion is by using the element’s position on the periodic table. Atoms seek a stable electron configuration, often achieving a full outer shell of eight valence electrons, known as the octet rule. Metals tend to lose electrons to achieve this stability, forming positively charged ions called cations. For instance, alkali metals in Group 1 lose their single valence electron to form a positive one charge (\(\text{Na}^+\)), while Group 2 metals form a positive two charge (\(\text{Mg}^{2+}\)).
Nonmetals, conversely, tend to gain electrons to fill their outer shell, forming negatively charged ions called anions. Halogens in Group 17 gain one electron to form a negative one charge (\(\text{Cl}^-\)). Elements in Group 16 gain two electrons to achieve a negative two charge.
Determining Atomic Charge in Complex Molecules
Determining the charge of an individual atom within a complex molecule or polyatomic ion requires calculating its oxidation state. The oxidation state is the apparent charge an atom would have if electrons were completely transferred in a bond. The sum of the oxidation states for all atoms in a species must equal the net charge of that species. This sum must be zero for a neutral molecule or equal to the ion’s charge for a polyatomic ion.
A hierarchy of rules exists for assigning these states. Fluorine is always assigned an oxidation state of negative one (-1). Oxygen is typically assigned negative two (-2) in most compounds, and Hydrogen is usually positive one (+1) when bonded to nonmetals.
For the sulfate ion, \(\text{SO}_4^{2-}\), the overall charge is negative two (-2). Since four oxygen atoms contribute -8 (4 x -2), the oxidation state of sulfur (S) is found by solving \(\text{S} + (-8) = -2\). Sulfur must have an oxidation state of positive six (+6) to balance the charge.
Converting Mass to the Number of Ions
Calculating the actual count of ions begins with the measured mass of the ionic compound. First, convert the mass from grams into moles by dividing by the compound’s molar mass. Molar mass is the sum of the atomic masses and serves as the bridge between mass and moles.
Next, determine the moles of the specific ion using the subscript ratio from the chemical formula. For example, one mole of magnesium chloride (\(\text{MgCl}_2\)) contains one mole of \(\text{Mg}^{2+}\) ions and two moles of \(\text{Cl}^-\) ions. The moles of chloride ions are thus twice the moles of the \(\text{MgCl}_2\) compound.
The final step is to convert the moles of the ion into a countable number of particles using Avogadro’s number (\(6.022 \times 10^{23}\) particles per mole). Multiplying the moles of the ion by this constant yields the total count of those ions in the sample.
Calculating Ion Concentration
Ion concentration in a liquid solution is typically expressed using Molarity (M), defined as the number of moles of solute per liter of solution. When an ionic compound dissolves, it undergoes dissociation, breaking apart into its constituent positive and negative ions. Consequently, the concentration of individual ions is often higher than the molarity of the original compound.
The calculation must account for the dissociation factor, which is the ion’s subscript in the chemical formula. For a 1 M solution of magnesium chloride (\(\text{MgCl}_2\)), the compound dissociates into one \(\text{Mg}^{2+}\) ion and two \(\text{Cl}^-\) ions. The concentration of \(\text{Mg}^{2+}\) is \(1 \times 1 \text{ M} = 1 \text{ M}\), and the concentration of \(\text{Cl}^-\) is \(2 \times 1 \text{ M} = 2 \text{ M}\).
To find the total ion concentration, the concentrations of all individual ions are summed. In the \(\text{MgCl}_2\) example, the total ion concentration is \(1 \text{ M} + 2 \text{ M} = 3 \text{ M}\). This distinction between the compound’s molarity and the individual ion concentrations is fundamental for solution chemistry.