Key concepts in chemistry Study Guide

The endings '-ide' and '-ate' are used in the names of chemical compounds to indicate their composition and the types of elements present. The '-ide' ending is typically used for binary compounds, meaning those formed from only two different elements. For example, in sodium chloride, chloride indicates that the compound contains chlorine ions (Cl⁻) combined with sodium. Similarly, magnesium oxide contains oxygen (O²⁻) ions. In contrast, the '-ate' ending signifies a compound containing a polyatomic ion that includes oxygen, often referred to as an oxoanion. These polyatomic ions are made up of more than two atoms from different elements, where oxygen is one of them. For instance, in copper sulfate, 'sulfate' refers to the sulfate ion (SO₄²⁻), which is composed of sulfur and oxygen atoms. Calcium carbonate contains the carbonate ion (CO₃²⁻), consisting of carbon and oxygen. These endings provide a systematic way to infer the composition of many inorganic compounds directly from their names.

Ionic compounds are formed when atoms transfer electrons to achieve stable electron configurations, resulting in the formation of positively charged ions (cations) and negatively charged ions (anions). These oppositely charged ions are then attracted to each other, forming an ionic bond. To deduce the formula of an ionic compound, the key principle is that the overall charge of the compound must be neutral. This means the total positive charge from the cations must balance the total negative charge from the anions. You need to know the charge of common ions. For example, Group 1 metals form +1 ions, Group 2 metals form +2 ions, and Group 7 non-metals typically form -1 ions. Polyatomic ions like sulfate (SO₄²⁻) or nitrate (NO₃⁻) have specific charges that must also be balanced. The 'criss-cross' method is a useful technique: write the symbols of the ions with their charges, then swap the numerical values of the charges (without their signs) to become the subscripts for the other ion. Reduce the subscripts to their lowest whole number ratio. If a polyatomic ion requires a subscript greater than 1, it must be enclosed in parentheses.