Ion is the term for an atom or a group of bonded atoms that carries a net electric charge. This happens when there’s an unequal number of protons and electrons: losing electrons gives a positive charge, while gaining electrons gives a negative charge. Monoatomic ions are single atoms with a charge, like Na+ or Cl−, while polyatomic ions are groups of bonded atoms that together carry a charge, such as NH4+ or SO4^2−. In contrast, a neutral atom has no overall charge, and a molecule is typically a neutral group of atoms bonded together, though charged groups of atoms are described as ions.

When atoms gain electrons, they become negatively charged ions. The species with that negative charge is called an anion. This contrasts with cations, which form when atoms lose electrons and carry a positive charge. An electron is a negatively charged particle, not an ion by itself, and a neutron is neutral. For example, chlorine that gains an electron forms Cl− (an anion), while sodium that loses an electron forms Na+ (a cation). The suffix -ide often appears in simple anions, like oxide (O2−) or chloride (Cl−). The key idea is that a negative charge identifies the species as an anion.

The rate of a reaction at a specific moment is the instantaneous rate. It is the slope of the concentration-versus-time curve at that exact moment, or, in calculus terms, the derivative of concentration with respect to time (often written as -d[A]/dt). This value can change as the reaction proceeds because reactant concentrations are changing, so the driving force for the reaction—and thus the rate—can increase or decrease over time. In practice, you determine it by looking at the tangent to the concentration–time plot at that moment or by evaluating the rate law with the current concentrations. The rate law describes how the rate depends on concentrations (and possibly other factors) and gives a relationship to calculate the rate, but the instantaneous rate is the actual numerical rate at a specific time. A catalyst, while it speeds up the reaction by providing a lower-energy pathway, changes the rate constant but is not itself a rate measured at an instant. The activated complex is the transient high-energy state in the transition from reactants to products, not a rate.

Enthalpy is a state function, so the energy change depends only on the starting and ending states, not on the path taken. Because of this, you can break a reaction into a sequence of steps, find the enthalpy change for each step, and add them together to get the overall enthalpy change. This is Hess's Law. It’s particularly useful for calculating the heat of a reaction by combining known step enthalpies, such as formation or combustion values, to obtain the total ΔH. The other laws describe different ideas: Le Châtelier's Principle explains how equilibria respond to stresses, Dalton's Law deals with partial pressures in gas mixtures, and Kirchhoff's Law concerns how enthalpy changes vary with temperature rather than summing steps.