Elements, compounds and mixtures Study Guide

Chemical bonding is the force that holds atoms together to form molecules or compounds. This force arises from the electrostatic attraction between oppositely charged particles. Atoms bond to achieve a stable electron configuration, typically resembling that of noble gases with a full outer shell of electrons. This stability is generally reached by having eight electrons in their outermost shell, often referred to as the octet rule (though for very small atoms like hydrogen, two electrons in the outer shell provide stability).

There are two primary ways atoms achieve this stable configuration: by transferring electrons (ionic bonding) or by sharing electrons (covalent bonding).

In ionic bonding, typically between a metal and a non-metal, one or more electrons are transferred from the outer shell of a metal atom to the outer shell of a non-metal atom. The metal atom loses electrons and becomes a positively charged ion (cation), while the non-metal atom gains electrons and becomes a negatively charged ion (anion). The strong electrostatic attraction between these oppositely charged ions forms the ionic bond.

In covalent bonding, typically between two non-metal atoms, electrons are shared between the atoms. Each atom contributes one or more electrons to form shared pairs. These shared electrons are simultaneously attracted to the nuclei of both atoms, creating a strong electrostatic force that holds the atoms together. The number of shared pairs determines the type of covalent bond (single, double, or triple).

The physical state of a substance – solid, liquid, or gas – is determined by its temperature relative to its melting point and boiling point. Every pure substance has specific melting and boiling points at standard pressure. The melting point is the temperature at which a solid turns into a liquid, and the boiling point is the temperature at which a liquid turns into a gas. Below its melting point, a substance exists as a solid. Above its melting point but below its boiling point, it exists as a liquid. Above its boiling point, it exists as a gas. Substances do not exist as liquids above their boiling point or below their melting point under normal conditions; they transition directly to gas or solid, respectively. At exactly the melting point, solid and liquid phases can coexist in equilibrium. Similarly, at exactly the boiling point, liquid and gas phases can coexist. This information is crucial for understanding how substances behave under different environmental conditions and for practical applications, like choosing materials for specific uses or understanding chemical processes. By using given data on melting and boiling points, you can predict the state of a substance at a specific temperature. For example, if a substance has a melting point of 10°C and a boiling point of 100°C: at 5°C, it's a solid; at 50°C, it's a liquid; and at 150°C, it's a gas. This predictive capability is a fundamental concept in chemistry.