Key concepts in chemistry Study Guide

Metals and non-metals represent two main categories of elements, each exhibiting distinct physical properties. Most metals are typically shiny solids at room temperature. They possess high melting points, meaning a significant amount of energy is required to change them from a solid to a liquid state. Metals also tend to have high densities, indicating that a large amount of mass is packed into a small volume. A key characteristic of metals is their excellent conductivity of electricity, due to the presence of delocalised electrons. In contrast, most non-metals generally have low boiling points, existing as gases, liquids, or brittle solids at room temperature. They are typically poor conductors of electricity, acting as insulators, because their electrons are tightly held within their atoms rather than being free to move.

The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms in its chemical formula. It's used to compare the masses of different compounds and is a fundamental concept in stoichiometry. The Ar values for elements are found on the periodic table. To calculate the percentage by mass of an element within a compound, you divide the total relative atomic mass of that element in the formula by the relative formula mass of the entire compound, then multiply by 100%. This calculation helps determine the proportion of a specific element present in a sample of the compound.