Elements, compounds and mixtures Study Guide

The empirical formula of a compound represents the simplest whole-number ratio of atoms of each element present in the compound. It is a fundamental concept in chemistry because it provides the most basic structural information about a chemical substance. Unlike a molecular formula, which shows the actual number of atoms of each element in a molecule, the empirical formula is always reduced to the simplest possible ratio. For example, the molecular formula for glucose is C₆H₁₂O₆, but its empirical formula is CH₂O, because the subscripts (6, 12, 6) can all be divided by 6 to give the simpler ratio (1, 2, 1). To deduce the empirical formula, you need to determine the ratio of the number of atoms of each element. This can be done by using the relative atomic masses of the elements and the masses of the elements in a compound, or by simply counting the atoms in a model or diagram. If you are given the masses, you would divide each mass by its element's relative atomic mass to find the number of moles. Then, divide all mole values by the smallest mole value to find the simplest whole-number ratio.

The position of an element in the Periodic Table gives clues about its atomic structure, specifically the number of valence electrons. This, in turn, dictates whether the element behaves as a metal or a non-metal. Metals are generally found on the left and in the middle of the Periodic Table, while non-metals are located on the right side. This division is separated by a diagonal line of metalloids.

Metals typically have one, two, or three electrons in their outermost electron shell (valence electrons). Due to the relatively weak attraction between these few valence electrons and the nucleus, metals tend to lose these electrons to achieve a stable electron configuration, typically a full outer shell. This loss of electrons results in the formation of positive ions (cations), which is characteristic of metallic bonding and explains properties like electrical conductivity.

Non-metals, in contrast, usually have four, five, six, or seven electrons in their outermost electron shell. They are located towards the right of the Periodic Table. These non-metals tend to gain or share electrons to achieve a stable, full outer shell. Elements with almost full outer shells (like halogens with seven valence electrons) readily gain one electron to form negative ions (anions). Elements with four, five, or six valence electrons often share electrons to form covalent bonds. This tendency to gain or share electrons, rather than lose them, explains why non-metals do not conduct electricity in the solid state (except for graphite) and typically form compounds with different properties compared to metals. The noble gases, in Group 0, have a full outer shell and are unreactive, representing the stable electron configuration that other elements aim to achieve through chemical reactions.