Elements, compounds and mixtures Study Guide

The relative formula mass (Mr) of a substance is a crucial concept in chemistry, representing the sum of the relative atomic masses (Ar) of all the atoms in its chemical formula. This value is used for both ionic and covalent compounds. To calculate the Mr, you need the chemical formula of the substance and the relative atomic mass of each element involved. These relative atomic masses are typically found on the periodic table. For example, to find the Mr of water (H₂O), you would add the Ar of two hydrogen atoms to the Ar of one oxygen atom. If the chemical formula includes brackets with a subscript outside, like in Ca(OH)₂, you must multiply everything inside the bracket by the subscript before adding it to the rest of the formula. For example, in Ca(OH)₂, there is one calcium atom, two oxygen atoms, and two hydrogen atoms. Knowing how to calculate Mr is fundamental for many chemical calculations, including determining reacting masses in balanced chemical equations, understanding concentrations, and working with moles. The Mr helps us to compare the 'heaviness' of different chemical substances and is a dimensionless quantity because it's a relative measure compared to 1/12th the mass of a carbon-12 atom.

The position of an element in the Periodic Table is directly linked to its atomic structure, specifically the number of protons (atomic number) and the arrangement of electrons in shells. Metals are typically found on the left and in the centre of the Periodic Table, while non-metals are located on the right side. This division is not arbitrary; it reflects fundamental differences in their electron configurations.

Metals are characterised by having few electrons in their outermost shell, typically one, two, or three. These valence electrons are relatively far from the positively charged nucleus and are not strongly attracted to it. This makes it energetically favourable for metal atoms to lose these electrons to achieve a stable, full outer shell, forming positive ions. The ability to easily lose outer shell electrons is what gives metals their characteristic properties, such as electrical conductivity and malleability, as the lost electrons become delocalised and can move freely throughout the metallic structure.

In contrast, non-metals tend to have many electrons in their outermost shell, usually four, five, six, seven, or eight (for noble gases). For non-metal atoms, it is energetically more favourable to gain or share electrons to achieve a stable, full outer shell. When they gain electrons, they form negative ions. Elements in Group 7, the halogens, have seven outer electrons and only need to gain one electron, making them highly reactive non-metals.

The atomic structure, as represented by the element's position in the Periodic Table (group number indicates valence electrons, period number indicates number of electron shells), helps us to predict whether an element will behave as a metal or a non-metal. Elements in the same group have similar chemical properties because they have the same number of valence electrons, influencing their reactivity and the type of ions they form. For example, Group 1 elements (alkali metals) all have one outer electron, making them very reactive metals.