Elements, compounds and mixtures Study Guide

The position of an element in the Periodic Table is directly linked to the structure of its atoms, specifically its atomic number and the arrangement of its electrons. The atomic number, represented by the proton number, uniquely identifies an element and determines the number of electrons in a neutral atom. Elements are arranged in order of increasing atomic number across a period (horizontal row). Each new period begins when a new electron shell starts to fill with electrons. The period number corresponds to the number of electron shells an atom has. For instance, elements in Period 1 have one electron shell, while those in Period 2 have two electron shells. The vertical columns are called groups. Elements within the same group have the same number of electrons in their outermost shell, also known as valence electrons. This similarity in the number of valence electrons is why elements in the same group often exhibit similar chemical properties. For example, all elements in Group 1 have one electron in their outer shell, making them very reactive metals that tend to lose this electron to form a positive ion. Conversely, elements in Group 7 have seven valence electrons and are highly reactive non-metals that tend to gain one electron to achieve a full outer shell. Therefore, an element's position on the Periodic Table neatly summarises its atomic structure and predicts its chemical behaviour.

The empirical formula of a chemical compound represents the simplest whole-number ratio of atoms of each element in the compound. It is a fundamental concept in chemistry, as it provides a concise way to describe the elemental composition of a substance without detailing the exact number of atoms in a molecule (which is the molecular formula). For instance, both ethene (C₂H₄) and cyclobutane (C₄H₈) have different molecular formulas, but they share the same empirical formula, CH₂, because the ratio of carbon to hydrogen atoms in both is 1:2. To determine the empirical formula from the relative numbers of atoms, you find the smallest whole-number ratio. If you are given a model or diagram of a molecule, you count the number of atoms of each type, then divide by the greatest common divisor to simplify the ratio. For example, if a model shows 4 carbon atoms and 10 hydrogen atoms, the ratio C:H is 4:10. Dividing both by 2 gives 2:5, so the empirical formula is CH₅. The empirical formula is crucial for understanding the basic composition of all chemical compounds, including ionic and covalent substances.