Chemical reactions Study Guide

In chemistry, elements are the fundamental building blocks of all matter. Each element has a unique name and a specific chemical symbol, which is typically one or two letters, with the first letter always capitalised. For example, 'H' for Hydrogen, 'O' for Oxygen, and 'Na' for Sodium. These symbols are universally recognised and are organised on the Periodic Table of Elements. The Periodic Table is a crucial tool that provides not only symbols but also other important information about each element, such as its atomic number and atomic mass. When elements combine, they form compounds. The composition of a compound is represented by its chemical formula, which uses the symbols of the elements involved along with subscripts to indicate the number of atoms of each element in one molecule or formula unit of the compound. For instance, the formula for water is H₂O, meaning one molecule of water contains two hydrogen atoms and one oxygen atom. Chemical reactions involve the rearrangement of atoms as reactants are converted into products. These reactions are represented by chemical equations. A balanced chemical equation uses chemical formulas to show the reactants on the left side and the products on the right side, separated by an arrow. Crucially, a balanced equation must obey the Law of Conservation of Mass, meaning the number of atoms of each element must be the same on both sides of the equation. This is achieved by placing coefficients (numbers) in front of the chemical formulas. For example, the reaction of hydrogen gas with oxygen gas to form water is written as 2H₂(g) + O₂(g) → 2H₂O(l) when balanced, ensuring two hydrogen atoms and one oxygen atom appear before and after the reaction.

Chemical formulae are a concise way to represent the composition of elements and compounds using chemical symbols and numbers. Each element is represented by a unique symbol, which can be one or two letters. For example, oxygen is O, and sodium is Na. When writing formulae, the first letter is always capitalised, and the second (if present) is lowercase.

For elements, the formula often reflects whether they exist as individual atoms or molecules. Many non-metal elements exist as diatomic molecules (two atoms bonded together), such as O₂ (oxygen gas), H₂ (hydrogen gas), N₂ (nitrogen gas), Cl₂ (chlorine gas), F₂ (fluorine gas), Br₂ (bromine liquid), and I₂ (iodine solid). Metals, like iron (Fe), sodium (Na), or copper (Cu), are generally represented by their symbol alone as they exist as a giant metallic lattice rather than discrete molecules.

For compounds, the formula shows the ratio of different elements present. There are two main types of compounds: simple covalent and ionic. Simple covalent compounds are formed between non-metal atoms sharing electrons. The formula indicates the number of atoms of each element in one molecule. For instance, water is H₂O (two hydrogen atoms and one oxygen atom), and carbon dioxide is CO₂ (one carbon atom and two oxygen atoms). The subscript number applies to the element immediately preceding it. If no subscript is present, it means there is one atom of that element.

Ionic compounds are typically formed between a metal and a non-metal, involving the transfer of electrons to form ions. The formula represents the simplest whole-number ratio of ions in the compound, ensuring the overall charge is neutral. For example, sodium chloride is NaCl (one sodium ion for every one chloride ion), and magnesium chloride is MgCl₂ (one magnesium ion for every two chloride ions). The valency (combining power) of each element or ion helps determine these ratios. Mastering chemical symbols and valencies is crucial for correctly writing and interpreting chemical formulae.