Valency plays a crucial role in the derivation of chemical formulae as it indicates the combining power of an element, essentially showing how many electrons an atom will lose, gain, or share when it forms chemical bonds. In simple terms, valency is the number of hydrogen atoms an element can combine with or displace in a chemical reaction. For instance, the valency of oxygen is 2, meaning it can combine with two hydrogen atoms to form water (H₂O). In a compound like methane (CH₄), carbon has a valency of 4, bonding with four hydrogen atoms. Understanding valency is especially important in forming compounds with the correct ratios of atoms. For example, in magnesium oxide (MgO), magnesium has a valency of 2 and oxygen a valency of 2, leading to a 1:1 ratio in the compound. A correct understanding of valency ensures that the derived chemical formulae accurately represent the compound's composition and conforms to the law of conservation of mass.

The Periodic Table is an invaluable tool in deriving compound formulae as it provides essential information about the elements that make up compounds. Each element's position in the Periodic Table indicates its atomic number (the number of protons), which directly relates to its atomic symbol used in chemical formulae. Additionally, the table groups elements with similar chemical properties, which is crucial in predicting how different elements will combine. For instance, elements in Group 1 (alkali metals) typically form +1 ions, while those in Group 17 (halogens) form -1 ions. This knowledge helps in determining the ratios in which different elements combine to form compounds. The Periodic Table also gives information about the valency of elements, which is the ability of an atom to bond with other atoms, and this is key in determining the ratios of atoms in a compound. By understanding these patterns and properties, students can more accurately derive the formulae of compounds.

A compound typically has a unique chemical formula that accurately represents its composition. However, certain compounds can be represented by more than one formula, each highlighting different aspects of its chemical structure. For instance, glucose has the molecular formula C₆H₁₂O₆, which shows the number of each type of atom. However, it also has a structural formula that illustrates how these atoms are bonded together, which can be represented in various ways (linear, ring structure, etc.), each offering insights into the molecule's geometry and reactivity. Another example is isomers, compounds with the same molecular formula but different arrangements of atoms, leading to different properties. Ethanol (C₂H₆O) and dimethyl ether (C₂H₆O) are isomers; they share the same molecular formula but have different structural arrangements and, consequently, different chemical and physical properties. This illustrates that while the molecular formula remains constant, the structural formula can vary, providing additional information about the compound's characteristics.

Understanding molecular geometry is crucial in deriving compound formulae as it provides insight into the arrangement of atoms within a molecule and how these atoms bond together. Molecular geometry affects the chemical and physical properties of the compound, including reactivity, polarity, phase of matter, colour, and magnetism. For instance, the angular geometry of water (H₂O) leads to its polarity, making it an excellent solvent. The linear geometry of carbon dioxide (CO₂) explains its non-polarity, affecting how it interacts with other substances. In organic chemistry, the tetrahedral geometry of methane (CH₄) or the planar geometry of ethene (C₂H₄) significantly influences their chemical behaviour and reactions. By understanding molecular geometry, students can better predict the properties of compounds and the types of chemical reactions they might undergo. This understanding is not only fundamental for deriving the right formulae but also for comprehending the broader implications of these formulae in real-world chemical contexts.

Determining the formula of a compound from a chemical diagram that includes ions requires understanding the charges of the ions and how they combine to form neutral compounds. Firstly, identify the ions and their respective charges. For example, in a diagram depicting sodium ions (Na⁺) and chloride ions (Cl⁻), the positive and negative charges should balance each other out. Since both ions have charges of ±1, they combine in a 1:1 ratio, forming NaCl. In a more complex scenario, such as calcium ions (Ca²⁺) and sulfate ions (SO₄²⁻), the ratio is also 1:1, resulting in CaSO₄. However, if the ions have different charges, such as aluminum ions (Al³⁺) and chloride ions (Cl⁻), a 1:3 ratio is needed to balance the charges, forming AlCl₃. It's crucial to understand that the total positive charge must equal the total negative charge to achieve a neutral compound. This approach is fundamental in deriving the formula of ionic compounds from chemical diagrams.