The structure (what atom is bound to what atom) for covalent compounds is not always easy to infer from the molecular formula. However, the bonding (what is bound to what and the order of the bonds: single, double or triple) is critically important for predicting the chemical and physical properties of the covalent compound. Chemists often use a simple tool called Lewis Structures to help correctly predict the structural formula of these compounds. The guiding principle is that atoms tend to arrange in ionic or molecular clusters to satisfy the octet rule (achieve Noble gas configuration).