Final answer:
Atoms in a molecule are designated as having a partial positive charge (δ+) or a partial negative charge (δ-) based on their electronegativity in relation to other atoms. Oxidation numbers illustrate electron movement during reactions. Balance of net charge on both sides of an equation is crucial, as well as the convention in physics to signify direction with positive and negative signs.
Step-by-step explanation:
We determine if an atom within a molecule has a partial positive or partial negative charge by examining the molecule's polar covalent bonds. Atoms with a higher electronegativity typically carry a partial negative charge (δ-), whereas atoms with a lower electronegativity carry a partial positive charge (δ+). For example, in a hydrochloric acid (HCl) molecule, chlorine (Cl) has a higher electronegativity than hydrogen (H), so we assign it a δ- and hydrogen a δ+. Similarly, in water (H₂O), the single oxygen atom assumes a δ- because it has a higher electronegativity than hydrogen atoms, which are assigned an oxidation number of +1, indicating a δ+.
The term oxidation numbers also helps clarify the concept of positive and negative charges within a molecule. By assigning oxidation numbers, we can track electron movement during reactions. Thus, magnesium (Mg), when it donates two electrons, becomes a cation with a charge of +2 (Mg²+), and selenium (Se), when it gains two electrons, becomes an anion with a charge of -2 (Se²-).
Finally, when balancing equations, we ensure the net charge is the same on both sides. For instance, if the left side of an equation has a total charge of -2 and the right also has -2, the charges are balanced. In physics, such as when studying electric charge or motion, positive and negative signs can also represent direction, with upward or rightward motion often considered positive (+), and downward or leftward as negative (-).