Atomic Structure

Protons carry a relative charge of $+1$ and a relative mass of approximately $1$ atomic mass unit (amu); the number of protons defines the identity of the element.

Neutrons are electrically neutral (charge $0$) with a relative mass of approximately $1$ amu, acting as a 'nuclear glue' that stabilizes the nucleus against electrostatic repulsion between protons.

Electrons have a relative charge of $-1$ and a negligible mass (approx. $\frac{1}{1836}$ amu), meaning they contribute to the atom's charge and volume but not significantly to its total mass.

The Atomic Number ($Z$) represents the total number of protons in the nucleus; in a neutral atom, this also equals the number of electrons.

The Mass Number ($A$) is the sum of protons and neutrons in the nucleus, expressed by the formula: $A = Z + N$ where $N$ is the number of neutrons.

To calculate the number of neutrons in a specific isotope, subtract the atomic number from the mass number ($N = A - Z$).

For Ions, the electron count is adjusted: a positive charge (cation) indicates electrons were lost, while a negative charge (anion) indicates electrons were gained.

Identify the Constant: Always remember that the number of protons ($Z$) NEVER changes for a given element; if the proton count changes, it is a different element entirely.

Mass vs. Weight: Do not confuse the 'Mass Number' (an integer for a specific atom) with 'Relative Atomic Mass' (a decimal value found on the periodic table representing an average of all isotopes).

Charge Calculation: When dealing with ions, use the logic: $\text{Charge} = \text{Protons} - \text{Electrons}$. A common mistake is adding electrons for a positive charge; remember that 'positive' means 'missing negative particles'.

Sanity Check: Ensure that the Mass Number is always greater than or equal to the Atomic Number (except for Hydrogen-1, where they are equal).

The 'Empty Space' Fallacy: Students often underestimate how much of an atom is empty space; if a nucleus were the size of a marble, the electrons would be kilometers away.

Neutron Miscounts: Avoid assuming the number of neutrons equals the number of protons; while often similar in light elements, heavier elements require significantly more neutrons for stability.

Shell Capacity: Remember the maximum capacity of electron shells (e.g., $2n^2$ rule) to avoid placing too many electrons in a single energy level during configuration exercises.