Q.The number of electrons, protons and neutrons in a species are equal to 18, 16 and 16 respectively. Assign the proper symbol to the species.

Atomic Notation: The ID Card of an Atom

Imagine you're at a huge stadium filled with people. To identify any one person, you'd need more than just a name — you'd need their jersey number, their team, maybe their position. An atom is the same way. Just saying "carbon" or "oxygen" tells you the kind of atom, but not the full story. Atomic notation is the shorthand that gives you the complete identity.

The Intuition: Three Numbers, One Symbol

Every atom is built from three key particles:

• Protons (positive charge) — these define what element it is.
• Neutrons (no charge) — these add mass and stability.
• Electrons (negative charge) — these orbit the nucleus and determine chemical behaviour.

Atomic notation packs all this information around the element's symbol. Think of it as a label with two numbers attached:

Where:

• X is the element symbol (e.g., C for carbon, O for oxygen).
• Z is the atomic number (number of protons).
• A is the mass number (protons + neutrons).

The Precise Statement

For any atom represented as $^{A}_{Z}\text{X}$:

• Atomic number $Z$ = number of protons = number of electrons (in a neutral atom).
• Mass number $A$ = number of protons + number of neutrons.
• Number of neutrons $N = A - Z$.

A neutral atom has equal protons and electrons. If the atom gains or loses electrons, it becomes an ion — but the notation for the nucleus stays the same.

Example: Carbon-12

The most common carbon atom is written as:

From this:

• $Z=6$ → 6 protons, and in a neutral atom, 6 electrons.
• $A=12$ → total nucleons (protons + neutrons) = 12.
• Neutrons $N = 12 - 6 = 6$.

Why Two Numbers? Why Not Just One?

If you only knew the mass number $A$, you wouldn't know the element — because different elements can have the same mass number (e.g., $^{14}\text{C}$ and $^{14}\text{N}$ both have $A=14$). If you only knew the atomic number $Z$, you wouldn't know how many neutrons are present — and that matters for isotopes.

A Quick Table for Clarity