Atomic structure

The idea

Every element's identity comes down to one number: how many protons sit in its nucleus. You already know matter is built from atoms; now you are looking inside one. The nucleus packs protons (positive) and neutrons (neutral) into a tiny, dense core, while electrons (negative) occupy the vast space around it. The atomic number Z counts protons and defines the element; the mass number A counts protons plus neutrons and tells you which version of the atom you have.

The counting rules are mechanical once you see them: protons always equal Z, neutrons equal A − Z, and electrons equal Z in a neutral atom — adjusted up or down when the atom carries a charge. A common trap is thinking a charged atom (an ion) gained or lost protons. It never does in ordinary chemistry: only electrons move. Strip a proton from the nucleus and you no longer have an ion of the same element — you have a different element entirely.

Keep the scale in mind: the nucleus holds essentially all the mass but almost none of the volume. Electrons weigh roughly 1/1800 of a proton, which is why mass number ignores them, yet those nearly massless electrons control all of the atom's chemistry, because they are what other atoms actually touch.

Worked example

A phosphorus ion is written ³¹P³⁻ — mass number 31, charge 3−. How many protons, neutrons, and electrons does it contain?

1. Identify the element first: the symbol P means phosphorus, and the periodic table gives phosphorus atomic number 15. So there are exactly 15 protons — that proton count is what makes it phosphorus at all.
2. Neutrons come from the mass number: A − Z = 31 − 15 = 16 neutrons. The mass number lumps together everything heavy in the nucleus, so subtracting the protons leaves the neutrons.
3. The 3− charge means the ion holds 3 more electrons than protons (extra negatives), so electrons = 15 + 3 = 18.
4. Sanity-check with the charge formula: charge = protons − electrons = 15 − 18 = −3, which matches the 3− in the symbol. Notice the protons never changed — only the electron count did.

Answer. The ³¹P³⁻ ion has 15 protons, 16 neutrons, and 18 electrons.

Check your understanding

• Why does changing the number of protons create a different element, while changing neutrons or electrons does not?
• If the charge on this ion had been 3+ instead of 3−, which particle count would change, in which direction, and why?
• Why can the mass number safely ignore electrons, and where in the atom is nearly all of the mass concentrated?
• How would you explain the difference between atomic number and mass number to a friend who confuses them?

Build the foundations first

Atomic structure builds on these concepts. If any feel shaky, start there.

Keep going

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