Atomic Number and Mass Number

Atomic number ($Z$) is the number of protons in an atom's nucleus. Because protons carry the positive charge that a neutral atom must exactly balance with electrons, a neutral atom always has electron count equal to $Z$ as well. Every atom of one element shares the same $Z$ — it is $Z$, not mass or appearance, that fixes which element an atom belongs to.

Mass number ($A$) is the total count of nucleons — protons plus neutrons — in the nucleus. Electrons are left out of this count because an electron's mass is roughly $1/1836$ of a proton's, negligible next to the nucleons.

where: $A$ is the mass number, $Z$ is the atomic number (proton count), and $N$ is the neutron number. Rearranged, this gives the neutron count directly from the two numbers usually printed on a symbol:

Standard notation. An atom of element X is written $_{Z}^{A}\text{X}$, with the mass number as a left superscript and the atomic number as a left subscript. Reading this notation correctly, in order, is itself half the skill the syllabus tests. Two atoms sharing the same $Z$ are always the same element, however many neutrons they carry — atoms of one element with different neutron counts (and so different $A$) are called isotopes, a topic met separately. Because $Z$, $A$, and $N$ are pure counts of whole particles, they are always whole numbers. This is why they differ from the atomic mass value printed on a periodic table (chlorine's $35.5\ \text{u}$, for instance), which averages over a natural mix of isotopes and need not be a whole number at all.

Question: An atom of an element has 20 protons and 20 neutrons. (a) Write its standard notation $_Z^A\text{X}$. (b) State its atomic number and mass number. (c) How many electrons does the neutral atom have, and what is its neutron-to-proton ratio?

Step 1. Read off the atomic number. The number of protons given is $20$, and atomic number is defined as the proton count, so $Z = 20$.

Step 2. Find the mass number using $A = Z + N$. Here $N = 20$ (the given neutron count), so $A = 20 + 20 = 40$.

Step 3. Assemble the standard notation. With $A = 40$ as the top-left number and $Z = 20$ as the bottom-left number, the notation is $^{40}_{20}\text{X}$. This happens to be the calcium atom, $^{40}_{20}\text{Ca}$.

Step 4. Read off the electron count and the ratio. Since the atom is neutral, electrons equal protons, so it has $20$ electrons. The neutron-to-proton ratio is $N/Z = 20/20 = 1$. Answer: atomic number 20, mass number 40, notation $^{40}_{20}\text{Ca}$, with 20 electrons and a neutron-to-proton ratio of 1.

Competitive stretch

Same calcium atom: if it lost two electrons to become a $\text{Ca}^{2+}$ ion, how many electrons would it now have, and would $Z$ or $A$ change? Losing electrons changes only the electron count, never the nucleus, so $Z = 20$ and $A = 40$ stay exactly as before; the ion now has $20 - 2 = 18$ electrons. This is the standard trap: atomic number and mass number describe the nucleus and are untouched by ionisation, which only adds or removes electrons.

• Swapping atomic number and mass number — atomic number counts protons only; mass number counts protons + neutrons.
• Leaving electrons out of mind entirely, or wrongly folding them into the mass number — their mass is negligible, so they are excluded from $A$ by definition.
• Assuming electron count always equals $Z$ — true only for a neutral atom; an ion has gained or lost electrons, so its electron count differs from $Z$ while $Z$ and $A$ themselves stay fixed.
• Reading $_Z^A\text{X}$ upside down — the top-left number is always the mass number $A$, and the bottom-left number is always the atomic number $Z$.
• Plugging the periodic-table atomic mass (an average across isotopes, often a decimal like $35.5\ \text{u}$ for chlorine) into $A = Z + N$ in place of the whole-number mass number of one specific atom.

Ask the tutor: if going back and forth between protons, neutrons, and the $_Z^A\text{X}$ notation still feels shaky, ask the AI tutor for a quick round of practice problems.