Atomic Structure
Simple Explanation
An atom is made of a central nucleus (containing protons and neutrons) surrounded by electrons. The atomic number (Z) is the number of protons — it uniquely identifies which element an atom is. The mass number (A) is the total number of protons AND neutrons. Isotopes are atoms of the same element (same Z, same number of protons) but with a different number of neutrons (and therefore a different mass number, A).
Why Do We Need It?
Knowing an atom's structure — and especially the idea of isotopes — is essential for understanding radioactivity, nuclear energy, and how elements are identified and distinguished from one another.
Worked Example
Find the number of protons, neutrons, and electrons in an atom
An atom has an atomic number of 17 and a mass number of 35. Find its number of protons, neutrons, and electrons (assuming a neutral atom).
Why Does This Work?
Since the mass number A is defined as the TOTAL of protons and neutrons together, subtracting the number of protons (Z) directly isolates the number of neutrons — this simple subtraction works for any atom, given its atomic and mass numbers.
Real-Life Example
Carbon-12 vs. carbon-14
Carbon-12 (used to define the atomic mass unit) and carbon-14 (used in radiocarbon dating) are both forms of carbon.
Both have exactly 6 protons (making them both carbon), but carbon-14 has 2 extra neutrons compared to carbon-12 — making them isotopes of the same element with different masses and different (in this case, radioactive) properties.
Practice
An atom has an atomic number of 11 and a mass number of 23. Find its number of neutrons.
EasyTwo atoms of the same element have different mass numbers. What must be true about them?
MediumCommon mistake
Confusing atomic number (Z, number of protons) with mass number (A, protons + neutrons) — Z identifies the element, while A varies between isotopes of the same element.
Quick Review
- Atomic number Z = number of protons (identifies the element).
- Mass number A = protons + neutrons.
- Isotopes: same Z (same element), different number of neutrons (different A).