Protons, neutrons, electrons, and how their arrangement determines an element's identity and behavior — the atomic number and mass number aren't just labels, they're the two numbers every other calculation in this subtopic builds from. Every question comes with a written explanation of exactly what each number represents.
Atomic number is protons only; mass number is protons plus neutrons — using mass number where atomic number is needed, or vice versa, gives the wrong particle count.
Isotopes of the same element have identical chemical behavior because they have the same number of protons and electrons — only the neutron count, and therefore the mass, differs.
Gaining electrons makes an ion more negative, not more positive — losing electrons is what creates a positive charge, since protons don't change during ion formation.
Straight from the bank — one per difficulty tier. Reveal the answer to see the explanation you'd get in a real session.
An atom of chlorine has an atomic number of 17 and a mass number of 35. How many neutrons does this atom have?
A — 18. Mass number equals protons plus neutrons: 35 = 17 + neutrons, so neutrons = 35 − 17 = 18. 35 (C) mistakes the mass number itself for the neutron count, ignoring that it already includes the 17 protons.
An ion has 20 protons, 20 neutrons, and 18 electrons. What is the charge of this ion?
A — +2. Charge is determined by the imbalance between protons and electrons: 20 protons minus 18 electrons leaves 2 more positive charges than negative, giving a charge of +2. -2 (B) has the sign backwards — this ion has fewer electrons than protons, which makes it positive, not negative.
Two isotopes of carbon, carbon-12 and carbon-14, both have an atomic number of 6. How many more neutrons does carbon-14 have compared to carbon-12?
A — 2. Carbon-12 has 12−6=6 neutrons, and carbon-14 has 14−6=8 neutrons. The difference is 8−6=2 more neutrons in carbon-14. 8 (C) is carbon-14's total neutron count, not the difference between the two isotopes.
Atomic structure questions are almost entirely arithmetic once the definitions are locked in — the fastest gains come from never confusing atomic number with mass number.
Before answering, list all three counts explicitly, even when the question only asks for one. Untimed practice is where that habit prevents mixing up the numbers.
Move to timed sessions once the atomic number/mass number distinction is automatic.
Pair atomic structure with the periodic table in a mock — atomic number is literally what the table is organized by.
Already have an account? Sign in →