atomic number

Meaning: The atomic number, Z, is the number of protons in an atom's nucleus, and it defines the element: every atom with Z = 6 is carbon. The periodic table is ordered by increasing Z. A neutral atom has Z electrons; an ion has gained or lost electrons while Z stays the same. Chemical reactions leave Z unchanged. Nuclear reactions change it, as in alpha decay (Z falls by 2) and beta-minus decay (Z rises by 1).

Equation: A = Z + N, where A is the mass number (protons plus neutrons), Z is the atomic number (protons), and N is the neutron number. All three are pure counts, so they have no SI unit.

On the MCAT: Given a nuclide symbol such as ¹⁴₆C, count 6 protons, 8 neutrons and, if neutral, 6 electrons. Balance nuclear equations by conserving both A and Z. Z also drives trends: across a period, effective nuclear charge rises with Z, and among isoelectronic ions, higher Z means a smaller radius.

Common confusion: Students mix up Z with the mass number A or with the atomic mass on the periodic table. Z is a whole-number proton count; atomic mass is an isotope-weighted average in amu. Isotopes share Z and differ in N, so extra neutrons do not change the element.