Atomic Number
The atomic number (symbol Z) is the number of protons in the nucleus of an atom and uniquely identifies the chemical element — all atoms of the same element have the same atomic number. In a neutral atom, the atomic number also equals the number of electrons, which determines the element's chemical behavior, reactivity, and position on the periodic table. The atomic number ranges from 1 (hydrogen) to 118 (oganesson) for currently known elements.
Key Formula
Z = number of protons = number of electrons (in a neutral atom)
LaTeX: Z = \text{number of protons} = \text{number of electrons (neutral atom)}
| Symbol | Meaning | Unit |
|---|---|---|
| Z | Atomic number | dimensionless |
Worked Example
Problem
Iron (Fe) has atomic number 26 and mass number 56. State the number of protons, neutrons, and electrons in a neutral iron atom.
Solution
Step 1: Number of protons = Z = 26. Step 2: Number of electrons (neutral atom) = Z = 26. Step 3: Number of neutrons = A - Z = 56 - 26 = 30.
Answer
Iron-56 has 26 protons, 26 electrons, and 30 neutrons.
Atomic Numbers of Common Elements
| Element | Symbol | Atomic Number (Z) | Period | Group |
|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | 1 |
| Carbon | C | 6 | 2 | 14 |
| Nitrogen | N | 7 | 2 | 15 |
| Oxygen | O | 8 | 2 | 16 |
| Sodium | Na | 11 | 3 | 1 |
| Iron | Fe | 26 | 4 | 8 |
Interactive Tools
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Related Terms
Proton
A proton is a positively charged subatomic particle found in the nucleus of every atom, with a charge of +1 elementary charge and a mass of approximately 1.673 × 10⁻²⁷ kg (about 1 atomic mass unit). The number of protons in an atom's nucleus defines the element and is called the atomic number, which determines the chemical identity of the atom. Protons are composed of two up quarks and one down quark held together by the strong nuclear force.
Mass Number
The mass number (symbol A) is the total number of nucleons — protons and neutrons — in the nucleus of an atom, and is always a whole number. It is used to distinguish between different isotopes of the same element, as isotopes have the same atomic number (Z) but different mass numbers (A) due to differing neutron counts. The mass number is approximately equal to the atomic mass in atomic mass units (u), though not exactly because of binding energy effects.
Electron Configuration
Electron configuration is the distribution of electrons of an atom or molecule among its atomic orbitals, described by the principal quantum number (n), sublevel (s, p, d, f), and the number of electrons in each sublevel. Electron configuration determines how an element behaves chemically — its valence electrons (outermost shell) govern reactivity, oxidation states, and bonding capacity. The Aufbau principle, Pauli exclusion principle, and Hund's rule govern how electrons fill orbitals.
The term "atomic number" was introduced by Antonius van den Broek in 1913, who proposed that the nuclear charge determines an element's position in the periodic table. Henry Moseley confirmed this in 1913–1914 by measuring characteristic X-ray frequencies, establishing the modern definition.