Atomic number
The atomic number Z is the number of protons in the nucleus. It uniquely identifies the element. Sodium has Z = 11, so every sodium nucleus contains 11 protons.
Start with the atomic number
The atomic number Z is the number of protons in the nucleus. It uniquely identifies the element. Sodium has Z = 11, so every sodium nucleus contains 11 protons.
Na is the chemical symbol for sodium. The decimal mass value shown in many tables is not the atomic number and should not be confused with the mass number of one isotope.
Then read the symbol, name and mass
Na is the chemical symbol for sodium. The decimal mass value shown in many tables is not the atomic number and should not be confused with the mass number of one isotope.
Chlorine has Z = 17, lies in period 3 and group 17, and its configuration ends in 3s² 3p⁵. It therefore has seven valence electrons in its ground state and commonly forms Cl⁻.
Position helps identify chemical families, electron patterns and broad trends. Precise reactivity, toxicity, melting point or the behaviour of a compound require additional data and models.