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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.
Z fixes the element
The atomic number Z equals the number of protons. Modern periodic tables are ordered by increasing Z, which aligns chemical periodicity with nuclear charge.
Isotopes keep Z but change neutron count
Carbon-12 and carbon-14 both contain six protons. Their different neutron counts alter mass and nuclear stability, not the element name.
Ions keep the nucleus while electrons change
Na and Na⁺ both have 11 protons. Losing an electron changes charge and chemical behaviour but does not turn sodium into another element.
An atom has 17 protons and 18 electrons. What element and charge is it?
Z = 17 identifies chlorine. One extra electron gives a −1 charge, so it is Cl⁻.
Common traps to avoid
- Ion formation does not change atomic number.
- Neutron number does not determine the element identity.