Mendeleev · science lesson

Atomic number defines the element

Changing neutron count creates an isotope. Changing electron count creates an ion. But changing proton count changes the element itself.

Start with the idea

Changing neutron count creates an isotope. Changing electron count creates an ion. But changing proton count changes the element itself.

6→C→12→14

A concrete example

11 protons means sodium. 12 protons means magnesium. This remains true regardless of isotope or electric charge.

Mendeleev

Mendeleev’s tip

Do not start with a formula. Identify what changes, what stays constant and which quantity you are trying to explain or calculate.

Go one step deeper

Carbon-12 and carbon-14 both have 6 protons. Their neutron counts differ, but both remain carbon.

Common trap

A trend or model is not an exceptionless law. Always check the quantity’s definition and the conditions being compared.

Check your understanding

Can you explain the phenomenon in your own words before looking at an equation?

Go deeper in Science →

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.

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.

Atomic number