Chapter 4 · Atoms and Elements
4.5Elements: Defined by Their Numbers of Protons
8 min · two checks
Predict
If you could change a sodium atom into a magnesium atom, what would you have to change?
The idea
- Define an element by atomic number.
- Explain why ions of sodium are still sodium.
The periodic table is ordered by atomic number, not by mass. Element 11 is sodium whether a particular atom has 10 electrons (the Na⁺ ion) or 11 electrons (the neutral atom). The common isotope, sodium-23, has 12 neutrons; a different neutron count is a different isotope of sodium, not a different element. Change the electron count and you have a sodium ion. Change the proton count and you have a different element.
That definition is what makes the table a table of elements rather than a table of particles. Symbols are abbreviations, not properties: Fe is iron from ferrum, Au is gold from aurum, K is potassium from kalium. Learn the common ones by use. The proton count is the fact; the symbol is the nickname.
Keep these
- Atomic number = element identity.
- Ions and isotopes do not change the element.
- Changing Z changes the element.