How many orbits are contained in an atom with atomic number 13?

Analyze the question: Atomic number 13 is aluminium. "Orbits" here means electron shells (energy levels) that hold electrons.

Concept: The atomic number equals the number of electrons in a neutral atom. Electrons fill shells in order: 1st shell holds up to 2, 2nd shell up to 8, 3rd shell up to 18 (it takes 8 first in the simple scheme used at this level).

Solution:

• Atomic number 13 means 13 electrons.
• 1st shell: 2 electrons (remaining 11)
• 2nd shell: 8 electrons (remaining 3)
• 3rd shell: 3 electrons (remaining 0)

Electron configuration: 2, 8, 3 (or 1s² 2s² 2p⁶ 3s² 3p¹)

The electrons occupy 3 shells, so the atom contains 3 orbits.

Tip: The number of occupied shells equals the period number of the element in the periodic table. Aluminium is in Period 3, so it has 3 shells. The last number in 2, 8, 3 is the number of valence electrons, which also gives the group (Group 3/13).