Chemistry
Atomic structure & electron configuration — quick study summary
Atoms have a tiny dense nucleus (protons + neutrons) surrounded by electrons in shells/orbitals. Atomic number = number of protons (defines the element). Mass number = protons + neutrons. Isotopes differ only in neutron count. Electrons fill orbitals in order of energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p… (Aufbau principle). The outermost shell (valence electrons) determines chemical behaviour, which is why the periodic table groups by valence.
Key points
- Atomic number Z = protons; defines the element
- Mass number A = protons + neutrons; isotopes share Z, differ in A
- Orbital filling order: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p…
- Hund's rule: fill each orbital singly with parallel spins before pairing
- Valence electrons (outer shell) determine chemistry and group on the periodic table
Practice quiz
Click each question to reveal the answer.
1. What is the electron configuration of a neutral oxygen atom (Z = 8)?
- 1s² 2s² 2p⁴
- 1s² 2s² 2p⁶
- 1s² 2s² 2p³
- 1s² 2s² 2p⁵
Answer: 1s² 2s² 2p⁴
Oxygen has 8 electrons: 2 fill 1s, 2 fill 2s, and the remaining 4 go into 2p.
2. How do isotopes of an element differ?
Answer: They have the same number of protons but different numbers of neutrons
Same Z (so same element and chemistry), different mass number A. ¹²C and ¹⁴C both have 6 protons but 6 vs 8 neutrons.
3. Which electrons determine an element's chemical reactivity?
Answer: Valence (outermost shell) electrons
Bonding involves the outer electrons. That's why elements in the same group (same valence count) react similarly.
Last reviewed: May 2026