ChemAtlas

Chapter 2

Atomic structure

Protons fix the element, neutrons fix the isotope, and electrons fix the chemistry. Everything here follows from those three counts.

Valence shell2 electrons, does the chemistryNucleusprotons + neutronsCoreshielded, inertOccupancy12283842
Fig. 1Shell structure of an atom — Calcium, Z = 20. Only the outermost electrons take part in chemistry.

0 for a neutral atom

Try one

Nuclear symbol

6630Zn

Mass number on top, atomic number underneath.

Protons

30

fixes the element

Neutrons

36

fixes the isotope

Electrons

30

equal to protons in a neutral atom

Mass number

66

protons + neutrons

Working

  1. 01The number after the symbol is the mass number A = 66.
  2. 02Zinc is element 30, so Z = 30 protons. That never changes.30 protons
  3. 03Neutrons = A − Z = 66 − 3036 neutrons
  4. 04The atom is neutral, so electrons equal protons.30 electrons