2. Atomic Number, Mass & Reading an Element Box

Each element has a box on the periodic table packed with information. Two numbers do most of the work.

• The atomic number is the number of protons. It identifies the element — every carbon atom has 6 protons, and anything with 6 protons is carbon.
• The mass number is the number of protons + neutrons (the atomic mass on the table is close to this).

From those, you can count all three particles in a neutral atom:

• Protons = atomic number.
• Electrons = protons (in a neutral atom).
• Neutrons = mass number − atomic number.

For example, carbon (atomic number 6, mass ≈ 12) has 6 protons, 6 electrons, and 6 neutrons.

⚠️ Common misconception: the atomic number is the number of protons, not the mass and not the electrons. (Protons are what truly define the element.)

💡 Tip: protons = atomic number; neutrons = mass − atomic number; electrons = protons (if neutral).