Atomic Structure: Nucleus, Electrons and the Periodic Table
A simple particle model explains why every element has its own identity.
Big idea: An atom has a tiny central nucleus containing protons and neutrons, with electrons occupying the space around it. The number of protons is the atomic number and identifies the element.
Learning goals
- name the three main subatomic particles and compare their charges;
- use proton and electron counts for a neutral atom;
- read atomic number, symbol and element name from a periodic-table cell;
- describe the simple first-shell and second-shell electron model.
Inside an atom
Protons have positive charge and neutrons have no electric charge. Both are packed into the nucleus. Electrons have negative charge and are found in the region around the nucleus. A proton and neutron each have much more mass than an electron.
| Particle | Charge | Place |
|---|---|---|
| Proton | positive | nucleus |
| Neutron | zero | nucleus |
| Electron | negative | around nucleus |
Atomic number and neutrality
The atomic number equals the number of protons. Changing that number changes the element. In a neutral atom, positive and negative charges balance, so the number of electrons equals the number of protons. Neutrons add mass but do not change the total charge.
Important: do not use the total number of particles as the atomic number; count protons only.