At both SL and HL, protons and neutrons are located in the atom’s dense, positively charged nucleus, while electrons occupy the space outside the nucleus. Protons and neutrons consist of quarks, whereas electrons have no known smaller constituents.
Particle Composition and Location
| Particle | What is it made of? | Location and properties |
|---|---|---|
| Proton | Two up quarks and one down quark, written as | In the nucleus; relative charge ; relative mass approximately |
| Neutron | One up quark and two down quarks, written as | In the nucleus; relative charge ; relative mass approximately |
| Electron | An elementary particle called a lepton; no known internal structure | Outside the nucleus; relative charge ; relative mass approximately |
The quarks within protons and neutrons are held together by the strong interaction. However, quark composition is beyond the assessable requirements of IB Chemistry Structure 1.2; you mainly need the particles’ charges, relative masses, and locations.
The nucleus is positively charged because it contains protons and neutral neutrons. Electrons occupy energy levels outside the nucleus, with orbitals describing regions where there is a high probability of finding them. They do not travel around the nucleus in fixed circular paths.
The number of protons is the atomic number, . The number of neutrons is , where is the mass number. In a neutral atom, the number of electrons equals the number of protons.
For example, contains:
- protons:
- neutrons:
- electrons:
A common misconception is that isotopes differ in electron number. Isotopes actually have the same number of protons but different numbers of neutrons; ions differ in electron number.
Exam Technique
For IB questions, use the nuclear symbol to determine each particle count and show the subtraction for neutrons. Remember: changing protons changes the element, changing neutrons changes the isotope, and changing electrons forms an ion.