IB Chemistry Sub-topic S1.2 Notes
Atoms
In Topic S1.2, the primary focus is understanding atoms and the behavior of their electrons. This is very important because all chemical reactions will involve interactions between atoms and their electrons.
Atoms are defined as the smallest unit of an element. The IB requires you to know the Bohr model of the atom, which is composed of:
- A dense positively charged nucleus with protons and neutrons, termed nucleons.
- Negatively charged electrons in orbitals around the nucleus, arranged into energy levels/shells.
Although the Bohr model is technically incorrect, as electrons do not orbit the nucleus like satellites, it clearly communicates the concept of electrons having different energy levels. This is explored in more detail in Topic S1.3.
Additionally, you need to be aware of the basic properties of each component of the atom. This is succinctly summarized in the table below:
| Proton | Neutron | Electron | |
|---|---|---|---|
| Relative mass | 1 | 1 | 0 |
| Relative charge | +1 | 0 | -1 |
Atomic notation
Since elements can appear in several forms (think Carbon-12, Carbon-13, and Carbon-14), it is important to clearly communicate which species is being used in any reaction. This is done by atomic notation, which communicates the following:
- Element (X) is the abbreviation of the element.
- Mass number (A) is equal to the number of nucleons.
- Atomic number (Z) is equal to number of protons.
- Charge (n) indicates the ionic state of the atom.
Note that for each element, the abbreviation and the atomic number will never change as they are linked to that element. If either changes, the element has changed.
On the other hand, mass number and charge regularly change in chemical reactions, giving rise to isotopes and ions, respectively.