Inside the atom: nucleus, isotopes and shells
An element is defined by its proton count. Neutrons can vary to form isotopes, while electrons occupy shells and control most chemical behavior.
Learning goals
- compare protons, neutrons and electrons;
- use atomic and mass numbers;
- identify isotopes;
- write simple shell arrangements for the first twenty elements.
1. Three particles, three roles
| Particle | Charge | Location | Main role |
|---|---|---|---|
| Proton | +1 | Nucleus | Proton count $Z$ identifies the element. |
| Neutron | 0 | Nucleus | Changes mass number and isotope. |
| Electron | −1 | Shells around nucleus | Outer electrons guide bonding and reactions. |
The nucleus is tiny compared with the whole atom but contains nearly all its mass. A neutral atom has equal numbers of protons and electrons, so positive and negative charges balance.
2. Atomic number and mass number
Nuclide notation may be written as $^{A}_{Z}X$. Here $X$ is the element symbol, $Z$ is atomic number and $A$ is mass number.
Core equations: $Z=\text{protons}$; $A=\text{protons}+\text{neutrons}$; therefore $\text{neutrons}=A-Z$. For a neutral atom, $\text{electrons}=Z$.
- Read $Z$ and identify the element.
- Read $A$.
- Subtract $A-Z$ to find neutrons.
- If the atom is neutral, set electron count equal to $Z$.