Atomic Structure

Overview

The atom is the fundamental building block of matter. Understanding atomic structure is essential for explaining chemical reactions and the properties of elements.

Structure of the Atom

An atom consists of:

• **Nucleus**: Contains protons (positive charge) and neutrons (no charge)
• **Electron shells**: Electrons (negative charge) orbit the nucleus in energy levels

Key Particles

| Particle | Charge | Mass (relative) | Location |

|----------|--------|-----------------|----------|

| Proton | +1 | 1 | Nucleus |

| Neutron | 0 | 1 | Nucleus |

| Electron | -1 | 1/1840 | Shells |

Atomic Number and Mass Number

• **Atomic number (Z)**: Number of protons in the nucleus
• **Mass number (A)**: Total number of protons and neutrons
• For a neutral atom: number of protons = number of electrons

Electron Configuration

Electrons occupy shells (energy levels) around the nucleus:

• **First shell**: Maximum 2 electrons
• **Second shell**: Maximum 8 electrons
• **Third shell**: Maximum 8 electrons (for first 20 elements)

Example: Oxygen (8 electrons) → 2,6

Exam Tips

• Always show electron configuration as numbers separated by commas
• Remember: mass number = protons + neutrons
• Atomic number defines the element
• Draw neat diagrams showing nucleus and electron shells