All matter is made of atoms. An atom is the smallest particle of an element that can take part in a chemical reaction. It consists of a central nucleus surrounded by electrons.
Sub-atomic Particles
| Particle | Symbol | Charge | Relative mass | Location |
|---|---|---|---|---|
| Proton | p | +1 | 1 | Nucleus |
| Neutron | n | 0 | 1 | Nucleus |
| Electron | e⁻ | −1 | 1/1840 (negligible) | Shells (orbits) |
Atomic Number and Mass Number
- Atomic number (Z) = number of protons (also equals the number of electrons in a neutral atom). It identifies the element.
- Mass number (A) = number of protons + neutrons.
- Number of neutrons = A − Z.
Example: Sodium has Z = 11 and A = 23. So it has 11 protons, 11 electrons and 23 − 11 = 12 neutrons.
Isotopes
Isotopes are atoms of the same element with the same atomic number but different mass numbers (different numbers of neutrons). Example: carbon-12 and carbon-14.
Electronic Configuration
Electrons are arranged in shells around the nucleus. The shells fill in the order 2, 8, 8, 18…
- Sodium (11 electrons): 2, 8, 1
- Oxygen (8 electrons): 2, 6
- Calcium (20 electrons): 2, 8, 8, 2
Summary
An atom has protons and neutrons in its nucleus and electrons in shells around it. Atomic number = protons, mass number = protons + neutrons. Isotopes differ in neutron number, and electronic configuration follows the 2, 8, 8… pattern.
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