Atomic Number and Mass Number
Fundamental concepts for identifying elements and understanding atomic structure
Atomic Number (Z) and Mass Number (A) are essential for identifying elements and calculating the number of subatomic particles in an atom.
Atomic Number (Z) = protons
1. Atomic Number (Z)
Definition
The atomic number (Z) is the number of protons in the nucleus of an atom.
Identity: Each element has a unique atomic number, which determines its position on the Periodic Table.
Hydrogen
Carbon
Oxygen
Sodium
Neutral Atoms
In a neutral atom, the atomic number equals the number of electrons:
Carbon atom (Z = 6) has 6 protons and 6 electrons
2. Mass Number (A)
Definition
The mass number (A) is the total number of protons and neutrons (nucleons) in the nucleus.
Formula: A = Z + N where N = number of neutrons
Rearranged: N = A - Z
Example: Sodium has A = 23, Z = 11
Neutrons = A - Z = 23 - 11 = 12 neutrons
Atomic Calculator
Protons
Neutrons
Electrons
(neutral atom)
Atomic Number (Z): 11 | Mass Number (A): 23
3. Standard Notation (Nuclide Notation)
Format: ᴬ₂X or ^{A}_{Z}X
Notation Practice
An atom has 8 protons and 8 neutrons. Write its nuclide notation.
4. Ions and Charge
| Ion Type | Formation | Charge | Example |
|---|---|---|---|
| Cation | Loses electrons | Positive (+) | Na⁺, Ca²⁺, Al³⁺ |
| Anion | Gains electrons | Negative (-) | Cl⁻, O²⁻, N³⁻ |
Sodium Cation
Charge = +1
Chloride Anion
Charge = -1
Charge Formula: Charge = Number of Protons - Number of Electrons
Ion Charge Calculator
A calcium atom has 20 protons. It loses 2 electrons. What is its charge?
Test Your Understanding
1. What does the atomic number (Z) represent?
2. How many neutrons are in an atom of ²³₁₁Na?
3. A cation forms when an atom:
4. What is the correct notation for an atom with 6 protons and 6 neutrons?
Key Terms
Key Takeaways
- Atomic number (Z) = number of protons = identifies the element
- Mass number (A) = protons + neutrons
- Neutrons = A - Z
- Nuclide notation: ᴬ₂X (A top left, Z bottom left)
- Neutral atom: protons = electrons
- Cation: loses electrons → positive charge
- Anion: gains electrons → negative charge
- Charge = protons - electrons
