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: A/Z X 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 Na-23 (Z=11)?
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: A is written at the top left and Z at the bottom left of the element symbol
- Neutral atom: protons = electrons
- Cation: loses electrons and becomes positively charged
- Anion: gains electrons and becomes negatively charged
- Charge = protons - electrons
Exam Focus: Finding Subatomic Particles
Use the same order every time: atomic number gives the number of protons, a neutral atom has the same number of electrons as protons, and neutrons are found by subtracting atomic number from mass number. Writing these three lines prevents most mistakes in atomic structure questions.
For ions, only the number of electrons changes. A positive ion has lost electrons, while a negative ion has gained electrons. The number of protons stays the same because changing protons would change the element itself.
