What Are Atomic Number and Mass Number?
Atomic number and mass number are two key numbers that describe an atom's nucleus. The atomic number tells you how many protons; the mass number tells you how many protons plus neutrons.
Atomic number Z = number of protons (defines the element). Mass number A = protons + neutrons (defines the isotope).
- •Counts protons only
- •Defines the element
- •Example: Z=6 → Carbon
- •Determines chemistry
- •Counts protons + neutrons
- •Defines the isotope
- •Example: A=14 → Carbon-14
- •Close to atomic mass
Step-by-step worked examples
Oxygen has atomic number Z=8. How many protons and electrons does a neutral oxygen atom have?
Z = 8 means 8 protons. Neutral atoms have equal electrons: 8 electrons. Oxygen has 8 protons + 8 electrons.
Carbon-12 (¹²C) has Z=6. How many neutrons does it have?
Z = 6 protons (from atomic number) A = 12 (mass number) Neutrons = A − Z = 12 − 6 = 6 neutrons
Uranium-235 (²³⁵U) has Z=92. How many neutrons?
Z = 92 protons A = 235 (mass number) Neutrons = 235 − 92 = 143 neutrons
Flashcards
Quick quiz
Q1.An atom has Z=20. How many protons?
Q2.¹⁶O has A=16, Z=8. How many neutrons?
Q3.Isotopes are atoms with the same Z but different…
Q4.Which determines the element?
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Common mistakes
Atomic number is the number of neutrons. — Correct: Atomic number is the number of protons; mass number is protons + neutrons.
All atoms of an element have the same mass number. — Correct: Isotopes have the same Z but different A (mass number).
Atomic number tells you the mass of an atom. — Correct: Mass number (A) approximates the mass in amu; atomic number identifies the element.
Changing the atomic number creates a new isotope. — Correct: Changing Z creates a different element; changing N creates a different isotope of the same element.
FAQ
What is the relationship between Z and A?
A = Z + N, where Z is protons, N is neutrons, and A is the total (mass number).
What is an isotope notation?
ᴬ_Z X shows the mass number (top) and atomic number (bottom), e.g., ¹⁴_6 C.
Do all atoms of an element have the same atomic number?
Yes, but they may have different mass numbers (different isotopes).
Why do we need both atomic and mass numbers?
Atomic number identifies the element; mass number identifies the specific isotope.