Atomic Number Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Atomic Number.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

The number of protons in an atom's nucleus, which uniquely identifies the element and determines its position in the periodic table.

The atom's ID number — Z = 6 always means carbon, no matter what else changes.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Atomic number uniquely identifies an element — no two elements share the same number.

Common stuck point: Atomic number counts protons only — not protons plus neutrons (that's mass number).

Sense of Study hint: When using atomic number, remember it equals protons and defines the element. First locate the element on the periodic table — the number above or beside the symbol is the atomic number. Then for a neutral atom, electrons equal the atomic number. Finally, do not confuse atomic number with mass number, which includes neutrons.

Worked Examples

Example 1

easy
Element X has 15 protons. Identify the element and determine the number of electrons in a neutral atom.

Solution

  1. 1
    Look up atomic number Z = 15 on the periodic table: this corresponds to phosphorus (P).
  2. 2
    The number of protons equals the atomic number, so phosphorus has 15 protons in its nucleus.
  3. 3
    A neutral atom carries no net charge, meaning the number of electrons equals the number of protons: 15 electrons.

Answer

\text{Phosphorus (P), 15 electrons}
The atomic number uniquely defines each element and equals the number of protons. In a neutral atom, the electron count matches the proton count.

Example 2

medium
Two atoms both have a mass number of 40. Atom A has 20 protons and Atom B has 19 protons. Are they the same element? How many neutrons does each have?

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy
What is the atomic number of an element with 29 protons? Name the element.

Example 2

easy
An atom has atomic number 15. How many protons does it have? If the atom is neutral, how many electrons does it have?
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Background Knowledge

These ideas may be useful before you work through the harder examples.

proton