ID Card of an Atom
Calculate atomic number and mass number from particle counts.
What this lesson covers
Why it matters
Imagine every atom wears a unique ID card. The number on the front tells you who it is, while the weight on the back tells you how heavy it is.
Predict first
If you change the number of neutrons in an atom, what happens to its identity?
Neutrons add mass but carry no charge. Only protons define the element's identity.
- It becomes a different element
- It stays the same element but gets heavier — correct
- It becomes an ion
- Nothing changes at all
What you do
Build an atom with 6 protons and 6 neutrons. Then, add one more neutron. Watch the mass number change while the atomic number stays fixed.
Check yourself
An atom has 11 protons and 12 neutrons. What is its atomic number?
Atomic number (Z) is strictly the count of protons. Here, Z = 11.
An atom of magnesium has 12 protons and 12 neutrons. What is its mass number?
Mass number = protons + neutrons = 12 + 12 = 24. Electrons are far too light to count.
Why do we say the atomic number is the 'identity' of an element?
The number of protons (atomic number) is unique to each element and dictates its chemical properties and place in the periodic table.
- 11 — correct
- 12
- 23
- 1
- 12
- 24 — correct
- 36
- 0
- Because it determines the element's position in the periodic table — correct
- Because it is the heaviest particle
- Because it is in the nucleus
- Because it is positive