Build the Period 3 Atoms
Construct atoms from protons, neutrons, and electrons
What this lesson covers
Why it matters
Imagine a row of houses where the house number dictates the family name, not the furniture inside. In the periodic table, the atomic number is that unique house number.
Predict first
If you change the number of neutrons in a sodium atom, does it become a different element?
Element identity is locked by the proton count. Changing neutrons creates an isotope, not a new element.
- Yes, because the mass changes
- No, because the proton count (Z) stays the same — correct
- Yes, because isotopes are different elements
What you do
Build the atoms of Period 3 (Na to Ar). Start with sodium — 11 protons, 12 neutrons and 11 electrons (mass number 23) — then add protons to see the element identity shift.
Check yourself
Which property uniquely identifies an element in the periodic table?
The atomic number (Z) equals the number of protons. This count is unique to each element and determines its position.
Why does Magnesium (Z=12) come after Sodium (Z=11) in Period 3?
Elements in a period are arranged by increasing atomic number. Mg has 12 protons, Na has 11.
An atom has 17 protons and 20 neutrons. What element is it?
Z=17 corresponds to Chlorine. The neutron count (20) makes it the isotope Cl-37, but it is still Chlorine.
- Atomic Mass
- Atomic Number (Proton Count) — correct
- Number of Neutrons
- Number of Electrons
- It has one more electron shell
- It has one more proton in the nucleus — correct
- It has fewer electron shells than Sodium
- It is more reactive
- Chlorine — correct
- Argon
- Sulfur
- Potassium