Atomic Identity Crisis
Understand atomic number, mass, and isotopes
What this lesson covers
Why it matters
Meet two atoms with the exact same number of protons, yet one is heavier than the other. They are siblings in identity but strangers in weight.
Predict first
Why do Carbon-12 and Carbon-14 behave chemically the same way despite having different masses?
Chemical behavior is dictated by electron configuration, which is determined by the number of protons (atomic number). Since isotopes share the same atomic number, they share the same chemistry.
- They have the same number of electrons — correct
- They have the same number of neutrons
- They are located in different groups of the periodic table
- Their nuclei are identical in size
What you do
Build carbon-12: 6 protons and 6 neutrons in the nucleus, 6 electrons in shells. Notice as you add each neutron the mass number climbs — but the element never stops being carbon.
Check yourself
Which particle count defines the element's identity?
The atomic number (proton count) is the unique fingerprint of an element. Change the protons, and you change the element.
If an atom has 6 protons and 8 neutrons, what is its mass number?
Mass number = Protons + Neutrons. 6 + 8 = 14.
Why are isotopes of the same element placed in the same position in the periodic table?
The periodic table is arranged by atomic number. Since isotopes share the same proton count, they occupy the same slot.
- Neutrons
- Protons — correct
- Electrons
- Total mass number
- 6
- 8
- 14 — correct
- 2
- They have the same mass
- They have the same atomic number — correct
- They have the same number of neutrons
- They are all radioactive