Inside the Nucleus

Why it matters

If an atom were a stadium, its nucleus would be a marble at the centre spot — holding 99.97% of the weight. Everything radioactive begins inside that marble.

The idea in plain words

Nuclear notation, decoded. Tap the terms.

ᴬzX : A = p + n, Z = p

³⁵Cl and ³⁷Cl: compare protons, neutrons, electrons (neutral atoms).

Identical chemistry, two extra neutrons in ³⁷Cl — textbook isotopes

• identity: Z — atomic number = protons — decides which element (and its chemistry)
• weight class: A — mass number = protons + neutrons — the nucleon count
• same element: isotopes — same Z, different A: ¹H/²H/³H, C-12/C-14, U-235/U-238
• the cousin term: isobars — same A, different Z — different elements at equal weight
• Both: Z = 17 → 17 p, 17 e⁻
• Neutrons: 35−17 = 18 vs 37−17 = 20

Predict first

Carbon-12 and carbon-14 are both carbon because they share the same…

Z is the element's identity card. Change neutrons and you get ISOTOPES — same chemistry, different mass (and sometimes, different stability).

• number of protons (Z = 6) — correct
• mass number
• number of neutrons

What you do

Build carbon-12, then add neutrons to make carbon-14 — same element, heavier heart.

Check yourself

The nucleus contains…

Electrons orbit far outside; the nucleus is a proton-neutron cluster bound by the strong force.

An atom of ²³⁸U (Z = 92) has how many neutrons?

n = A − Z = 238 − 92 = 146.

Isotopes of an element have identical…

Chemistry can't tell C-12 from C-14 — but the nucleus can: one is forever, one ticks.

• protons and neutrons — correct
• protons and electrons
• only protons
• 146 — correct
• 92
• 238
• chemical properties (same electrons/Z) — correct
• nuclear stability
• mass