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General Chemistry I · Atoms, Molecules & Ions

Subatomic Architecture: Protons, Neutrons, Electrons, and Nuclide Notation

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

Every atom is built from three subatomic particles: protons (positive, in the nucleus), neutrons (neutral, in the nucleus), and electrons (negative, outside the nucleus). The number of protons defines which element an atom is and is called the atomic number (Z). The mass number (A) is the total count of protons plus neutrons. Atoms of the same element always have the same Z but can differ in A — these variants are isotopes. Because the proton and neutron are nearly 2000 times heavier than the electron, virtually all of an atom's mass lives in its tiny nucleus, while the electrons occupy almost all of its volume.

Why this matters

The atomic number is the organizing principle of the entire periodic table — change Z and you change the element. Isotopes explain why atomic masses on the periodic table are not whole numbers, power carbon-14 dating and medical imaging, and are the fuel of nuclear reactors. Ion formation (electron gain/loss) is the starting point of all chemical bonding and every reaction between ions. Reading ᴬZX fluently is a prerequisite for stoichiometry, nuclear chemistry, and spectroscopy.

The college version

Key Ideas

  • Proton (p⁺): charge +1, mass ≈ 1.0073 u (amu), located in the nucleus.
  • Neutron (n⁰): charge 0, mass ≈ 1.0087 u, located in the nucleus.
  • Electron (e⁻): charge −1, mass ≈ 0.000549 u, located outside the nucleus.
  • Atomic number Z = number of protons = identity of the element.
  • Mass number A = number of protons + neutrons.
  • Neutron number N = A − Z.
  • Isotopes: same Z (same element), different A (different neutron count), nearly identical chemistry but different mass.
  • Nuclide notation: ᴬZX (e.g., ¹²₆C) — superscript A on top, subscript Z below, element symbol X.
  • In a neutral atom, protons = electrons; an ion has gained (anion) or lost (cation) electrons.

Equations and Variables

A = Z + N, equivalently N = A − Z

  • A = mass number (protons + neutrons), a whole number with no unit
  • Z = atomic number (number of protons), a whole number with no unit
  • N = number of neutrons

For any neutral atom: number of protons = number of electrons = Z. For an ion with charge q (e.g., +2, −1): number of electrons = Z − q. (A +2 cation has lost two electrons: e⁻ = Z − 2.)

How It Works or Problem-Solving Method

Given a nuclide symbol ᴬZX (or an element name and A), find p⁺, n⁰, e⁻:

  1. Protons = Z (the subscript; if only the name is given, look up Z on the periodic table).
  2. Neutrons = A − Z.
  3. Electrons:
    • Neutral atom → electrons = Z.
    • Ion of charge q → electrons = Z − q.

To write nuclide notation from data: identify Z from the element name, use A = protons + neutrons, and place them as superscript/subscript around the symbol.

Worked Example

Problem 1: For ⁶⁵₂₉Cu, find the number of protons, neutrons, and electrons in the neutral atom.

  • Protons = Z = 29.
  • Neutrons = A − Z = 65 − 29 = 36.
  • Electrons (neutral) = 29.

Problem 2: The isotope uranium-235 is written ²³⁵₉₂U. How many neutrons does it have?

Uranium's Z = 92 (protons). Neutrons = A − Z = 235 − 92 = 143.

Problem 3: A neutral atom has 17 protons and 18 neutrons. Write its nuclide symbol and name it.

Z = 17 → chlorine (Cl). A = 17 + 18 = 35. Nuclide symbol: ³⁵₁₇Cl (chlorine-35).

Problem 4: The ³²₁₆S²⁻ ion has how many electrons?

Z = 16 protons. Charge q = −2 means two extra electrons: electrons = Z − q = 16 − (−2) = 18 electrons.

Problem 5: Which pair are isotopes — (a) ¹²₆C and ¹⁴₆C, or (b) ¹⁴₆C and ¹⁴₇N?

(a) Same Z = 6, different A → isotopes. (b) Different Z (6 vs 7) → different elements, not isotopes.

Common Confusions

  • "The atomic number tells you the number of neutrons." Wrong — Z is the number of protons; neutrons come from A − Z.
  • "Isotopes are atoms with different numbers of protons." Wrong — different proton counts mean different elements. Isotopes differ in neutrons only (same Z, different A).
  • "The mass number is the same as the atomic mass on the periodic table." Wrong — A is a whole number for one specific isotope; the periodic-table mass is a weighted average over all naturally occurring isotopes (see the next note).
  • "Electrons live in the nucleus." Wrong — electrons occupy the space outside the nucleus; the nucleus holds only protons and neutrons.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of an atom as a tiny solar system where the "sun" is the nucleus (made of protons and neutrons) and the "planets" are electrons buzzing far away. The protons are the atom's ID card — count them and you know the element's name; change the count and it's a different element. Neutrons are extra "weight" you can add without changing the ID: carbon-12 and carbon-14 are the same element wearing a lighter or heavier backpack. Isotopes are siblings — same name, different backpack weight. Limit of the analogy: electrons are not little balls on neat planetary orbits (that's the old Bohr picture), and the nucleus is far tinier relative to the atom than the sun is to a solar system.

Key takeaways

  • Proton: +1, ~1 u, nucleus. Neutron: 0, ~1 u, nucleus. Electron: −1, ~0.0005 u, outside nucleus.
  • Z = protons = element identity; A = protons + neutrons; N = A − Z.
  • Isotopes = same Z, different A (different neutrons).
  • Neutral atom: e⁻ = p⁺ = Z. Ion: e⁻ = Z − (charge).
  • Nuclide notation: superscript = A (mass number), subscript = Z (atomic number).
  • Nearly all atomic mass is in the nucleus; nearly all atomic volume is the electron cloud.
  • The proton and neutron are ~1800× more massive than the electron.
  • Three particles: proton (+1, nucleus), neutron (0, nucleus), electron (−1, outside).
  • Z = number of protons (defines the element).
  • A = protons + neutrons (mass number).
  • N = A − Z (number of neutrons).
  • Isotopes: same Z, different A.
  • Neutral atom: p⁺ = e⁻; ions gain/lose electrons.
  • Nuclide notation ᴬZX: A on top, Z on bottom.

Keep learning

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Practice General Chemistry I

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • State the charge, mass, and location of the proton, neutron, and electron.
  • Define atomic number (Z), mass number (A), and isotope, and write nuclide notation (ᴀᴢX).
  • Calculate the number of neutrons from mass number and atomic number (N = A − Z).
  • Explain why atoms are electrically neutral and how ions form by gaining or losing electrons.

Sources & references

  1. OpenStax, *Chemistry 2e*, Ch. 2.3, "Atomic Structure and Symbolism."
  2. NIST, "Fundamental Physical Constants" (proton, neutron, and electron masses).

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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