Unit 1 Atomic Structure and Properties

AP Chemistry Atomic Structure

The parts of an atom, where its mass and charge sit, how to count particles in atoms and ions, and how Coulomb’s law holds the atom together, in four worked examples.

Every explanation in AP Chemistry, from periodic trends to bonding, comes back to the same picture: a small positive nucleus holding negative electrons by electrostatic attraction. This page builds that picture carefully, then uses it to count particles in atoms and ions and to reason with Coulomb’s law, the tool the course expects you to use instead of rules of thumb.

The Three Particles That Make Up an Atom

Particle	Charge	Mass (amu)	Location
Proton	+1	1.0073	Nucleus
Neutron	0	1.0087	Nucleus
Electron	−1	0.000549	Shells around the nucleus

The charges are relative to the elementary charge, 1.602 × 10⁻¹⁹ C. A proton is about 1836 times heavier than an electron, so for most purposes protons and neutrons each count as 1 amu and electrons as almost nothing. A neutral atom has equal numbers of protons and electrons, so its charges cancel.

The Nucleus and the Electron Shells

Fig. 1 · A sodium-23 atom in the shell model

Nucleus: tiny, dense, and positively charged, it holds the protons and neutrons and nearly all of the atom’s mass.
Shells: electrons occupy shells, or energy levels, at increasing average distances from the nucleus. Inner (core) electrons shield outer electrons from part of the nuclear charge.
Valence electrons: the electrons in the outermost shell decide how an atom bonds. Sodium’s single valence electron is why it forms Na⁺. See valence electrons.

The shell model is a simplification. Electrons do not move in fixed circular paths; each shell is split into subshells, which electron configurations describe, and the energies of those levels are measured directly by photoelectron spectroscopy.

What Sets an Atom’s Identity, Isotopes, and Ions

Atomic number (Z): the number of protons. It alone decides the element.
Mass number (A): protons plus neutrons. Neutrons = A − Z.
Isotopes: same number of protons, different numbers of neutrons, such as carbon-12 and carbon-14. Reading these numbers from symbols is covered in isotope notation.
Ions: same number of protons, different number of electrons. Losing electrons makes a positive cation; gaining them makes a negative anion.

How Coulomb’s Law Holds the Atom Together

The force between two charged particles is proportional to the product of their charges and inversely proportional to the square of the distance between them: F ∝ q₁q₂ / r². Opposite charges attract, so the positive nucleus pulls on every electron.

More protons in the nucleus means a stronger pull on each electron.
Greater distance means a weaker pull: doubling the distance cuts the force to one quarter.
Shielding by inner electrons reduces the charge an outer electron effectively feels.

These three ideas explain atomic radius, ionization energy, and electron affinity, so every periodic trend argument starts here.

Worked Example 1: Counting Particles in Atoms and Ions

Find the numbers of protons, neutrons, and electrons in sodium-23, chloride-35 (Cl⁻), and iron-56 as Fe³⁺.

Na-23: Z = 11, so 11 protons, 23 − 11 = 12 neutrons, and 11 electrons in the neutral atom.
Cl⁻-35: 17 protons, 35 − 17 = 18 neutrons, and 17 + 1 = 18 electrons.
Fe³⁺-56: 26 protons, 56 − 26 = 30 neutrons, and 26 − 3 = 23 electrons.

The charge changes only the electron count, and the mass number changes only the neutron count. The proton count, and so the element, never moves.

Worked Example 2: Showing That the Nucleus Holds Almost All the Mass

A hydrogen-1 atom has one proton (1.0073 amu) and one electron (0.000549 amu). What percentage of its mass is the electron?

Total: 1.0073 + 0.000549 = 1.0078 amu.
Electron share: 0.000549 ÷ 1.0078 × 100 = 0.054%.

Even in the lightest atom, the electron is about one-twentieth of one percent of the mass. That is why mass numbers count only protons and neutrons, and why a mass spectrum measures the nucleus, as in average atomic mass problems.

Worked Example 3: Picturing How Small the Nucleus Is

An atom is about 10⁻¹⁰ m across and its nucleus about 10⁻¹⁵ m. If the nucleus were a 1 cm marble, how wide would the atom be?

Ratio: 10⁻¹⁰ ÷ 10⁻¹⁵ = 10⁵ = 100,000.
Scaled atom: 1 cm × 100,000 = 100,000 cm = 1 km.

The atom is almost entirely empty space around a tiny, dense center, the conclusion of Rutherford’s gold foil experiment. It also explains why Fig. 1 cannot be drawn to scale.

Worked Example 4: Comparing Attractions With Coulomb’s Law

(a) An electron is moved twice as far from a nucleus. How does the attractive force change? (b) Which electron feels the stronger pull: one 1.0 × 10⁻¹⁰ m from a nucleus of charge +1, or one at the same distance from a nucleus of charge +2?

(a) F ∝ 1/r², so doubling r makes the force (1/2)² = one quarter as large.
(b) F ∝ q₁q₂, so at the same distance the +2 nucleus pulls twice as hard.

These are the two levers in every periodic trend explanation: nuclear charge (stronger across a period) and distance (larger down a group). Free-response answers earn credit for naming both.

Five Atomic Structure Practice Questions With Answers

How many protons, neutrons, and electrons are in oxygen-18?
Show answer
8 protons, 18 − 8 = 10 neutrons, 8 electrons.
An ion has 20 protons, 20 neutrons, and 18 electrons. Identify it.
Show answer
Calcium (Z = 20), mass number 40, charge +2: Ca²⁺.
Do carbon-12 and carbon-14 have the same chemical properties? Explain.
Show answer
Nearly the same. They have the same number of protons and electrons, which control chemistry; only the neutron count, and so the mass, differs.
An electron moves from 2r to r from the same nucleus. By what factor does the attraction change?
Show answer
It becomes 4 times stronger, because F ∝ 1/r².
Which particle could be removed from an atom without changing its mass number or its identity?
Show answer
An electron. Removing it makes a cation of the same element and the same mass number.

Common Atomic Structure Mistakes

Changing the protons to make an ion. Ions form by gaining or losing electrons; the protons stay fixed.
Getting the ion’s electron count backward. A positive charge means fewer electrons than protons, not more.
Counting electrons in the mass number. Mass number is protons plus neutrons only.
Treating the shell model as literal orbits. It is a simplified picture of energy levels.
Explaining with “atoms want a full shell.” Use Coulomb’s law: charge, distance, and shielding.
Thinking isotopes are different elements. Same proton count means the same element.

The atom-level picture behind Coulomb’s law earns points across Units 1 and 2. To see how stronger Unit 1 answers would move your overall estimate, try the AP Chem Score Calculator.

Frequently Asked Questions

What is an atom?

An atom is the smallest unit of an element that keeps its chemical identity. It has a tiny, dense, positively charged nucleus made of protons and neutrons, surrounded by negatively charged electrons.

What are the three parts of an atom?

Protons (charge +1, mass about 1 amu) and neutrons (no charge, mass about 1 amu) make up the nucleus. Electrons (charge −1, mass about 1/1836 of a proton) occupy shells around the nucleus.

What determines which element an atom is?

The number of protons, called the atomic number. Every carbon atom has 6 protons; an atom with 7 protons is nitrogen. Changing the number of neutrons makes an isotope, and changing the number of electrons makes an ion, but neither changes the element.

Where is most of the mass of an atom?

In the nucleus. Protons and neutrons are each about 1836 times heavier than an electron, so in a hydrogen atom the electron is only about 0.05% of the mass, even though the electrons take up almost all of the atom's volume.

What is an isotope?

Isotopes are atoms of the same element with different numbers of neutrons, and so different mass numbers. Carbon-12 and carbon-14 both have 6 protons but have 6 and 8 neutrons. They behave almost identically in chemical reactions.

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