Inside the Atom: Protons, Neutrons, Electrons原子的结构：质子、中子、电子

Big Idea · Structure and Properties (SAP)AP Science Practice 1 · Models and RepresentationsAP link · Unit 1~25 min

Learning goals

  • Describe an atom as a nucleus (protons and neutrons) surrounded by electrons, with the charge and relative mass of each particle
  • Explain why an atom has no overall charge
  • Use scientific notation to write and compare the tiny numbers of the atomic world

Before you start

  • Molecules and atoms (1.3.1)

Can an atom be split?

In the last lesson, the atom was the smallest particle in a chemical change. But in the late 1800s and early 1900s, scientists found even smaller particles hiding inside atoms: first the negatively charged electron, then the nucleus at the atom’s center. An atom isn’t a solid little ball; it has an inner structure. (Lesson 1.3.7 tells the story of how this was discovered.)

What an atom is made of

  • Atom
    • Nucleus原子核 (at the center, positively charged)
      • Protons质子: each has a charge of +1
      • Neutrons中子: no charge
    • Electrons核外电子: each has a charge of −1 and moves in the large space around the nucleus
ParticleChargeMassRelative massLocation
Proton+11.6726×10−271.6726 \times 10^{-27} kgabout 1In the nucleus
Neutron01.6749×10−271.6749 \times 10^{-27} kgabout 1In the nucleus
Electron−19.109×10−319.109 \times 10^{-31} kgabout 11836\frac{1}{1836}Outside the nucleus

Three things follow from this table:

  1. An atom has no overall charge. In an atom, the number of protons equals the number of electrons. Their charges are equal in size and opposite in sign, so they cancel out.
  2. Almost all of an atom’s mass is in the nucleus. An electron has only about 11836\frac{1}{1836} the mass of a proton, small enough to ignore.
  3. The nucleus is incredibly small. Its diameter is only about 1/100,000 of the atom’s. If an atom were the size of a football stadium, its nucleus would be about the size of an ant on the 50-yard line. Most of an atom is empty space.

Also, not every atom has neutrons: the most common kind of hydrogen atom has just one proton in its nucleus.

Very large and very small numbers

Numbers in the atomic world are so small that writing them as decimals means counting a long string of zeros. Scientific notation fixes that:

🔧 Math tool card · Scientific NotationSkip it if you already know this

The form: write the number as a×10na \times 10^n, where 1≤a<101 \le a < 10 and nn is a whole number.

Large numbers have a positive nn: 602 000=6.02×105602\,000 = 6.02 \times 10^5 (the decimal point moved 5 places left).

Small numbers have a negative nn: 0.000 000 000 1=1×10−100.000\,000\,000\,1 = 1 \times 10^{-10} (the decimal point moved 10 places right).

Multiplying and dividing: when you multiply powers of 10, add the exponents; when you divide, subtract them.

10a×10b=10a+b10a÷10b=10a−b10^{a} \times 10^{b} = 10^{a+b} \qquad 10^{a} \div 10^{b} = 10^{a-b}

Example: how many times heavier is a proton than an electron? 1.67×10−279.11×10−31=1.679.11×10−27−(−31)≈0.183×104≈1.8×103\dfrac{1.67 \times 10^{-27}}{9.11 \times 10^{-31}} = \dfrac{1.67}{9.11} \times 10^{-27-(-31)} \approx 0.183 \times 10^{4} \approx 1.8 \times 10^{3}, or about 1800 times (more precisely, about 1836).

An atom is about 0.000 000 000 1 m across. How is that written in scientific notation?

Build an atom

Use the tool below to add or remove protons, neutrons, and electrons. Which particle changes the element? How do you work out the charge? Then try the challenges:

Try it: add or remove protons, neutrons, and electrons to build an atom
Protons
Neutrons
Electrons

Challenge: build each particle below (a check mark appears when you get it)
Hydrogen atom X11X2121H\ce{^{1}_{1}H}Helium atom X24X2224He\ce{^{4}_{2}He}Lithium atom X37X2327Li\ce{^{7}_{3}Li}Carbon atom X612X26212C\ce{^{12}_{6}C}

Not to scale: a real nucleus is only about 1/100,000 the diameter of its atom.

You’ll notice that the number of protons decides which element it is. Changing the number of neutrons keeps the element the same and just makes the atom heavier or lighter (isotopes, lesson 1.3.4). Changing the number of electrons gives the atom a charge and turns it into an ion (lesson 1.3.6).

Example 1: Is an oxygen atom charged?

An oxygen atom has 8 protons and 8 neutrons in its nucleus. How many electrons does it have? What is the charge of the nucleus? Of the whole atom?

Step-by-step solution
  1. Nuclear charge. Neutrons have no charge, so all of the nucleus’s charge comes from protons: 8 protons give the nucleus a charge of +8.
  2. Electrons. In an atom, electrons = protons = 8.
  3. The whole atom. +8 and −8 cancel, so the atom has no overall charge.
  4. Careful. The atom is neutral not because it contains no charged particles, but because its positive and negative charges are exactly equal.

Example 2: How much bigger is an atom than its nucleus?

An atom is about 1×10−101 \times 10^{-10} m across, and its nucleus is about 1×10−151 \times 10^{-15} m across. How many times bigger is the atom’s diameter?

Step-by-step solution
  1. Set up. Ratio = atom diameter ÷ nucleus diameter = 1×10−101×10−15\dfrac{1 \times 10^{-10}}{1 \times 10^{-15}}.
  2. Subtract the exponents. 10−10−(−15)=10510^{-10-(-15)} = 10^{5}.
  3. Conclusion. The atom is about 10510^5, or 100,000, times wider than its nucleus. If the nucleus were a 1 cm marble, the whole atom would be 1 km across.
Practice: which particle is it?
  1. It has a charge of +1.
  2. It has no charge.
  3. It has a charge of −1.
  4. It has the smallest mass of the three.
  5. Its number decides which element an atom belongs to.
  6. It moves in the large space outside the nucleus.
  7. The most common kind of hydrogen atom has none of these.
  8. It has about the same mass as a proton but no charge.

Common mistakes

Mistake 1: “Every nucleus contains neutrons.” The most common hydrogen atom has no neutrons, just a single proton.

Mistake 2: “The nucleus takes up most of the atom.” The nucleus holds nearly all of the atom’s mass, but it’s tiny. Most of an atom is empty space.

Mistake 3: “Atoms are neutral because they contain no charged particles.” Atoms contain positive protons and negative electrons. Their charges are equal and opposite, so they cancel.

Quick check

Why does an atom have no overall charge?

Where is most of an atom's mass?

Summary

  • An atom is a positively charged nucleus surrounded by negatively charged electrons. The nucleus is made of protons and neutrons (the most common hydrogen atom has no neutrons).
  • In an atom, nuclear charge = protons = electrons, so the atom is neutral overall.
  • Protons and neutrons each have a relative mass of about 1, electrons about 1/1836, so nearly all the mass is in the nucleus. The nucleus is tiny, and most of the atom is empty space.
  • Scientific notation, a×10na \times 10^n with 1≤a<101 \le a < 10, handles the tiny numbers of the atomic world. Multiply: add exponents; divide: subtract exponents.
On to AP (for later)

AP topic 1.5 uses Coulomb’s law to explain atomic structure: the attraction between the nucleus and an electron grows with the charges and falls off with the square of the distance. Topic 1.6, photoelectron spectroscopy (PES), gives experimental evidence that electrons are arranged in shells. And you’ll use scientific notation in almost every AP calculation.

Practice this chapter: Module 3 · The Particle World: Atoms and Molecules · 10 questions

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