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Atoms and the Periodic Table
Everything around you is built from atoms, and the periodic table sorts them all.
Your desk, the air, and you are all made of atoms, far too small to see. In this unit you'll meet the three particles inside an atom, learn why the number of protons decides which element an atom is, find out what isotopes and ions are, and see how the periodic table organizes all 118 known elements.
What is an atom?
An atom is the smallest piece of an element that still acts like that element. In the Matter unit you learned that all matter is made of tiny moving particles. Those particles are atoms, or groups of atoms joined together called molecules.
An element is a substance made of only one kind of atom, such as gold, oxygen, or carbon.
Protons, neutrons and electrons
- Protons have a positive charge (+1) and sit in the center of the atom, the nucleus.
- Neutrons have no charge and also sit in the nucleus. A neutron has about the same mass as a proton.
- Electrons have a negative charge (−1) and move around the nucleus. They are very light: it takes about 1,800 electrons to equal the mass of one proton.
So almost all of an atom's mass is packed into its nucleus, yet the nucleus is tiny. If an atom were blown up to the size of a football stadium, the nucleus would be about the size of a blueberry. The rest is mostly empty space where the electrons move.
Try the interactive atom builder at jugami.com/physical-science/atoms-and-periodic-table/
Try it: build an atom
Drag the sliders to add protons, neutrons, and electrons. Watch the element, mass number, and charge change.
Carbon-12: 6 protons, 6 neutrons, 6 electrons.
Key: red = proton, grey = neutron, blue = electron. This is a simplified drawing, not to scale. Real electrons don't travel in neat circles, and not every combination you can build is a stable atom found in nature.
Atomic number and elements
The number of protons in an atom is its atomic number, and it decides which element the atom is. Every atom with 6 protons is carbon. Add one more proton and it becomes nitrogen.
The mass number is the number of protons plus neutrons. A carbon atom with 6 protons and 6 neutrons has a mass number of 12, so we call it carbon-12.
Each element has a one- or two-letter symbol: H for hydrogen, O for oxygen, Na for sodium (from its Latin name, natrium).
Isotopes and ions
- Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12 has 6 neutrons and carbon-14 has 8, but both have 6 protons, so both are carbon.
- Ions are atoms that have gained or lost electrons, so their charges no longer balance. Lose an electron and you get a positive ion. Gain one and you get a negative ion. Table salt is made of sodium ions (+) and chloride ions (−).
Changing protons changes the element. Changing neutrons makes a different isotope. Changing electrons makes an ion.
How we learned about atoms
Nobody can see an atom with their eyes, so scientists figured out what's inside through clever experiments. In the early 1800s, John Dalton argued that each element is made of its own kind of atom. Later, J. J. Thomson discovered the electron. Then Ernest Rutherford's team fired tiny particles at thin gold foil. A few bounced back, which showed that atoms have a small, dense nucleus.
The periodic table
In 1869, Dmitri Mendeleev in Russia arranged the elements known at the time into a table, and Lothar Meyer in Germany did similar work. Mendeleev even left gaps for elements that hadn't been discovered yet. Today's periodic table lists all 118 known elements in order of atomic number.
- Periods are the rows. Atomic number goes up by one as you move left to right.
- Groups are the columns. Elements in the same group have similar chemical properties. For example, the noble gases in group 18, such as helium and neon, hardly react with anything.
- Metals fill the left and middle and are usually shiny and good conductors. Nonmetals are on the upper right. Metalloids, such as boron and silicon, sit along the zigzag line between them and have properties of both.
Activities to try at home
Ask an adult before you start. Never taste or eat craft materials, and keep small pieces away from little kids, because they're a choking hazard.
1. Model atom
You need: modeling clay in three colors (or three colors of dried beans), a paper plate, a marker.
Build a carbon-12 atom: roll 6 proton balls and 6 neutron balls and press them together in the middle of the plate. Draw two circles around them and put 2 electrons on the inner circle and 4 on the outer one.
Why: Then swap in 2 more neutrons to make carbon-14, or take away an electron to make an ion.
2. Element hunt
You need: paper, a pencil, and a periodic table (there's a free one on the IUPAC site in Sources).
Look for things made mostly of one element, or labeled with one: aluminum foil (Al), copper wire (Cu), steel paper clips (mostly iron, Fe), and a helium balloon (He). Find each on the table and write its atomic number.
Why: You'll see that metals you use every day sit on the left and middle of the table.
3. Be Mendeleev
You need: 10 index cards and a pen.
Write the first 10 elements, hydrogen to neon, one per card with its atomic number. Shuffle them, then put them back in order. Next, set lithium and sodium, or helium and neon, side by side and look up what they have in common.
Why: Ordering by atomic number and grouping similar elements is exactly how the periodic table is built.
Self-guided quiz
Talk about it
- Scientists described atoms long before anyone could take a picture of one. How can you know something is real if you can't see it?
- Mendeleev left gaps for elements nobody had found yet. Why was that a bold move?
- Why do you think we use one table for all the elements instead of a long list?
Sources
- OpenStax, Chemistry 2e, 2.2: Evolution of Atomic Theory (Dalton, Thomson, Rutherford).
- OpenStax, Chemistry 2e, 2.3: Atomic Structure and Symbolism (particles, charges, the blueberry and stadium comparison, isotopes).
- OpenStax, Chemistry 2e, 2.5: The Periodic Table (Mendeleev and Meyer in 1869, groups, periods, metals, nonmetals and metalloids).
- IUPAC, Periodic Table of Elements (the official table, including elements up to 118).
Credits: All illustrations on this page are original, hand-coded SVG by Galileo. The share image was drawn in code.