Notes

You know that changing something's size, shape, or state is called a physical change. When you cut up a log, build a tower out of playing cards, or freeze juice to make ice pops, you are making physical changes.

You also know that sometimes molecules break apart and recombine into new molecules through chemical reactions. New substances are made during these chemical changes. Iron reacts with oxygen in the air and turns into rust. Sodium metal and chlorine gas combine with the release of a lot of energy to form ordinary table salt (sodium chloride). Wood, when it burns, combines its molecules with oxygen and breaks apart. These new substances often have very different properties than the substances from which they came.

ENTER_A11y_Description_asset Use the right arrow key to move forward and the left arrow key to move backward. If using NVDA hold down the Shift, Control, and Alt keys. Atoms are the building blocks of matter. You can't see individual atoms--they're much too small. How did scientists learn about atoms without being able to see atoms? It took a very long time and plenty of careful experiments.

A man named John Dalton worked on the idea that matter was made up of tiny building blocks, which he called atoms. He suggested that some stuff was made up of just one kind of atom (an element), while other stuff was made up of combinations of atoms. We now know that atoms themselves are made up of even smaller pieces, but Dalton’s discoveries still hold true. An element’s chemical properties are determined by what atoms make it up and how they are arranged.In ancient times, people thought everything on Earth was made of just four elements: earth, air, fire, and water. We now know these people were wrong. None of these is an element.

Elements are substances that have only one type of atom. Sodium metal, chlorine gas, gold, and helium are examples of elements. They cannot be broken into simpler substances by chemical means. Elements can be found on the periodic table.

All of an element's atoms are of the same. For example, all sodium atoms have the same number of protons and electrons. Every atom of sodium has the same properties. All chlorine atoms have the same number of protons and electrons, but this number is different from sodium atoms.

The Periodic Table of Elements
All the elements are organized in a chart called the Periodic Table of the Elements.