Reading Comprehension · Science
Atoms and Molecules
A science reading lesson about atoms and molecules — the building blocks of all matter — including protons, neutrons, electrons, chemical bonds, elements, and compounds.
Subject
Reading Comprehension
Grade Level
Grades 4–7
Format
Lesson + Printable PDF
Access
Full Member
The Building Blocks of Matter
Have you ever built something with LEGOs? You start by dumping all the pieces on the floor — many different sizes, shapes, and colors. You can build something simple using just one kind of piece, or something complex using many different pieces together. Atoms work in a very similar way. Atoms are the building blocks of matter. All matter — everything around you — is made of atoms.
Atoms are the smallest unit of matter that can be identified as a specific substance. All atoms are made of three smaller particles: protons, neutrons, and electrons. Protons carry a positive (+) electrical charge. Electrons carry a negative (−) charge. Neutrons have no charge at all — they are neutral. The protons and neutrons are packed together in the center of the atom, called the nucleus. Electrons move rapidly around the nucleus in regions of space called orbitals or shells.
The number of protons in an atom determines what element it is. For example, every atom that has one proton is a hydrogen atom. Every atom with six protons is a carbon atom. Every atom with eight protons is an oxygen atom. Scientists have organized all known elements in the periodic table, which lists each element by its number of protons — called its atomic number. There are more than 100 known elements, and each one has its own unique properties.
Molecules, Elements, and Compounds
Some atoms have room to hold more electrons around their nucleus, and some atoms have more electrons than they need. Atoms will sometimes join together and share electrons with each other. When atoms share electrons, a chemical bond forms between them. A molecule is created when two or more atoms are connected by chemical bonds. Molecules are the smallest unit of a substance that still has all the properties of that substance.
When two or more of the same kind of atom form a molecule, it is called an element. For example, oxygen gas is made of two oxygen atoms bonded together — written as O₂. When two or more different kinds of atoms form a molecule, it is called a compound. Water is a compound made of two hydrogen atoms and one oxygen atom — written as H₂O. Table salt is a compound made of one sodium atom and one chlorine atom — written as NaCl.
Some compounds are very simple, like water and table salt. Others are extremely complex and contain hundreds or even thousands of atoms. The molecules that make up human blood, for example, are enormous and complicated structures. DNA — the molecule that carries genetic information in every living cell — is one of the most complex molecules known to science. Understanding atoms and molecules helps scientists explain why substances look, feel, and behave the way they do.
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