In this topic you will learn about the atoms and elements that make up the matter we use.

There are about 110 pure substances that cannot be broken down further into anything simpler. These substances are called elements. All matter in the world is made of elements. Elements can be solids, liquids, or gases.

Elements are made of very tiny particles called atoms. An atom is the smallest particle of an element that has the same chemical properties as the element. The atoms of one element are different from the atoms of any other element. Because each element is made of a particular type of atom, it has its own special properties. In 1803, John Dalton presented a simple atomic theory, concluding that gases were made up of solid particles with spaces between them.

An atom's dense center, where most of its mass is located, is called a nucleus. The nucleus contains particles called protons and neutrons. A proton is a positively charged particle inside an atom's nucleus. A neutron is a particle with no charge inside an atom's nucleus. Both of these particles have about the same mass. The nucleus is held together by nuclear force.

An electron is a negatively charged particle that moves around an atom's nucleus. Electrons are about 1,800 times less massive than protons. They are held near the nucleus by electrical attraction to the protons. An atom has an equal number of protons and electrons, which makes the atom electrically neutral.

The atoms of each element have a unique number of protons. The number of protons in an atom is called its atomic number. The number of protons in an atom tells us what element it is.

All the elements are arranged by increasing atomic number to form the Periodic Table of Elements. In 1868, Dmitry Mendeleyev arranged elements according to atomic mass, and discovered a repetitive pattern to several properties, including density, metal character, and ability to react with other elements. Any repeating pattern is called periodic.

According to properties, elements can be placed into one of three groups: metals, metalloids, and nonmetals. Metals are any group of elements that conduct heat and electricity, are shiny when polished, and bend rather than break. Nonmetals are just the opposite. Metalloids have only some of the properties of metals. Each of these groups falls in a certain part of the periodic table.