Introduction to the Chemical Elements

liquid in vial on table of elements
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An element or chemical element is the simplest form of matter in that it cannot be further broken down using any chemical means. Elements are made up of smaller particles, but you can't take an atom of an element and perform any chemical reaction that will break it apart or join its subunits to make a bigger atom of that element. Atoms of elements may be broken down or fused together using nuclear reactions.

Element Basics

So far, 118 chemical elements have been found. Of these, 94 are known to occur in nature, while the others are manmade or synthetic elements. Eighty elements have stable isotopes, while 38 are purely radioactive. The most abundant element in the universe is hydrogen: In the Earth (as a whole), it's iron. In the Earth's crust and the human body, the most abundant element by mass is oxygen.

The term "element" may be used to describe atoms with a given number of protons or any amount of a pure substance made up of atoms of one element. It doesn't matter whether the number of electrons or neutrons varies throughout the sample.

What Makes Elements Different From Each Other?

How can you tell if two chemicals are the same element?

Sometimes examples of a pure element look very different from each other. For example, diamond and graphite (pencil lead) are both examples of the element carbon. You wouldn't know it based on appearance or properties. However, atoms of diamond and graphite each share the same number of protons. The number of protons, particles in an atom's nucleus, determines the element. Elements on the periodic table are arranged in order of increasing numbers of protons. The number of protons is also known as an element's atomic number, which is indicated by the number Z.

The reason different forms of an element (called allotropes) can have different properties even though they have the same number of protons is that the atoms are arranged or stacked differently. Think of it in terms of a set of blocks. If you stack the same blocks in different ways, you get different objects.

Examples of Elements

Pure elements can be found as atoms, molecules, ions, and isotopes. Examples of elements include a hydrogen atom (H), hydrogen gas (H2), the hydrogen ion H+, and isotopes of hydrogen (protium, deuterium, and tritium).

The element with one proton is hydrogen. Helium contains two protons and is the second element. Lithium has three protons and is the third element, and so on. Hydrogen has the smallest atomic number (1), while the largest known atomic number is that of the recently discovered element oganesson (118).

Pure elements contain atoms that all have the same number of protons. If the number of protons of the atoms in a sample is mixed, you have a mixture, called a compound. Examples of pure substances that are not elements include water (H2O), carbon dioxide (CO2) and salt (NaCl). Note how the chemical composition of these materials includes more than one type of atom. If the atoms had been the same type, the substance would have been an element even though it contained multiple atoms. Oxygen gas (O2) and nitrogen gas (N2) are examples of elements.