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2.11: Elements and Compounds

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    What's happening in this beaker?

    The bubbles indicate that vapor is being formed, which lets you know that a chemical reaction is taking place. Many chemical reactions are going on constantly inside your body. In fact, there are probably thousands of chemical reactions occurring every second in every one of your cells. And as all living things are comprised of chemicals, understanding how chemicals work is essential to understanding how living things work.

    Chemicals of Life

    The Elements

    If you pull a flower petal from a plant and break it in half, and then take that piece and break it in half again, and take the next piece and break it half, and so on, and so on, until you cannot even see the flower anymore, what do you think you will find? We know that the flower petal is made of cells, but what are cells made of? Scientists have broken down matter, or anything that takes up space and has mass—like a cell—into the smallest pieces that cannot be broken down anymore. Every physical object, including rocks, animals, flowers, and your body, are all made up of matter.

    Matter is made up of a mixture of things called elements. Elements are substances that cannot be broken down into simpler substances. There are more than 100 known elements, and 92 occur naturally around us. The others have been made only in the laboratory.

    Inside of elements, you will find identical atoms. An atom is the simplest and smallest particle of matter that still has chemical properties of the element. Atoms are the building block of all of the elements that make up the matter in your body or any other living or non-living thing. Atoms are so small that only the most powerful microscopes can see them.

    Atoms themselves are composed of even smaller particles, including positively charged protons, uncharged neutrons, and negatively charged electrons. Protons and neutrons are located in the center of the atom, or the nucleus, and the electrons move around the nucleus. How many protons an atom has determines what element it is. For example, hydrogen (H) has just one proton, helium (He) always has two protons (Figure below), while sodium (Na) always has 11.

    All the atoms of a particular element have the exact same number of protons, and the number of protons is that element's atomic number. An atom usually has the same number of protons and electrons, but sometimes an atom may gain or lose an electron, giving the atom a positive or negative charge. These atoms are known as ions and are depicted with a "+" or "-" sign. Ions, such as H+, Na+, K+, or Cl- have significant biological roles.

    Drawing of an atom of helium
    Figure \(\PageIndex{1}\): An atom of Helium (He) contains two positively charged protons (red), two uncharged neutrons (blue), and two negatively charged electrons (yellow).

    The Periodic Table

    In 1869, a Russian scientist named Dmitri Mendeleev created the periodic table, which is a way of organizing elements according to their unique characteristics, like atomic number, density, boiling point, and other values (Figure below). Each element is represented by a one or two letter symbol. For example, H stands for hydrogen, and Au stands for gold. The vertical columns in the periodic table are known as groups, and elements in groups tend to have very similar properties. The table is also divided into rows, known as periods.

    The periodic table
    Figure \(\PageIndex{2}\): The periodic table groups the elements based on their properties. The table begins with Hydrogen, atomic number 1.

    Chemical Reactions

    A molecule is any combination of two or more atoms. The oxygen in the air we breathe is two oxygen atoms connected by a chemical bond to form O2, or molecular oxygen. A carbon dioxide molecule is a combination of one carbon atom and two oxygen atoms, CO2. Because carbon dioxide includes two different elements, it is a compound as well as a molecule.

    A compound is any combination of two or more different elements. A compound has different properties from the elements that it contains. Elements and combinations of elements (compounds) make up all the many types of matter in the Universe. A chemical reaction is a process that breaks or forms the bonds between atoms of molecules and compounds. For example, two hydrogens and one oxygen bind together to form water, H2O. The molecules that come together to start a chemical reaction are the reactants. So hydrogen and oxygen are the reactants. The product is the end result of a reaction. In this example, water is the product.

    Atoms also come together to form compounds much larger than water. It is some of these large compounds that come together to form the basis of the cell. So essentially, your cells are made out of compounds, which are made out of atoms.


    • Elements are substances that cannot be broken down into simpler substances with different properties.
    • Elements have been organized by their properties to form the periodic table.
    • Two or more atoms can combine to form a molecule.
    • Molecules consisting of more than one element are called compounds.
    • Reactants can combine through chemical reactions to form products.

    Explore More

    Use the resource below to answer the following questions.

    1. What is the atomic number of nitrogen? When and where was it identified? In what state of matter does nitrogen exist at room temperature?
    2. What is the atomic number of oxygen? When and where was it identified? In what state of matter does oxygen exist at room temperature?
    3. What is the atomic number of carbon? When and where was it identified? In what state of matter does it exist at room temperature?
    4. What is the atomic number of phosphorus? From what was phosphorus originally isolated? In what state of matter does it exist at room temperature?


    1. What is an element?
    2. What is the difference between the terms molecule and compound?
    3. Describe the composition of an atom.
    4. Who is credited with developing the periodic table?

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