READ: Chemistry of Life

Elements: The building blocks of matter

Objective 

• List the major chemical elements in cells (i.e. carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur, trace elements)

Introduction

Living things are made of matter. In fact, matter is the “stuff” of which all things are made. Anything that occupies space and has mass is known as matter. Matter, in turn, consists of chemical substances.

Chemical Substances 

A chemical substance is a material that has a definite chemical composition. It is also homogeneous, so the same chemical composition is found uniformly throughout the substance. A chemical substance may be an element or a chemical compound.

Elements 

Elements (pure substances that cannot be broken down into different types of substances) include carbon, oxygen, hydrogen, and iron. The most abundant elements in cells are carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur. The acronym CHNOPS can be used to remember these elements. Trace elements are elements that are required by living things in small amounts, but play a role in helping the body function properly. An example of a trace element is iron.

Each element is made up of just one type of atom. An atom is the smallest particle of an element that still characterizes the element. As shown in the figure, at the center of an atom is a nucleus. The nucleus contains positively charged particles called protons and electrically neutral particles called neutrons. Surrounding the nucleus is a much larger electron cloud consisting of negatively charged electrons.

Chemical Compounds

A chemical compound is a new substance that forms when atoms of two or more elements react with one another. A chemical reaction is a process that changes some chemical substances into other chemical substances. A compound that results from a chemical reaction always has a unique and fixed chemical composition. The substances in the compound can be separated from one another only by another chemical reaction. The atoms of a compound are held together by chemical bonds. There are different types of chemical bonds, ionic and covalent. Ionic bonds form by transferring electrons between atoms. Covalent bonds form when atoms share electrons.

Energy is required when bonds are formed and energy is released when bonds are broken. For example, when parents walk across the street with a young child they will make a bond by “holding hands” which allows them to contain their child’s energy. If they release it the energy is released with the child and they may run out in front of traffic. An example of a chemical compound is water. A water molecule forms when oxygen (O) and hydrogen (H) atoms react and are held together by a chemical bond. Like other compounds, water always has the same chemical composition: a 2:1 ratio of hydrogen atoms to oxygen atoms. This is expressed in the chemical formula H2O. A model of a water molecule is shown in the Figure below.

Summary 

• Matter consists of elements and compounds. The most abundant elements in living things are carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. A compound forms when elements combine in fixed proportions and undergo a chemical reaction.