Living and Nonliving Matter and the Chemicals of Life Study Guide
Most of us naturally sort objects around us into living (animate) beings or nonliving (inanimate) objects. However, sometimes it is hard to tell the difference or we may have to stretch our definitions of living versus nonliving. This lesson looks at some of the principles of chemistry underlying living systems.
Atoms and Elements
Living organisms and inanimate objects are all composed of atoms. The way in which atoms are arranged into more complex molecules and how those molecules interact determines whether something is alive or not. Around one hundred elements can be found on Earth and in the rest of the universe. If you divide any element into its smallest part, you will have what we call an atom. An element is made up of one and only one type of atom (although trillions of these same atoms exist in even a tiny piece of the element).
Elements are arranged according to the number of protons they have in their nucleus. Thus, hydrogen, with only one proton, is the first element. The number of electrons equals the number of protons. Electrons are exchanged and shared in chemical reactions, but protons remain untouched during such reactions. The neutrons in the nucleus also remain untouched. The number of neutrons varies and, along with the protons, contributes to the mass of the atom. The electrons are so small that their mass is not included in the mass of the whole atom.
Particles of an Atom
An atom is composed of even smaller particles called protons, neutrons, and electrons. These particles are common to all atoms. The number of these particles will determine the uniqueness of an atom and thus an element.
The neutrons and protons are combined in the center of the atom, a region called the nucleus. The electrons are located in cloud-like layers where they spin around the nucleus.
The electrons in atoms concentrate into layers surrounding the nucleus. These layers are called shells. Each atom needs to fill out the number of electrons in each of its shells. Atoms with complete shells are "content" and do not easily participate in chemical reactions. Chemical reactions occur when electrons are shared or transferred between atoms. Atoms that do not have complete electron shells tend to be more reactive and participate in chemical reactions.
Atoms can complete their electron shells in one of two ways. They can acquire them through a transfer, or they can share them with other atoms. When two or more atoms combine, we call the resulting compound a molecule.
When electrons are transferred between atoms, each becomes an ion with either a positive or negative electric charge. The opposite charges then attract each ion to the other. Sodium and chlorine form ions that are attracted to each other in a molecule called sodium chloride, otherwise known as table salt. Bonds between ions are called ionic bonds.
When atoms share electrons, they are said to have formed a covalent bond. A good example of a molecule with covalent bonding is water. Water consists of one oxygen atom and two hydrogen atoms. The oxygen atom needs two electrons to complete its outer shell, and each hydrogen atom needs one electron. The oxygen atom can thus form a covalent bond with each of the two hydrogen atoms.
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