Atoms The structure of an atom Protons and neutrons are located in the center of the atom, called the nucleus. Electrons are located outside the nucleus.
Elements An element is a pure substance that cannot be broken down into other substances by physical or chemical means. There are over 100 known elements, 92 of which occur naturally. Each element has a unique name and symbol.
Elements The periodic table of elements Horizontal rows are called periods, vertical columns are called groups.
Elements Isotopes Atoms of the same element that have the same number of protons and electrons but have a different number of neutrons are called isotopes.
Elements Radioactive isotopes Changing the number of neutrons in an atom does not affect the charge, but does affect the stability. When a nucleus breaks apart, it gives off radiation that can be detected and used for many applications. Isotopes that give off radiation are called radioactive isotopes.
Compounds A compound is a pure substance formed when two or more different elements combine. Compounds are always formed from a specific combination of elements in a fixed ratio. Compounds cannot be broken down into simpler compounds or elements by physical means, but can be broken down by chemical means.
Chemical Bonds The force that holds substances together is called a chemical bond. The electrons of an atom are responsible for forming chemical bonds. Electrons travel around the nucleus of an atom in areas called energy levels. Atoms become more stable by losing electrons or attracting electrons from other atoms.
Chemical Bonds Covalent bonds The chemical bond that forms when electrons are shared is called a covalent bond. A molecule is a compound in which the atoms are held together by covalent bonds.
Chemical Bonds Ionic Bonds An atom that has lost or gained one or more electrons is an ion. Ions carry an electric charge. An ionic bond is an electrical attraction between two oppositely charged atoms.
Chemical Bonds Ionic bonds Some atoms tend to donate or accept electrons more easily than other atoms. The elements identified as metals tend to donate electrons. The elements identified as nonmetals tend to accept electrons. Most ionic compounds dissolve in water, are crystalline at room temperature, and have higher melting points than compounds formed by covalent bonds.
Reactants and Products A chemical reaction is the process by which atoms or groups of atoms in substances are reorganized into different substances. Chemical bonds are broken or formed during a chemical reaction. Clues that a chemical reaction has taken place: production of heat or light formation of a new gas, liquid, or solid Substances can also undergo physical changes, which change the appearance but not the composition. Chemical Reactions
Reactants and Products Chemical equations In written chemical equations, chemical formulas describe the substances in the reaction and arrows indicate the process of change. Reactants are the starting substances, on the left side of the arrow. Products are the substances formed during the reaction, on the right side of the arrow. The arrow can be read as yields or react to form. Chemical Reactions
Reactants and Products Chemical equations Balanced equations In chemical reactions, matter cannot be created or destroyed. All chemical equations must show this balance of mass. The number of atoms of each elements on the reactant side must equal the number of atoms of the same element on the product side. C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O Chemical Reactions
Energy of Reactions Activation energy The minimum amount of energy needed for reactants to form products is called activation energy. Some reactions rarely happen due to the high activation energy required. Chemical Reactions
Energy of Reactions Energy change in chemical reactions Reactions that release energy in the form of heat are exothermic. Reactions that absorb energy in the form of heat are endothermic. Chemical Reactions
Enzymes All living things are driven by chemical reactions. Additional substances are needed to reduce activation energy and reaction time in living organisms. A catalyst is a substance that lowers the activation energy needed to start a chemical reaction. Catalysts do not change the amount of product produced, nor are they used up during the reaction. Chemical Reactions
Enzymes Special proteins called enzymes are the biological catalysts that speed up the rate of chemical reactions in biological processes. Most enzymes are specific to one reaction. Chemical Reactions
Enzymes The reactants that bind to an enzyme are called substrates. The specific location where a substrate binds on an enzyme is called the active site. Factors such as ph, temperature, and other substances affect enzyme activity. Chemical Reactions