Chemistry2 Two Types of Substances• Of the known elements, only about 90 occur naturally on Earth. The remainder are synthesized, usually in barely detectable amounts, in high-energy nuclear experiments.• Less than half of the 90 naturally occurring elements are abundant enough to play a significant role in the chemistry of everyday stuff.
Chemistry2 Organizing the Elements• The periodic table organizes elements in a way that provides a wealth of chemical information—much more than is evident to you now. It shows the chemical symbols for the elements.• Their symbols usually correspond to their names in Latin.
Chemistry2 Compounds Are More Than One Element• A more complete definition is that a compound is a chemical combination of two or more different elements joined together in a fixed proportion with a unique set of chemical and physical properties.• More than 10 million compounds are known and the number keeps growing.
Chemistry2 Compounds Are More Than One Element• The properties of the compound are different from the properties of the elements that compose the compound.silver + bromine = silver bromide
Chemistry2 Compounds Are More Than One Element• More than 10 million compounds are known and the number keeps growing.• New compounds are discovered and isolated from natural chemical sources such as plants and colonies of bacteria and are synthesized in laboratories for many different uses.
Chemistry2 Compounds Are More Than One Element• A formula is a combination of the chemical symbols that show what elements make up a compound and the number of atoms of each element. Compound Formula Caffeine C8H10N4O2
Chemistry2 States of Matter• Some substances are described as volatile, which means that they change to a gas easily at room temperature.• Alcohol and gasoline are more volatile than water.• Density is the amount of matter (mass) contained in a unit of volume.• Styrofoam has a low density or small mass per unit of volume.
Chemistry2 States of Matter• Stones have a large density or a large mass per unit of volume.• In science, the density of solids and liquids is usually measured in units of grams (mass) per milliliter (volume) or g/mL.
Chemistry2 Chemical Properties• Chemical properties are those that can be observed only when there is a change in the composition of the substance.• Rusting is a chemical reaction in which iron combines with oxygen to form a new substance, iron oxide.• Inability to react is also a chemical property.
Chemistry2 Chemical Changes• A chemical property always relates to a chemical change, the change of one or more substances into other substances.• Another term for chemical change is chemical reaction.
Chemistry2 Chemical Changes• All matter is made of atoms, and any chemical change involves only a rearrangement of the atoms. Atoms do not just appear. Atoms do not just disappear.• This is an example of the law of conservation of mass, which says that in a chemical change, matter is neither created nor destroyed. It would be equally correct to call this the law of conservation of matter.
Chemistry2 Chemical Reactions and Energy• All chemical changes also involve some sort of energy change.• Energy is either taken in or given off as the chemical change takes place. Energy is the capacity to do work.• Work is done whenever something is moved.
Chemistry2 Chemical Reactions and Energy• Many reactions give off energy.• For example, burning wood is a chemical change in which cellulose, and other substances in the wood, combine with oxygen from the air to produce mainly carbon dioxide and water.
Chemistry2 Chemical Reactions and Energy• Energy is also produced and released in the form of heat and light.• Chemical reactions that give off heat energy are called exothermic reactions.• Chemical reactions that absorb heat energy are called endothermic reactions.