Elements Combine to Form Compounds

Elements Combine to Form Compounds Although there are just over 100 different elements, there are millions of different substances. That is because most substances are compounds. A compound is a substance made from atoms of two or more different elements.

Properties of Compounds depend on: Which atoms the compounds contains Water (H2O) and Carbon Dioxide (CO2) are very different even though they both contain oxygen.

Properties of Compounds depend on: How many of each atom there is Water (H2O) and Hydrogen Peroxide (H2O2) are very different even though they both contain hydrogen and oxygen.

Properties of Compounds depend on: How the atoms are arranged Carbon and hydrogen can combine to form thousands of different compounds. These compounds include natural gas, components of automobile gasoline, and many plastics. methane propane octane polyethylene

Compounds are often very different from the elements that make them. Sodium = very reactive metal – reacts violently with water, soft metal, can be cut with a knife Chlorine = poisonous, yellow-green gas, sharp odor

Compounds are often very different from the elements that make them. When these two react, they form a substance that is familiar to you – sodium chloride – table salt 2Na(s)  +  Cl2(g)  ——>  2NaCl(s)

Atoms combine in predictable numbers. A given compound always contains atoms of elements in a specific ratio. For example: the compound ammonia always has three hydrogen atoms for every one nitrogen atom. 3:1 ratio A substance with a different ratio of hydrogen to nitrogen atoms is not ammonia.

Chemical Formulas A chemical formula uses chemical symbols to represent the atoms of the elements and their ratios in a chemical compound. Some examples: H2O - water CO2 – carbon dioxide C6H12O6 – glucose (table sugar)

How to write chemical formulas Find the symbols for the elements in the compound on the Periodic Table Carbon (C) Oxygen (O) To indicate that there is more than one atom of an element, place subscripts slightly below and to the right of the element’s symbol O2 If there is only one atom of an element, you do not need to use a subscript. CO2

Chemical Formulas show the ratios of atoms in a chemical compound The chart shows names, atoms, ratios, and chemical formulas for several chemical compounds. Compound Name Atoms Atomic Ratio Chemical Formula Hydrogen Chloride 1:1 HCl Water 2:1 H2O Ammonia 1:3 NH3 Methane 1:4 CH4 Propane 3:8 C3H8

Chemical Formulas Name Formula # of Different Elements Names of Elements # of Atoms of Each Element Total # of Atoms in One Molecule Magnesium oxide MgO 2 Magnesium 1 Oxygen

Name That Molecule!

Bonds Can Make the Same Element Look Different The element Carbon can exist in very different forms: Diamond Graphite Both substances are made only of carbon. The properties of each form depend on how the carbon atoms are bonded to each other.

Diamond Diamond is the hardest natural substance. This makes them useful for cutting other substances Each carbon atom forms strong bonds with four other carbon atoms The pattern of linked atoms extends throughout the entire diamond crystal. This three-dimensional structure gives diamonds their strength.

Graphite Graphite is the dark, slippery component of pencil “lead”. Each carbon atom in graphite forms strong bonds with three other atoms to form flat layers. These layers stack up on top of one another like sheets of paper, with weak bonds between the layers. The layers slide past each other easily.

Water is Special Water is very unique because its solid phase (ice) is less dense than its liquid phase (water). This structure has spaces in it that make ice less dense than water.

Water is Special Because of the shape of the water molecule, it forms a six sided structure when frozen. This is why snowflakes are six sided.

Most Molecules are Very Complex Although we have talked about some simple molecules, most are very complex. Vanilla, C8H8O3 Acetaminophen, C8H9NO2  Aspartame, C14H18N2O5 Caffeine, C8H10N4O2 Sucrose, C6H12O6 DNA (Deoxyribonucleic Acid), (too complex to list)

Acetaminophen (Tylenol) vanillin Acetaminophen (Tylenol) Aspartame (Nutra Sweet)

caffeine sugar

(Deoxyribonucleic acid) DNA (Deoxyribonucleic acid)