c101-lecture-3 - Chemical Formulas A The Elements The...

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1/24/11 1 Chemical Formulas A chemical formula gives the numbers and types of atoms that are found in a substance. When the substance is a discrete molecule, then the chemical formula is also its molecular formula. Fe (iron) is a chemical formula Fe 2 O 3 is a molecular formula The Elements The chemical formulas of most of the elements are simply their elemental symbol: Na (sodium) Fe (iron) He (helium) U (uranium) These chemical formulas are said to be monatomic—only an atom in chemical formula The Elements There are seven elements that occur naturally as diatomic molecules— molecules that contain two atoms: H 2 (hydrogen) N 2 (nitrogen) O 2 (oxygen) F 2 (fluorine) Cl 2 (chlorine) Br 2 (bromine) I 2 (iodine) The last four elements in this list are in the same family of the Periodic Table Binary Compounds A binary compound is one composed of only two different types of atoms. Rules for binary compound formulas 1. Element to left in Periodic Table comes first except for hydrogen: KCl PCl 3 Al 2 S 3 Fe 3 O 4 Binary Compounds 2. Hydrogen comes last unless other element is from group 16 or 17: LiH, NH 3 , B 2 H 4 , CH 4 3. If both elements are from the same group, the lower element comes first: SiC, BrF 3 Other Compounds For compounds with three or more elements that are not ionic , if it contains carbon, this comes first followed by hydrogen. Other elements are then listed in alphabetical order: C 2 H 6 O C 4 H 9 BrO CH 3 Cl C 8 H 10 N 4 O 2
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1/24/11 2 Other Compounds However, the preceding rule is often ignored when writing organic formulas (molecules containing carbon, hydrogen, and maybe other elements) in order to give a better idea of how the atoms are connected: C 2 H 6 O is the molecular formula for ethanol, but nobody ever writes it this way—instead the formula is written C 2 H 5 OH to indicate one H atom is connected to the O atom. Structural Formulas Very often, chemists use structural formulas to show where the atoms in a molecule are positioned. The atoms are connected to each other with bonds . Bonds represent the attractive forces that hold molecules together. There are three types of structural formulas: line structures, ball-and-stick models, and spacing-filling models.
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c101-lecture-3 - Chemical Formulas A The Elements The...

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