c101_topic-4 - 10/2/11 Periodic Table Periodic Table  ...

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Unformatted text preview: 10/2/11 Periodic Table Periodic Table   Many more chemical reac�ons exist than anyone can imagine. Certain pa�erns of chemical reac�vity have been recognized for > 100 years. Each chemical element shows a pa�ern of chemical reac�vity and are the basis for the periodic table. Periodic Table   One way to list the elements would be in a long horizontal line broken into seven rows. Each row is placed below the previous row so that elements with similar chemical proper�es appear in the same column (group) of the table. As we move across a row (period) of the periodic table (PT), the elements generally increase in mass and change drama�cally in their chemical proper�es. Periodic Table Molecular Formulas   In 1869, Dmitri Mendeleev and Julius Meyer independently discovered how to arrange the chemical elements in a table so that elements with similar chemical proper�es were in the same columns.   Based primarily on chemical reac�vity pa�erns and elemental masses.   Today’s PT retains this form but we can explain the arrangement of elements in the PT in terms of atomic structure.   Answers to the ques�on “What is it?” may take the form of pictures or drawings as chemists use chemical formulas, chemical names and various types of molecular pictures to describe molecules. Structural formula Ball-and-stick model Space-filling model Line structure 1 10/2/11 Chemical Formulas Chemical Formulas   A chemical compound is a combina�on of atoms of different elements. Because a compound contains more than one element, there is more than one way to write its formula. HCl   To avoid confusion, chemists have adopted a standard order for wri�ng chemical formulas. For binary compounds-­‐those containing only two elements-­‐the following rules apply:   Except for H, the element farther to the le� in the PT appear first: KCl, PCl3, Al2S3 and Fe3O4.   If H is present, it appears last except when the other element is from column 16/17: LiH, NH3, B2H6 and CH4 but H2O2, H2S, HCl and HI.   If both elements are from the same column of the PT, the lower one appears first: SiC and BrF3. Chemical Formulas Structural Formulas   The formulas of C-­‐containing compounds start with C followed by H. A�er that, any other elements appear in alphabe�cal order: C2H6O, C4H9BrO, CH3Cl and C8H10N4O2.   Gives the # of atoms and how the atoms are connected to one another. The atoms in molecules have specific arrangements because they are held together by a�rac�ve forces called bonds. A bond is the result of coulombic a�rac�on between posi�vely charged nuclei and nega�vely charged electrons. We will discuss this later in the course. For now, it is sufficient to know that a pair of electrons shared between two atoms generates a chemical bond. Structural Formulas Three-­‐Dimensional Models   In a structural formula, the bonds between atoms are represented by lines connec�ng the elemental symbols.   Examples   A molecule is a 3D array of atoms. Many of a molecule’s most important proper�es, such as odor and chemical reac�vity, depend on its 3D shape. In ball-­‐and-­‐s�ck models, balls represent atoms, and s�cks represent chemical bonds. The balls are labeled with elemental symbols or different colors (each atom is a different color). 2 10/2/11 Three-­‐Dimensional Models Line Structures   Describing and understanding C chemistry requires the ability to visualize 3D molecular structure but wri�ng complete structural formulas becomes very tedious for all but the simplest molecules. Hence, line structures, have been developed. Rules:   1. All bonds except C-­‐H are shown as lines.   2. C-­‐H bonds are not shown in the line structure. Line Structures   3. Single bonds are shown as a single line; double bonds are shown as two lines; triple bonds are shown as three lines.   4. C atoms are not labeled.   5. All atoms except C and H are labeled with the elemental symbols.   6. H atoms are labeled when they are a�ached to any atoms other than C. Naming Chemical Compounds   Guidelines for naming binary compounds are as follows:   1. The element that appears first retains its elemental name.   2. The second element bears a root derived from its elemental name and ends with the suffix –ide. Line Structures   Examples Naming Chemical Compounds Element As Br C Cl F H I N O P S Se Root ArsenBromCarbChlorFluorHydrIodNitrOxPhosphSulfSelen- 3 10/2/11 Naming Chemical Compounds   3. When there is more than one atom of a given element in the formula, the name of the element usually contains a prefix that specifies the # of atoms present. If the numerical prefix ends with the le�er o or a and the name of the element begins with a vowel, the last le�er of the prefix is dropped. For example, use monoxide instead of monooxide and tetroxide instead of tetraoxide. Naming Chemical Compounds # 1 2 3 4 5 6 7 Prefix Mon(o)DiTriTetr(a)Pent(a)Hex(a)Hept(a)- Example CO SiO2 NI3 CCI4 PCI5 SF6 IF7 Name Carbon monoxide Silicon dioxide Nitrogen triiodide Carbon tetrachloride Phosphorus pentachloride Sulfur hexafluoride Iodine heptafluoride Ionic Compounds Ionic Compounds   Some substances exist as collec�ons of ca�ons and anions arranged so that the + and – charges are balanced. Any stable sample of ma�er is electrically neutral. This principle helps determine how ionic compounds are organized.   Many ionic compounds contain atomic ions. The elements classified as metals have a strong tendency to lose electrons and form atomic ca�ons. Almost every compound whose formula contains a metallic element from group 1/2 is ionic.   The transi�on metals (TMs) o�en form ionic species but these elements also form numerous compounds by sharing electrons. Al, Sn and Pb also tend to form atomic ca�ons.   Examples Li+, Na+, K+, Cs+ Mg2+, Ca2+, Ba2+ Cu+ or Cu2+ Ag+, Cd2+, Zn2+ Ionic Compounds Ionic Compounds   Although many elements form stable atomic ca�ons, only six form stable atomic anions. Examples F-­‐, Cl-­‐, Br-­‐, I-­‐, O2-­‐ and S2-­‐   Molecular ions are also called polyatomic ions to dis�nguish them from neutral molecules and atomic ions. Examples H3O+ hyydronium ion NH4+ ammonium ion hydroxide OH-­‐ cyanide CN-­‐ 4 10/2/11 Ionic Compounds Ionic Compounds   Most polyatomic anions, called oxyanions, contain a central atom surrounded by one or four oxygen atoms. Rules:   1. The name has a root taken from the central atom (carbonate, CO32-­‐ and nitrite, NO2-­‐).   2. When an element forms two different oxyanions, the one with fewer oxygen atoms ends in –ite and the other ends in –ate (SO32-­‐, sulfite and SO42-­‐, sulfate)   3. Cl, Br and I each form four different oxyanions that are dis�nguished by prefixes and suffixes. BrO-­‐, hypobromite; BrO2-­‐, bromite; BrO3-­‐, bromate; BrO4-­‐, perbromate.   4. Polyatomic anions with a charge more nega�ve than -­‐1 may add a hydrogen ion (H+) to give another anion. HCO3-­‐, hydrogencarbonate; HPO42-­‐, hydrogenphosphate; H2PO4-­‐, dihydrogenphosphate. 5 ...
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