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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
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 #
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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- Fall '11