Chapter Outline. 3 Elements and Compounds. Elements and Atoms. Elements. Elements. Elements 9/4/2013

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1 3 Elements and Compounds Chapter Outline 3.1 Elements A. Distribution of Elements Foundations of College Chemistry, 14 th Ed. Morris Hein and Susan Arena Copyright This reclining Buddha in Thailand is made of gold. B. Names of the Elements C. 3.2 Introduction to the Periodic Table A. Natural States of the Elements B. Diatomic Elements 3.3 Compounds and Formulas A. Molecular and Ionic Compounds B. Writing Formulas of Compounds C. Composition of Compounds Elements and Atoms Elements An element is a fundamental substance that cannot be broken down by chemical means into a simpler substance. Elements are the building blocks of matter. Elements can occur naturally or be synthesized in labs. The smallest unit of an element that retains its properties and chemical behavior is called an atom. Atoms are made up of subatomic particles, but they do not have the properties of the element. Currently, 118 elements are known. 88 of these elements are naturally occurring. At room temperature, only bromine (Br) and mercury (Hg) are liquids. At room temperature, 11 elements are gases: hydrogen (H 2 ), nitrogen (N 2 ), oxygen (O 2 ), fluorine (F 2 ), chlorine (Cl 2 ), helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). Elements Ten elements make up almost 99 % of the mass of the Earth s crust, seawater and atmosphere. Elements The most prevalent elements in the human body are oxygen (65 %), carbon (18 %) and hydrogen (10 %). Some elements in the body are present in tiny amounts, like chromium and copper, but are crucial for life. Oxygen accounts for about 20 % of the atmosphere and is found in nearly all rocks, sand and soil. 1

2 Names of Elements Many element names are derived from Greek, Latin or German words that describe a property of the element. Iodine comes from the Greek word iodes meaning violetlike and iodine is violet colored as a vapor. Some elements are named for the location of their discovery, such as germanium which was discovered in 1886 by a German chemist. Other elements commemorate famous scientists, like curium which is named after Marie Curie. Each element has an abbreviation called a symbol. Fourteen elements have single letter symbols and the rest have two letter symbols. iodine (I) barium (Ba) What element has the following symbol? nitrogen gold The table below contains symbols for the most common elements. The table below contains symbols of elements derived from early names. The first letter of a symbol must always be capitalized. If a second letter is needed, it should be lowercase. Consider the element cobalt: Cobalt has the symbol Co Introduction to the Periodic Table The Periodic Table was designed by Dimitri Mendeleev in 1869 to organize elements based on their properties. Elements with similar chemical properties are placed in columns called groups. Four groups have special identifying names, like Noble Gases, which are all unreactive gases. Writing CO would symbolize elements C (carbon) and O (oxygen) and would represent carbon monoxide. In order to succeed in chemistry, you should work to learn many of the elements on the periodic table. 2

3 Introduction to the Periodic Table The eight tall columns are called representative elements, or main group elements. These are shown in red. Introduction to the Periodic Table Elements can be further classified as metals, metalloids and nonmetals. The elements in the center are called the transition metals. These are shown in purple. Metals Metals are: solids at room temperature (except mercury) shiny good conductors of heat and electricity malleable (can be shaped) ductile (can be drawn into wires) Most metals have a high melting point and density. Common metals include aluminum, gold, platinum, silver, tin and iron. Metals Metals readily combine with nonmetals to form compounds but rarely combine with other metals. Some metals are found in their free states, like copper, gold and silver. Some metals are mixed to form homogeneous mixtures called alloys, like brass, bronze, steel and coinage metals. Nonmetals Nonmetals: are not shiny have fairly low melting points and densities are poor conductors of heat and electricity Nonmetals combine with each other to form molecular compounds and with metals to form ionic compounds. Common nonmetals found naturally include carbon, nitrogen, oxygen and sulfur. Metalloids Metalloids have properties between metals and nonmetals. These elements are positioned diagonally on the Periodic Table separating the metals and nonmetals. Certain metalloids, like boron, silicon and germanium are the materials used in electronic devices. 3

4 Let s Practice! Natural States of the Elements a. S b. Si c. Ag d. Au e. Sr What is the symbol for silver? sulfur silicon silver gold strontium Which of the following elements are metals? Na, Mo, Cl, S, Mg, Pt, Kr, I, C, Cu Elements tend to be reactive and combine with other elements to form compounds. It is rare to find elements in nature in their pure forms. Gold, silver and platinum (the noble metals) have low reactivity and are found uncombined in nature. Air is composed mainly of nitrogen and oxygen, a homogeneous mixture of diatomic elements. Na, Mo, Mg, Pt, Cu Metals are typically found on the left of the Periodic Table. Diatomic Elements Seven elements exist as diatomic molecules. These molecules contain exactly two atoms. Compounds and Formulas A compound is a substance containing two or more elements that are chemically combined in a definite proportion by mass. Compounds, unlike elements, can be decomposed chemically into simpler substances. Diatomic elements can be separated. In water (H 2 O), neither hydrogen or oxygen is diatomic. Elements in compounds are always combined in whole number ratios. Molecular and Ionic Compounds Compounds fall into two general types, molecular and ionic. Molecules and Ions Molecules are the smallest unchanged individual unit of a compound formed by two or more atoms. A molecule cannot be further divided without destroying its identity. Ions are charged atoms or groups of atoms. Ions can be positively or negatively charged. Molecular compounds are held together by covalent bonds and ionic compounds are held together by attractive forces between the positive and negative charges. A positively charged ion is called a cation. A negatively charged ion is called an anion. 4

5 Compounds Writing Formulas of Compounds More than 50 million compounds are currently known. Each compound has a unique set of properties. Consider water (H 2 O) and hydrogen peroxide (H 2 O 2 ). Property H 2 O H 2 O 2 color clear pale blue boiling point 100 C 150 C density 1.00 g/ml 1.44 g/ml Chemical formulas are abbreviations for compounds. A chemical formula contains the symbols and ratio of the atoms of the elements in a compound. Consider sulfuric acid. 1 molecule contains 2 H atoms, 1 S atom and 4 O atoms. One possible formula for sulfuric acid is HHSOOOO. A better formula is H 2 SO 4 H-two-S-O-four. The numbers after the element symbols are subscripts. Writing Formulas of Compounds 1. The formula of a compound contains the symbols of all the elements in the compound. 2. When a formula contains only one atom of an element, the number 1 is not needed as a subscript. 3. When a formula contains more than one atom of an element, the number is indicated by a subscript written after the symbol. Writing Formulas of Compounds 4. When the formula contains more than one of a group of atoms that occur as a unit, parentheses are placed around the group and the number of units is represented by a subscript outside the parentheses. 5. Formulas show the number and kind of atoms in a compound, but not the connectivity of the elements. Let s Practice! How many of each element is present in K 3 PO 4? 3 K atoms 1 P atom 4 O atoms How many of each element is present in Mg(OH) 2? 1 Mg atom 2 O atoms 2 H atoms Composition of Compounds A particular compound always contains the same elements in the same proportions by mass no matter what the origin. H 2 O 11.2 % H 88.8 % O 2 H + 1 O H 2 O % H 94.1 % O 2 H + 2 O The Law of Definite Composition states that a compound always contains two or more elements chemically combined in a definite proportion by mass. 5

6 Composition of Compounds Laws and Theories The Law of Multiple Proportions states that atoms of two or more elements may combine in different ratios to produce more than one compound. A law is a summary of observed behavior. A theory or model is an attempt to explain the observed behavior. Laws remain constant and do not undergo modification, while theories sometimes fail and are modified or discarded over time. 6