Chapter 4

4.1 The Elements 4.2 Symbols for the Elements 4.3 Dalton’s Atomic Theory 4.4 Formulas of Compounds 4.5 The Structure of the Atom 4.6 Introduction to the Modern Concept of Atomic Structure 4.7 Isotopes 4.8 Introduction to the Periodic Table 4.9 Natural States of the Elements 4.10 Ions 4.11 Compounds That Contain Ions

Alchemists Alchemist tried to make lead into gold. They studies many materials and used mysterious symbols to represent these materials.

115 known: 88 found in nature, others are man made. Just as you had to learn the 26 letters of the alphabet before you learned to read and write, you need to learn the names and symbols of the chemical elements before you can read and write chemistry.

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How the Term Element is Used Could mean a single atom of that element (Ar or H). Could mean molecules of an element (H2, N2, O2, F2, Cl2, Br2, I2), which are found in their natural states. Could mean atoms of elements are present in some form (sodium found in the human body). Look at each particular case to determine its proper use. Copyright © Cengage Learning. All rights reserved

Each element has a unique one- or two-letter symbol. First letter is always capitalized and the second is not. The symbol usually consists of the first one or two letters of the element’s name. Examples: Oxygen O Krypton Kr Sometimes the symbol is taken from the element’s original Latin or Greek name. Examples: Gold Au aurum Lead Pb plumbum Copyright © Cengage Learning. All rights reserved

Names and Symbols of the Most Common Elements You Should Know plus: Kr-krypton Se-selenium Cs-cesium Po-polonium Rn-Radon Fr-Francium

Most natural materials are mixtures of pure substances. Pure substances are either elements or combinations of elements called compounds. A given compound always contains the same proportions (by mass) of the elements. Copyright © Cengage Learning. All rights reserved

Law of Constant Composition A given compound always has the same composition, regardless of where it comes from. Water always contains 8 g of oxygen for every 1 g of hydrogen. Carbon dioxide always contains 2.7 g of oxygen for every 1 g of carbon. Copyright © Cengage Learning. All rights reserved

Dalton’s Atomic Theory (1808) Elements are made of tiny particles called atoms. 2. All atoms of a given element are identical. 3. The atoms of a given element are different from those of any other element. Atoms of one element can combine with atoms of other elements to form compounds. A given compound always has the same relative numbers and types of atoms. Copyright © Cengage Learning. All rights reserved

Dalton’s Atomic Theory (continued) 5. Atoms are indivisible in chemical processes. Atoms are not created or destroyed in chemical reactions. A chemical reaction simply changes the way the atoms are grouped together. Copyright © Cengage Learning. All rights reserved

Scanning Tunneling Microscope

Concept Check Which of the following statements regarding Dalton’s atomic theory are still believed to be true? Elements are made of tiny particles called atoms. All atoms of a given element are identical. A given compound always has the same relative numbers and types of atoms. IV. Atoms are indestructible. Statements I and III are true. Statement II is not true (due to isotopes and ions). Statement IV is not true (due to nuclear chemistry). Copyright © Cengage Learning. All rights reserved

Chemical Formulas Describe Compounds Compound – distinct substance that is composed of the atoms of two or more elements and always contains exactly the same relative masses of those elements. Chemical Formulas – expresses the types of atoms and the number of each type in each unit (molecule) of a given compound. Copyright © Cengage Learning. All rights reserved

Rules for Writing Formulas Each atom present is represented by its element symbol. The number of each type of atom is indicated by a subscript written to the right of the element symbol. When only one atom of a given type is present, the subscript 1 is not written. Copyright © Cengage Learning. All rights reserved

Exercise The pesticide known as DDT paralyzes insects by binding to their nerve cells, leading to uncontrolled firing of the nerves. Before most uses of DDT were banned in the U.S., many insects had developed a resistance to it. Write out the formula for DDT. It contains 14 carbon atoms, 9 hydrogen atoms, and 5 atoms of chlorine. C14H9Cl5 C14H9Cl5 Copyright © Cengage Learning. All rights reserved

Postulated the existence of electrons using cathode-ray tubes. J. J. Thomson (1856 -1940) Postulated the existence of electrons using cathode-ray tubes. The atom must also contain positive particles that balance exactly the negative charge carried by particles that we now call electrons. Copyright © Cengage Learning. All rights reserved

Cathode-Ray Tube - e- +

William Thomson (Lord Kelvin) Plum Pudding Model 1824-1907 Reasoned that the atom might be thought of as a uniform “pudding” of positive charge with enough negative electrons scattered within to counterbalance that positive charge.

Rutherford’s Experiment http://micro.magnet.fsu.edu/electromag/java/rutherford/

Explained the nuclear atom. Ernest Rutherford (1911) 1871-1937 Nobel Prize 1908 Explained the nuclear atom. Atom has a dense center of positive charge called the nucleus. Electrons travel around the nucleus at a relatively large distance. A proton has the same magnitude of charge as the electron, but its charge is positive.

Rutherford and Chadwick (1932) Most nuclei also contain a neutral particle called the neutron. A neutron is slightly more massive than a proton but has no charge.

The atom contains: Electrons – found outside the nucleus; negatively charged Protons – found in the nucleus; positive charge equal in magnitude to the electron’s negative charge; atomic number Neutrons – found in the nucleus; no charge; virtually same mass as a proton-atomic mass p+n

The nucleus is: Small compared with the overall size of the atom like a basketball in a horse track. Extremely dense; accounts for almost all of the atom’s mass.

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Why do different atoms have different chemical properties? The chemistry of an atom arises from its electrons. Electrons are the parts of atoms that “intermingle” when atoms combine to form molecules. It is the number of electrons that really determines chemical behavior. Copyright © Cengage Learning. All rights reserved

In nature most elements contain mixtures of isotopes. Atoms with the same number of protons but different numbers of neutrons. Show almost identical chemical properties; chemistry of atom is due to its electrons. In nature most elements contain mixtures of isotopes. Copyright © Cengage Learning. All rights reserved

Two Isotopes of Sodium Copyright © Cengage Learning. All rights reserved

Isotope Notation Protons Neutrons Electrons 6 6 6 6 7 6 6 8 6 6 6 6 6 7 6 6 8 6 How many protons, neutrons and electrons in each of the following: protons neutrons electrons 23Na 14N 38Ar 35Cl 36Cl-1 56Fe 11 12 11 7 7 7 18 20 18 17 18 17 17 19 18 26 30 26

What is the mass number of this isotope? Identify the element. Exercise A certain isotope X contains 23 protons and 28 neutrons. What is the mass number of this isotope? Identify the element. Mass Number = 51 Vanadium The mass number is 51. Mass Number = # protons + # neutrons. Mass Number = 23 + 28 = 51. The element is vanadium. The number of protons determines the identity of the element.

Mendeleev’s Table Mendeleev’s Table Russian chemist Dmitri Mendeleev developed and published the basic arrangement of the periodic table between 1869 and 1871. Mendeleev arranged the elements in order of increasing relative atomic mass (protons had not been discovered yet). The elements on the modern periodic table are arranged in order of increasing atomic number. He also grouped elements with similar properties into columns and rows so that the properties of the elements varied in a regular pattern (periodically). Figure 2.18 2 -

Mendeleev’s Table 2 -

Most elements are metals and occur on the left side. The Periodic Table Most elements are metals and occur on the left side. The nonmetals appear on the right side. Metalloids are elements that have some metallic and some nonmetallic properties. Copyright © Cengage Learning. All rights reserved

Periods – horizontal rows of elements The Periodic Table Groups or Families – elements in the same vertical columns; have similar chemical properties Periods – horizontal rows of elements Copyright © Cengage Learning. All rights reserved

Physical Properties of Metals Efficient conduction of heat and electricity Malleability (they can be hammered into thin sheets) Ductility (they can be pulled into wires) A lustrous (shiny) appearance Magnesium Platinum Sliver Gold Copper

Most elements are very reactive. Section 4.9 Natural States and Elements Most elements are very reactive. Elements are not generally found in uncombined form. Exceptions are: Noble metals – gold, platinum and silver Noble gases – Group 8 Helium Neon Argon Krypton Xenon

Natural States and Elements Section 4.9 Natural States and Elements Diatomic Molecules Nitrogen gas contains N2 molecules. Oxygen gas contains O2 molecules.

Natural States and Elements Section 4.9 Natural States and Elements Diatomic Molecules Copyright © Cengage Learning. All rights reserved

Nonmetals Liquid Nitrogen Phorphorus Fluorine Selenium Graphite Chlorine Bromine Sulfur

Different forms of a given element. Example: Section 4.9 Natural States and Elements Carbon Allotropes Different forms of a given element. Example: Solid carbon occurs in three forms. Diamond Graphite Buckminsterfullerene

Atoms can form ions by gaining or losing electrons. Section 4.10 Ions Atoms can form ions by gaining or losing electrons. Metals tend to lose one or more electrons to form positive ions called cations. Cations are generally named by using the name of the parent atom. Copyright © Cengage Learning. All rights reserved

Section 4.10 Ions Nonmetals tend to gain one or more electrons to form negative ions called anions. Anions are named by using the root of the atom name followed by the suffix –ide. Copyright © Cengage Learning. All rights reserved

Ion Charges and the Periodic Table Section 4.10 Ions Ion Charges and the Periodic Table The ion that a particular atom will form can be predicted from the periodic table. Group or Family Charge Alkali Metals (1A) 1+ Alkaline Earth Metals (2A) 2+ Halogens (7A) 1– Noble Gases (8A) Grand Secret of Chemistry-All atoms want to have the same number of electrons as the Noble Gases.

Conductivity of Solutions Containing Ions

Ion Charges and the Periodic Table Section 4.10 Ions Ion Charges and the Periodic Table Copyright © Cengage Learning. All rights reserved

An ion with a 3+ charge contains 23 electrons. Which ion is it? Section 4.10 Ions Exercise An ion with a 3+ charge contains 23 electrons. Which ion is it? a) Fe3+ b) V3+ c) Ca3+ d) Sc3+ The 3+ charge on the ion means that the ion has lost 3 electrons, therefore the number of protons is 26 (23+3). The ion is Fe3+. Copyright © Cengage Learning. All rights reserved

A certain ion X+ contains 54 electrons and 78 neutrons. Section 4.10 Ions Exercise A certain ion X+ contains 54 electrons and 78 neutrons. What is the mass number of this ion? 133 The mass number is 133. The plus charge in X+ means that the ion has lost an electron, therefore the number of protons is 55 (54+1). The ion is Cs+ with a mass number of 133 (55+78). Copyright © Cengage Learning. All rights reserved

Ions combine to form ionic compounds. Properties of ionic compounds Section 4.11 Compounds that Contain Ions Ions combine to form ionic compounds. Properties of ionic compounds High melting points Conduct electricity If melted If dissolved in water

Ionic compounds are electrically neutral. Section 4.11 Compounds that Contain Ions Ionic compounds are electrically neutral. The charges on the anions and cations in the compound must sum to zero.

Formulas for Ionic Compounds Section 4.11 Compounds that Contain Ions Formulas for Ionic Compounds Write the cation element symbol followed by the anion element symbol. The number of cations and anions must be correct for their charges to sum to zero. Copyright © Cengage Learning. All rights reserved

a) Ti2+ b) Sc+ c) Ca2+ d) Cr2+ Compounds that Contain Ions Section 4.11 Compounds that Contain Ions Concept Check A compound contains an unknown ion X and has the formula XCl2. Ion X contains 20 electrons. What is the identity of X? a) Ti2+ b) Sc+ c) Ca2+ d) Cr2+ The charge on the cation must be +2 since there are two Cl’s each with a –1 charge (giving an overall charge of –2 for the anion side). The cation can now be represented as X2+, containing 20 electrons. Therefore, 22 protons must be present to give a charge of +2 (+22p – 20e = +2). The element with 22 protons is titanium. Copyright © Cengage Learning. All rights reserved

a) CaBr2 b) KrBr c) RbBr d) SrBr2 Compounds that Contain Ions Section 4.11 Compounds that Contain Ions Concept Check A member of the alkaline earth metal family whose most stable ion contains 36 electrons forms a compound with bromine. What is the correct formula for this compound? a) CaBr2 b) KrBr c) RbBr d) SrBr2 An element in the alkaline earth metal family (Group 2) forms a +2 charge when forming a compound. Therefore two Br– ions will be required to give a net zero charge overall. 38 protons must be present to give a charge of +2 (+38p – 36e = +2). The element with 38 protons is strontium. The compound is therefore SrBr2. Copyright © Cengage Learning. All rights reserved