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CHAPTER 2 Atoms and the Periodic Table General, Organic, & Biological Chemistry Janice Gorzynski Smith CHAPTER 2: Atoms & the Periodic Table Learning Objectives: ! Elemental Symbols ! Metals vs Nonmetals vs Metalloids or Semimetals ! Subatomic Particles: properties & location ! Formulae of Compounds ! Models to represent particles ! Nucleus and structure of atom ! Atomic number, Mass Number, Isotopes, Atomic Weight, & Atomic Mass ! Periodic Table: groups & periods: similar properties within groups ! Electron structure: valence electrons and electron dot symbols ! Periodic trends: atomic size and ionization energy 2 Smith. General Organic & Biolocial Chemistry 2nd Ed. Matter Definition 3 http://ridenourmhs.wikispaces.com/ESUnit2 Matter Elements • An element is a pure substance that cannot be broken down into simpler substances by a chemical reaction. • Each element is identified by a one- or two-letter symbol. • Elements are arranged in the periodic table. • The position of an element in the periodic table tells us much about its chemical properties. diamond = carbon Smith. General Organic & Biolocial Chemistry 2nd Ed. gold sulfur 4 Definition Matter 5 http://ridenourmhs.wikispaces.com/ESUnit2 Compounds Matter Compound: a pure substance formed by chemically combining two or more elements together. A chemical formula consists of: • Element symbols to show the identity of the elements forming a compound. • Subscripts to show the ratio of atoms in the compound. H 2O 2 H atoms 1 O atom Smith. General Organic & Biolocial Chemistry 2nd Ed. C 3H 8 3 C atoms 8 H atoms 6 Compounds Matter Compounds can be drawn many ways: Different elements are represented by different colors: 7 Smith. General Organic & Biolocial Chemistry 2nd Ed. Depicting Compounds Matter CH4 methane H H C H H Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 8 Atoms Subatomic Particles All matter is composed of the same basic building blocks called atoms. Atoms are composed of three subatomic particles: Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E Atoms 9 Subatomic Particles Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 10 Subatomic Particles Atoms Opposite charges attract while like charges repel each other. Protons and electrons attract each other, but two electrons repel each other. 11 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E Subatomic Particles Atoms From the periodic table: 3 Li Atomic number (Z) is the number of protons in the nucleus. • Every atom of a given element has the same number of protons in the nucleus. • Different elements have different atomic numbers. • A neutral atom has no net overall charge, so Z = number of protons = number of electrons Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 12 Atoms Subatomic Particles Isotopes are atoms of the same element that have a different number of neutrons. Mass number (A) = the number of protons (Z) + the number of neutrons Mass number (A) 35 Atomic number (Z) 17 # of protons Cl = 17 # of electrons = 17 # of neutrons 35 – 17 = 18 = 13 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E Atoms Atomic Weight The atomic weight is the weighted average of the masses of the naturally occurring isotopes of a particular element reported in atomic mass units. From the periodic table: 6 C 12.01 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E atomic number element symbol atomic weight (amu) 14 Determine the Atomic Weight of an Element Atoms Example What is the atomic weight of chlorine? Step [1] List each isotope, it s mass in atomic mass units, and it s abundance in nature. Isotope Mass (amu) Isotopic Abundance Cl-35 34.97 75.78% = 0.7578 Cl-37 36.97 24.22% = 0.2422 15 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E Determine the Atomic Weight of an Element Atoms Step [2] Multiply the isotopic abundance by the mass of each isotope, and add up the products. The sum is the atomic weight of the element. 34.97 x 0.7578 = 26.5003 amu 36.97 x 0.2422 = 8.9541 amu 35.4544 amu = 4 sig. figs. Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 35.45 amu Answer 4 sig. figs. 16 Periodic Table Elements Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E Periodic Table 17 Elements 18 Periodic Table Groups 1A 2A B B 7A 8A Alkali Metals Alkaline Earth Metals Transition Metals Lanthanide & Actinide Halogens Nobel Gases Very reactive Reactive Metals Metals except for H Form ions with several different charges (oxidation states) +1 ions React with Oxygen to form compounds that dissolve into alkaline solutions in water +2 ions Oxygen compounds are strongly alkaline Many are not water soluble Tend to form +2 and +3 ions Lanthanides 58 – 71 Actinides 90 – 103 Reactive Inert Form diatomic molecules in elemental state Heavier elements have limited reactivity -1 ions Actinides are Salts with radioactive alkali metals Smith. General Organic & Biolocial Chemistry 2nd Ed. Periodic Table Do not form ions Monoatomic gases 19 Metals, Nonmetals, Metalloids Smith. General Organic & Biolocial Chemistry 2nd Ed. 20 Periodic Table Metals, Nonmetals, Metalloids Metals Nonmetals Metalloids • Brittle, dull • Metallic luster, malleable, ductile, hardness variable • Properties intermediate between metals and nonmetals • Insulators, nonconductors of electricity and heat • Conduct heat and electricity • Chemical reactivity varies • Metallic shine but brittle FIRST PAGES ELECTRONIC STRUCTURE • Solids at room temperature with the exception of Hg • Semiconductors: conduct electricity but • Exist mostly as 49 compounds rather not as well as metals: then pure elements examples are silicon 2.4C The Unusual Nature of Carbon • Chemical reactivity and germanium Carbon, a second-row element in 4A ofare the periodic • group Many gases,table, is different from most other elevaries greatly: Au, Pt ments in that it has three elemental forms (Figure The two some are2.6). solids at most common forms of carbon are unreactive Na, K is hard because it contains a dense three-dimensional network of diamond andwhile graphite. Diamond room temp, only Br2 is carbon atoms in six-membered rings. Graphite, on the other hand, is a slippery black substance very reactive a liquid. used as a lubricant. It contains parallel sheets of carbon atoms in flat six-membered rings. Buckminsterfullerene, also referred to as a bucky ball, is a third form that contains 60 carbon atoms joined together in a sphere of 20 hexagons and 12 pentagons in a pattern that resembles a Smith. General Organic & Biolocial Chemistry 2nd Ed. soccer ball. A component of soot, this form of carbon was not discovered until 1985. Its unusual name stems from its shape, which resembles the geodesic dome invented by R. Buckminster Fuller. 21 Carbon’s ability to join with itself and other elements gives it versatility not seen with any other element in the periodic table. In the unscientific but eloquent description by writer Bill Bryson in A Short History of Nearly Everything, carbon is described as “the party animal of the atomic world, latching on to many other atoms (including itself) and holding tight, forming molecular conga lines of hearty robustness—the very trick of nature necessary to build proteins and DNA.” As a result, millions of compounds that contain the element carbon are known. The chemistry of these compounds is discussed at length in Chapters 11–24. Carbon Atoms Figure 2.6ability Three Elemental Forms of Carbon Carbon’s to join with itself and other elements a. Diamond b. Graphite c. Buckminsterfullerene gives it a versatility not seen with any other element in the periodic table. Elemental forms of carbon include the following carbon-only structures: • Diamond consists of an intricate three-dimensional network of carbon atoms. diamond • Graphite contains parallel sheets of carbon atoms. graphite • Buckminsterfullerene contains a sphere with buckminsterfullerene 60 carbon atoms. Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 2.5 Electronic Structure Why do elements in a group of the periodic table have similar chemical properties? The chemical 22 Atoms Electron Configurations & Orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E Atoms 23 Electron Configurations & Orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 24 Electron Configurations & Orbitals Atoms 25 http://chemwiki.ucdavis.edu/ Valence Electrons Atoms • The chemical properties of an element depend on the number of electrons in the valence shell. • The valence shell is the outermost shell (the highest value of n). • The electrons in the valence shell are called valence electrons. Be Cl 1s22s2 1s22s22p63s23p5 valence shell: n = 2 valence shell: n = 3 # of valence electrons = 2 # of valence electrons = 7 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 26 Valence Electrons Atoms • Elements in the same group have similar electron configurations. • Elements in the same group have the same number of valence electrons. • The group number, 1A–8A, equals the number of valence electrons for the main group elements. • The exception is He, which has only 2 valence electrons. • The chemical properties of a group are therefore very similar. 27 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E Valence Electrons Atoms Group number: 1A 2A 3A 4A Period 1: H 1s1 5A 6A 7A 8A He 1s2 Period 2: Li 2s1 Be B C N O F Ne 2 2 1 2 2 2 3 2 4 2 5 2s 2s 2p 2s 2p 2s 2p 2s 2p 2s 2p 2s22p6 Period 3: Na Mg Al Si P S Cl Ar 1 2 2 1 2 2 2 3 2 4 2 5 3s 3s 3s 3p 3s 3p 3s 3p 3s 3p 3s 3p 3s23p6 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 28 Atoms Valence Electrons • Dots representing valence electrons are placed on the four sides of an element symbol. • Each dot represents one valence electron. • For 1 to 4 valence electrons, single dots are used. With > 4 valence electrons, the dots are paired. H C O Cl # of Valence electrons: 1 4 6 7 Electron-dot symbol: H C O Cl Element: 29 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E Atoms Periodic Trends Increases The size of atoms increases down a column, as the valence e! are farther from the nucleus. Decreases • The size of atoms decreases across a row, as the number of protons in the nucleus increases, pulling the valence electrons in closer. Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 30 Atoms Periodic Trends The ionization energy is the energy needed to remove an electron from a neutral atom. Na + energy Na+ + e– Decreases Increases • Ionization energies decrease down a column as the valence e! get farther away from the positively charged nucleus. • Ionization energies increase across a row as the number of protons in the nucleus increases. Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 31