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Table 2-1 Concept 2.2: An element’s properties depend on the structure of its atoms • Each element consists of unique atoms • An atom is the smallest unit of matter that still retains the properties of an element. • Atoms of the same element share similar properties. • Atoms have symbols which are the same as that of the element made up of those atoms. Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Subatomic Particles • Atoms are composed of subatomic particles • Relevant subatomic particles include: – Neutrons (no electrical charge, mass of one atomic mass unit, or amu) – Protons (positive charge, mass of 1 amu) – Electrons (negative charge, mass of 1/2000 amu) ____________________________________ Dalton is a unit used to express mass at the atomic level. 1 Dalton = 1.67 x 10-24 gram. Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Subatomic Particles • Neutrons and protons form the atomic nucleus at the center of the atom. • Electrons form a cloud around the nucleus Fig. 2-5 Cloud model Shell model Cloud of negative charge (2 electrons) Electrons Nucleus (a) (b) Subatomic Particles Nucleus Protons (+ charge) determine element Neutrons (no charge) determine isotope Electrons (– charge) form negative cloud and determine chemical behavior Atom An atom’s nucleus contains __________. • protons • neutrons • electrons • protons and neutrons An Element’s Atomic Number • Atoms of the various elements differ in number of subatomic particles • An atom before interacting with other atoms is electrically neutral, i.e. in this atom: the # of protons = the # of electrons • An element’s atomic number is the number of protons in its nucleus. Therefore, Atomic number = # of protons = # of electrons Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Atomic Mass number • An element’s mass number is the sum of protons plus neutrons in the nucleus • Mass number = # of protons + # of neutrons • Atomic mass, the atom’s total mass, can be approximated by the mass number The element lithium has 3 protons and 4 neutrons in its nucleus. Its mass number is __________. • 1 • 3 • 7 • 12 Bohr Model What is the atomic # and Mass # of each of these atoms in the Bohr models Isotopes • All atoms of an element have the same number of protons but may differ in number of neutrons • Isotopes are two or more atoms of an element that differ in number of neutrons. Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Electron Distribution and Chemical Properties • The distribution of electrons in the electron shells follow usually the formula: X = 2n2 Where: X is the maximum number of electrons in an electron shell and n is the number of that electron shell in terms of its distance from the nucleus. Hydrogen 1H Atomic mass First shell 2 He 4.00 Atomic number Helium 2He Element symbol Electrondistribution diagram Lithium 3Li Beryllium 4Be Boron 5B Carbon 6C Nitrogen 7N Oxygen 8O Fluorine 9F Neon 10Ne Silicon 14Si Phosphorus 15P Sulfur 16S Chlorine 17Cl Argon 18Ar Second shell Sodium Magnesium Aluminum 12Mg 11Na 13Al Third shell The Octet Rule • Valence electrons are those in the outermost shell, or valence shell • The chemical behavior of an atom is mostly determined by the valence electrons • Elements with a full valence shell are chemically inert • The Octet Rule: Rule that a valence shell is complete when it contains eight electrons except for H and He Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Chemically Inert Elements • Stable and unreactive • Inert elements have their outermost energy level fully occupied by electrons Chemically Reactive Elements • Reactive elements do not have their outermost energy level fully occupied by electrons Concept 2.3: The formation and function of molecules depend on chemical bonding between atoms • Atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms • These interactions usually result in atoms staying close together, held by attractions called chemical bonds. Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Losing, Gaining, or Sharing Electrons • Atoms having 1, 2, or 3 electrons in their valence shell tend to lose electrons. • Atoms having 4 or 5 electrons in their valence shell tend to share electrons. • Atoms having 6 or 7 electrons in their valence shell tend to gain or share electrons depending on their partner atom. The most important determinant of an atom’s bonding behavior is __________. • the number of protons in the nucleus • the total number of electrons • the number of valence shell electrons • the number of neutrons in the nucleus Types of Chemical Bonds 1- Covalent bonds (the strongest). 2- Ionic Bonds (strong, but fragile). http://www.youtube.com/watch?v=QqjcCvzW www 3- Hydrogen Bonds (weak). 4- Van der Waals Interactions (weak). 5- Hydrophobic Interactions (weak). Covalent Bonds • A covalent bond is the sharing of a pair of valence electrons by two atoms • In a covalent bond, the shared electrons count as part of each atom’s valence shell • A molecule consists of two or more atoms held together by covalent bonds Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Covalent Bonds • The atoms of four elements of the human body form covalent bonds most commonly: – oxygen (O) – carbon (C) – hydrogen (H) – nitrogen (N) Covalent Bonds • Covalent bonds can form between atoms of the same element or atoms of different elements • Every atom has a characteristic total number of covalent bonds that it can form, this number is called an atom’s valence. – The valence of H=1, O=2, N=3, and C=4 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings An Atom’s Valence Hydrogen (valence = 1) Oxygen (valence = 2) Nitrogen (valence = 3) Carbon (valence = 4) H O N C Electrons Are Shared in Covalent Bonds Single, double, and triple covalent bonds • Single covalent bond – One pair of electrons shared • e.g., between two hydrogen atoms • Double covalent bond – Two pairs of electrons shared • e.g., between two oxygen atoms • Triple covalent bond – Three pairs of electrons shared • e.g., between two nitrogen atoms Nonpolar and polar covalent bonds • Atoms in a covalent bond may share electrons equally or unequally • How they share is determined by electronegativity • Electronegativity: • Is the relative attraction of particular atom for the electrons of a covalent bond. • High electronegativity = electrons spend more time orbiting the nucleus • Is determined by the number of protons in the nucleus and the proximity of valence electrons to the nucleus Fig. 2-9 Hydrogen 1H Atomic mass First shell 2 He 4.00 Atomic number Helium 2He Element symbol Electrondistribution diagram Lithium 3Li Beryllium 4Be Boron 5B Carbon 6C Nitrogen 7N Oxygen 8O Fluorine 9F Neon 10Ne Silicon 14Si Phosphorus 15P Sulfur 16S Chlorine 17Cl Argon 18Ar Second shell Sodium Magnesium Aluminum 12Mg 11Na 13Al Third shell Copyright © The McGraw–Hill Companies, Inc. Permission required for reproduction or display. IIA IIIA Increasing electronegativity 1 IVA VA VIA VIIA 2 H He 1.008 3 4 Li Be 6.941 9.012 11 12 Na Mg 22.99 24.31 19 VIIIA 5 1 Atomic number H Element symbol B 20 K Ca 39.10 40.08 22 23 24 25 Fe 13 14 Al Si 26.98 26 21 C 12.01 10.81 Atomic mass number 1.008 6 27 30 4.003 8 7 O N 9 F 10 Ne 14.01 15.99 15 16 17 P S Cl 30.97 2.07 35.45 35 36 Br Kr 79.90 83.80 53 54 19.00 28.09 20.18 18 Ar 39.95 28 29 31 32 33 34 Co Ni Cu 58.93 58.69 63.55 Zn Ga Ge As Se 65.38 69.72 46 47 48 49 50 51 Sc Ti V Cr Mn 44.96 47.87 50.94 52.00 54.94 41 42 43 44 45 Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe 98.00 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3 37 38 39 40 Rb Sr Y Zr Nb Mo 85.47 87.62 88.91 91.22 92.91 95.94 55.85 72.64 74.92 78.96 52 55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At 132.9 137.3 138.9 178.5 180.9 183.8 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 209.0 210.0 222.0 87 88 89 104 105 106 107 108 109 110 111 112 112 114 115 116 117 118 Fr Ra Ac Rf Db Sg Bh Hs Mt Ds Rg Uub Uut 223.0 226.0 227.0 267.0 268.0 271.0 272.0 270.0 276.0 281.0 60 61 62 63 64 274 85 86 Rn Uuq Uup Uuh Uus Uuo 277 277 289.0 288.0 293.0 292.0 294.0 65 66 67 68 69 70 71 58 59 Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 140.1 140.9 144.2 145.0 150.4 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.0 175.0 90 91 92 103 Th Pa 232.0 231.0 U 238.0 93 94 Np Pu 237.0 244.0 95 96 Am Cm 243.0 247.0 97 Bk 247.0 98 99 100 101 102 Cf Es Fm Md No Lr 251.0 252.0 257.0 258.0 259.0 262.0 Increasing electronegaativity IA Nonpolar and polar covalent bonds (continued) • Atoms in a covalent bond may share electrons equally or unequally • Two atoms of the same element have the same electronegativities and share electrons equally, • This results in forming a nonpolar covalent bond • Atoms with different electronegativity share electrons unequally • This results in a polar covalent bond. Exception is the bond between carbon and hydrogen, considered nonpolar Nonpolar Covalent Bond • In a nonpolar covalent bond, the atoms share the electron equally Polar Covalent Bond • In a polar covalent bond, one atom is more electronegative, and the atoms do not share the electron equally Polar Covalent Bond • More electronegative atom develops a partial negative charge • Less electronegative atom develops a partial positive charge – In a bond between oxygen (more electronegative) and hydrogen (less electronegative), oxygen is slightly negative, hydrogen slightly positive – – O + H H H2O + Covalent Bonds What type of bond is formed between two oxygen atoms? Covalent Bonds Why are some covalent bonds nonpolar and others polar? Different atoms have different electronegativities and share electrons unequally, causing polar bonds. Bonds between atoms of the same type are nonpolar. (An exception is the carbon-hydrogen bond, which is considered nonpolar, though the atoms are of different types.) Covalent bonds occur when __________. • electrons are shared between atoms • electrons are transferred between atoms • electrons are lost • opposite charges attract atoms together Ionic Bonds • Atoms sometimes strip electrons from their bonding partners • An example is the transfer of an electron from sodium to chlorine • After the transfer of an electron, both atoms have charges • A charged atom (or molecule) is called an ion Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Ionic Bonds • A cation is a positively charged ion • An anion is a negatively charged ion • An ionic bond is an attraction between an anion and a cation When atoms gain electrons __________. • the atoms become electrically neutral • the atoms become positively charged • their atomic mass significantly increases • the atoms become negatively charged Fig. 2-14-2 Ionic Bonds Na Cl Na Cl Na Sodium atom Cl Chlorine atom Na+ Sodium ion (a cation) Cl– Chloride ion (an anion) Sodium chloride (NaCl) Figure 2.6 Ionic Attraction between Sodium and Chlorine Ionic bonds connect atoms together by __________. • overlap of valence shells • charge attractions • overlap of the nucleus • Ionic bonds do not connect atoms together. Ionic Compounds • Compounds formed by ionic bonds are called ionic compounds, or salts • Salts, such as sodium chloride (table salt), are often found in nature as crystals Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Comparison of Ionic, Polar Covalent, and Nonpolar Covalent Bonds Figure 2.9 Hydrogen Bonds • A hydrogen bond • forms between polar molecules • attraction between a partially positive hydrogen atom and a partially negative atom • individually weak, collectively strong Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Hydrogen Bond in Water Figure 2.4 Hydrogen Bonds Can Form between or within Molecules Copyright © The McGraw–Hill Companies, Inc. Permission required for reproduction or display. Hydrogen atom C H + O C O Hydrogen bond O C C Oxygen atom C O H + C O O O Glucose molecule Water molecule Fig. 2-16 + Water (H2O) + Hydrogen bond Ammonia (NH3) + + + Hydrogen bonds are similar to ionic bonds because __________. • both hydrogen bonds and ionic bonds are due to opposite charge attractions • they both occur between like charged atoms • they both form molecules • they are both very strong bonds