CHM1045 Exam 3 CHM 1045 General Chemistry Chapters 6 7 8 9 Exam 3 Chapter 8 Periodic Classification of the Elements Looking at a periodic table o Starting with hydrogen we see that subshells are filled in order According to the type of subshell being filled the elements can be divided into categories the representative elements the noble gases the transition elements or transition metals the lanthanides and the actinides The representative elements also called main group elements are the elements in Groups 1A through 7A all of which have incompletely filled s or p subshells of the highest principle quantum number With the exception of helium the noble gases the Group 8A elements all have a completely filled p subshell o The electron configurations are 1s2 for helium and ns2np6 for the other noble gases where n is the principal quantum number for the outermost shell o Behave very similarly Helium and neon are chemically inert Lack of chemical activity due to completely filled ns and np subshells correlates with great stability Transition metals are the elements in Groups 1B and 3B through 8B which have incompletely filled d subshells or readily produce cations with incompletely filled d subshells CHM1045 Exam 3 The Group 2B elements Zn Cd and Hg are neither representative elements nor transition metals o There is no special name for this group of metals The lanthanides and actinides are sometimes called f block transition elements because they have incompletely filled f subshells Refer to Figure 8 3 in the text Valence electrons outermost electrons electrons o Chemical reactivity of the elements largely determined by these o For the representative elements the valence electrons are those o All elements in a given group have the same number and type in the highest occupied n shell of valence electrons Similarity in valence electron configurations is what makes elements in the same group resemble one another in chemical behavior Core electrons all nonvalence electrons in an atom Representing Free Elements in Chemical Equations o Because metals do not exist in discrete molecular units we always use their empirical formulas in chemical equations The empirical formulas are the same as the symbols that represent the elements o For nonmetals there is no single rule For example carbon exists as an extensive three dimensional network of atoms and so we use its empirical formula C to represent elemental carbon in chemical reactions But hydrogen nitrogen oxygen and the halogens exist as diatomic molecules and so we use their molecular formulas H2 N2 O2 F2 Cl2 Br2 I2 in equations The stable form of phosphorus is molecular P4 and so we use P4 For sulfur chemists often use the empirical formula S in chemical equations rather than S8 which is the stable form CHM1045 Exam 3 All the noble gases are monatomic species thus we use their symbols The metalloids like the metals all have complex three dimensional networks and we represent them too with their empirical formulas Electron Configurations of Cations and Anions o Ions derived from representative elements Ions formed from atoms of most representative elements have the noble gas outer electron configuration of ns2np6 In the formation of a cation from the atom of a representative element one or more electrons are removed from the highest occupied n shell The electron configurations of some atoms and their corresponding cations are as follows Na Ne 3s1 Ca Ar 4s2 Al Ne 3s23p1 Na Ne Ca2 Ar Al3 Ne Note that each ion has a stable noble gas configuration In the formation of an anion one or more electrons are added to the highest partially filled n shell Consider the following examples H 1s1 H 1s2 or He F 1s22s22p5 F 1s22s22p6 or Ne O 1s22s22p4 O2 1s22s22p6 or Ne N 1s22s22p3 N3 1s22s22p6 or Ne All of these anions also have stable noble gas configurations Notice that F Na and Ne and Al3 O2 and N3 have the same electron configuration They are said to be isoelectronic because they have the same number of electrons and hence the same ground state electron configuration o Thus H and He are also isoelectronic o Cations derived from transition metals First row transition metals Sc to Cu the 4s orbital is always filled before the 3d orbitals CHM1045 Exam 3 Electron electron and electron nucleus interactions in a neutral atom can be quite different from those in its atoms Thus whereas the 4s orbital is always filled before the 3d orbital in Mn electrons are removed from the 4s orbital in forming Mn2 because the 3d orbital is more stable than the 4s orbital in transition metals When a cation is formed from an atom of a transition metal electrons are always removed first from the ns orbital and then from the n 1 d orbitals Keep in mind that most transition metals can form more than one cation and that frequently the cations are not isoelectronic with the preceding noble gases Problems 20 22 26 28 30 32 Periodic Variation in Physical Properties Effective Nuclear Charge o The effective nuclear charge Zeff is the nuclear charge felt by an electron when both the actual nuclear charge Z and the repulsive effects shielding of the other electrons are taken into account In general Zeff is given by Zeff Z lowercase sigma o Where lowercase sigma is called he shielding constant also called the screening constant The shielding constant is greater than zero but smaller than Z Atomic Radius o Atomic radius is one half the distance between the two nuclei in two adjacent metal atoms or in a diatomic molecule CHM1045 Exam 3 o Atomic radii in picometers of representative elements according to their positions in the periodic table Note that there is no general agreement on the size of atomic radii We focus only on the trends in atomic radii not on their precise values o Ionic radius is the radius of a cation or an anion o When a neutral atom is converted to an ion we expect a change Ionic Radius in size If the atom forms an anion its size or radius increases because the nuclear charge remains the same but the repulsion resulting from the additional electron s enlarges the domain of the electron charge On the other hand removing one more electrons from an atom reduces electron electron repulsion but the nuclear charge remains the same so the electron cloud shrinks and the cation is smaller than the atom CHM1045 Exam 3 Ionization Energy The ionization energy is the energy change H or IE associated with the following process g e o Xg X state Or the energy required to remove and
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