Periodic Relationships Chang Chapter 8 Bylikin et al. Ch 3

Chapter 8 Outline Development of the Periodic Table Periodic Classification of Elements Periodic Trends in Physical Properties –Effective Nuclear Charge –Atomic Radius –Ionic Radius –Ionization Energy –Electron Affinity –Electronegativity –Metallic Character Trends in Chemical Properties

Development of the Periodic Table In the early 1800s chemists began organizing elements by mass In 1864 John Newlands noticed that when the known atoms were arranged by mass, every 8 th element had similar properties –“Law of Octaves” In 1869 Dmitri Mendeleev & Lothar Meyer independently proposed a tabulation of elements based on the regular, periodic reoccurrences of properties in addition to mass. In 1913 Henry Moseley arranged the atoms by atomic number

Periodic Clarification of the Elements Representative elements (s- and p- block) –Elements that have incompletely filled s or p subshells of the highest principal quantum number Transition Metals (aka d-block transition elements) –Elements that have incompletely filled d subshells Lanthanide & Actinide Series (aka f-block transition elements) –Elements that have incompletely filled f subshells Nobel Gases –Elements that have completely filled p subshell Elements that exist as diatomic molecules –H 2,N 2,O 2,F 2,Cl 2,Br 2,I 2

Periodic Classification of the Elements Groups based on electronic character –Alkali Metals Li: [He]2s 1 Na: [Ne]3s 1 K: [Ar]4s 1 –HalogensF: [He]2s 2 2p 5 Cl: [Ne]3s 2 3p 5 Br: [Ar]4s 2 4p 5 *Elements in the same group have similar properties Outermost electrons are the ones involved in bonding – they are called the VALENCE ELECTRONS

Periodic Trends in Physical Properties Effective Nuclear Charge (Z eff ) Electrons close to the nucleus of an atom produce a shielding effect on outer shell electrons (not the other way around) –This shielding effect reduces the electrostatic attraction between the nucleus and the outer shell electrons The repulsive forces between electrons in a multi-electron atom further offset the attractive force Z eff = Z –  where Z is the actual nuclear charge (atomic number) and  is the shielding or screening constant

Periodic Trends in Physical Properties Effective Nuclear Charge (Z eff ) Example: He –The two electrons in helium shield one another –Neither electron “feels” the full charge of the nucleus –Z eff is less than 2+ –It takes 2373 kJ to strip the first e - off He but 5251 kJ to strip the second e - off He. Why? Moving across a period, Z eff increases for valence e - –same # inner shielding electrons, but increasing Z Moving up a group, Z eff increases very little

Periodic Trends in Physical Properties Atomic Radius –One half the distance between two nuclei in two adjacent metal atoms –Determined by how tightly the outer electrons are held to the nucleus –The larger Z eff, smaller atomic radii

Periodic Trends in Physical Properties

Ionic Radius –The radius of a cation or anion If the atom forms a cation, the radius will decrease –Z remains constant,  decreases If the atom forms an anion, the radius will increase –Z remains constant,  increases For atoms carrying the same charge, size increases down a group

Periodic Trends in Physical Properties Ionic Radius –If atoms are in different groups, a comparison is only meaningful if the atoms are Isoelectronic (have the same number of electrons) Example: How do the ionic radii of Al 3+, Mg 2+, Na + compare? Example: How do the ionic radii of Na +, F - compare? Example: How do the ionic radii of F -, O 2-, N 3- compare?

Periodic Trends in Physical Properties AOL

Periodic Trends in Physical Properties Ionization Energy –Minimum amount of energy (kJ/mol) required to remove an electron from a gaseous atoms in its ground state X (g)  X + (g) + e - first ionization energy X+ (g)  X 2+ (g) + e - second ionization energy X 2+ (g)  X 3+ (g) + e - third ionization energy –The greater the Ionization Energy, the harder it is to “pull” the e - off the atom I 1 < I 2 <I 3 … Why? (hint: think electrostatics)  H for these rxns is always positive

Periodic Trends in Physical Properties Ionization Energy –An inner shell electrons requires much more energy to remove than a valance electron Due to greater Z eff –Moving right across a period, ionization energy increase Due to increases Z eff –Moving down a group, ionization energy decreases Outer e- further away, Z eff decreases

Periodic Trends in Physical Properties

Electron Affinity (E ea ) Enthalpy change when an electron is removed from an anion of a given element X -  X + e -  H = E ea –Positive  H: removal of an e - from an anion requires an input of energy –Negative  H: removal of an e - from an anion release energy –Halogens have the greatest  H values-why?

Periodic Trends in Physical Properties Electron Affinity

Periodic Trends in Physical Properties Electronegativity (  ) –The relative attraction that an atom has for the shared pair of electrons in a covalent bond There are several different scales, each relative –Moving right across a period, electronegativity increases Due to Z eff increase –Moving down a group, electronegativity decreases Due to Z eff decrease

Periodic Trends in Physical Properties Electronegativity

Periodic Trends in Physical Properties Metal Character –Metals (left) Good conductors of heat and electricity, malleable, ductile, have lustre; typically oxidized in chemical reactions –Non-metals (right) Poor conductors of heat and electricity; typically reduced in chemical reactions –Metalloids (step) Have both metallic and non-metallic characteristic (B, Si, Ge, As, Sb, Te, At); some are semiconductors

Periodic Trends in Physical Properties Metallic Character

Periodic Trends in Physical Properties Summary Electronegativity

Trends in Chemical Properties Elements in the same group have similar chemical properties –If the metallic character of a particular group of elements changes significantly, there will be deviations

Trends in Chemical Properties Oxides Halogens Na 2 OMgOAl 2 O 3 SiO 2 P 4 O 10 SO 3 Cl 2 O 7 Type of Compound Ionic Molecular StructureExtensive 3D StructureDiscrete Molecular Units Melting Point ( o C) Boiling Point ( o C)? ? Acid-Base NatureBasic Amphoteric Acidic X 2 (aq)Cl - (aq)Br - (aq)I - (aq) Cl 2 (aq)NRCl 2 + Br -  Br 2 + Cl - (observation: formation of green color) Cl 2 + I -  I 2 + Cl - (observation: formation of dark red/brown color) Br 2 (aq)NR Br 2 + I -  I 2 + Br - (observation: formation of dark red/brown color) I 2 (aq)NR