Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Periodic Table Allows Us to not only predict electron configurations, but many trends including -  Atomic size  ion radius  ionization energy  electronegativity 1

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Atomic size varies consistently through the periodic table. – Down a group, the atoms become larger – Across a period, atoms become smaller What influences atomic size? – The number of electrons in an atom How? Two opposing forces: – principal quantum number, n – the effective nuclear charge, Z eff

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Atomic Radii

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Principal Quantum Number, n As n increases, electrons are farther from the nucleus, therefore atoms are larger

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Effective Nuclear Charge The charge experienced by an electron Not the same as nuclear charge (Z; atomic number; number of p + ) because of the effect of the inner electrons – “Shielding” – Outer electrons are attracted to the nucleus, but repelled by the inner electrons that shield them from the nucleus – Z eff is less than Z

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Effective Nuclear Charge Z eff for a given electron depends on: – Distance of the electron from the nucleus – Number of core electrons Z eff increases across a period – Z increases across a period – Shielding stays the same because only adding electrons to the outer shell (no change to inner shell) – Therefore, Z eff increases As Z eff increases, outer electrons are held more tightly, therefore atoms are smaller.

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Atomic Radii Electrons are moving in atomic orbitals Overlap of orbitals = sphere Radius of sphere = atomic radius – Measurable quantity – One-half distance between identical adjacent nuclei – Metal = metallic radius – Nonmetal = covalent radius

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Ionic Radii Ionic radius = estimated size of ion in a crystalline ionic compound Cations are always smaller than their neutral parent atoms – Electron is removed – Electron repulsions decrease – Nucleus-electron interaction increases – Electrons pulled closer to the nucleus and atom is smaller Anions are always larger than their neutral parent atoms – Electron is added – Election repulsions increase – Electrons occupy more space and atom is larger

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Trends in Ionic Size Increases down a group Decreases across a period Decreases with increasing + charge (isoelectronic ions -a series of ions/atoms containing the same number of electrons) example: N 3- > O 2- > F - > Na + > Mg 2+ > Al 3+ Decreases with increasing + charge (multiple ions of the same element) example: Au 3+ < Au + < Au

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Ionic Radii 10

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Atomic Radii

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Problem Just by looking at the periodic table, rank the following atoms from largest to smallest: Cl, Se, Br. Se > Br(Se is to the left in period 4) Br > Cl (Br is below Cl in group 17) From largest to smallest: Se > Br > Cl

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Ionization Energy (IE) Minimum amount of energy required to remove an electron from an isolated gaseous atom – measure of an element’s ability to form positive ions First ionization energy (IE 1 ): removes outermost e - atom + IE 1 → ion + + e - Second ionization energy (IE 2 ): removes next e - Ion + + IE 2 → ion 2+ + e - IE 2 > IE 1 Atoms with low IE 1 tend to form cations Atoms with high IE 1 tend to form anions

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Trends in Ionization Energy Generally increases as you go across a period – Smaller atomic size, more difficult to remove e - – Lowest = alkali metals – Highest = noble gases – Important exceptions at Be & Mg, N & P Generally decreases as you go down a group – Larger atomic size, easier to remove e -

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Ionization Energy

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Electron Affinity Energy change that occurs when an electron is added to an isolated gaseous atom – measure of an element’s ability to gain an electron (ability to form an anion) Equation: atom + e - → ion - (  E = Electron affinity) Electron affinity is usually negative –  E < 0 – exothermic process Atoms with small negative EA tend to form cations Atoms with large negative EA tend to form anions

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Trends in Electron Affinity Generally become more negative across a period – Smaller atomic size, more attraction of e - to nucleus – Noble gases have EA > 0 – Important exceptions at Be & Mg, N & P No trends within groups – Greater attraction of e - to nucleus with smaller atoms, but greater e - repulsion as well

Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Electron Affinity