Lecture 19 © slg CHM 151 PERIODIC TRENDS: atomic size, ionization energy electronegativity ionic size Overview TOPICS:

Properties of the Elements: Periodic trends #1. Atomic size: Atomic size is evaluated in terms of distance between nuclei in compounds, and is often termed “covalent radius:

Na, Z=11; Ne 10 3s 1 Ar, Z=18; Ne 10 3s 2 3p 6 Atomic size increases down a column as number of shells increase. Atomic size decreases across a period as nucleus becomes more effective: more protons in one concentrated location, attracting diffuse, spread out e’s going into same shell.

ATOMIC SIZE Determined by : Increasing number of shells, down a column Increasing “effective nuclear charge” across a period Measured experimentally by “Covalent Radius”

ION SIZES Ionic sizes do differ from the size of the parent ion: Cations, formed from atoms that have lost their outer shell electrons, are smaller than the parent atom. The larger the charge, the smaller the cation. Anions, which have more electrons than protons, reflect a reduced effective nuclear charge and are larger than the parent atom. The higher the negative charge, the larger the anion.

55 p’s, 54 e’s 55 p’s, 55 e’s 7 p’s, 7 e’s 7 p’s, 10 e’s

Ionic Size and “Isoelectric Series” Ionic sizes can best be remembered if considered in the context of the “isoelectric series”: all the ions which contain the same number of electrons as a neighboring noble gas: N 3- O 2- F - Ne Na + Mg 2+ In this series, all the elements have gained or lost e’s to achieve the neon configuration 1s 2 2s 2 2p 6

Neon Isoelectric Series (Z=10, 1s 2 2s 2 2p 6 ) All ions: same number of e’s, increasing number of p’s

Ar, Kr, Xe Series: Size in pm Ar Kr Xe

IONIZATION ENERGY Amount of energy required to remove one electron from an atom in gas phase: atom (g) ----> atom + (g) + e - Indicates atom’s relative ability to lose an electron from its outermost shell: Highest value at top for column (smallest atom) and at end of period (smallest atom): where nucleus is most effective, most attractive to outer shell electrons.

ELECTRONEGATIVITY The trends in ionization energies and size can be thought of as summarized in a single property called “electronegativity” (en or X). Electronegativity is a unit-less set of assigned values on a scale of 0 --> 4 describing the ability of an atom to attract electrons to itself. The values reaches a maximum at fluorine, with an X =4. Nonmetals have the largest values, metals the lowest. Noble gases have no assigned X value.

The most active metals have X values of 1.0 or less values for most transition and p block metals are in the range 1.5 to 1.9 the metalloids run around 2.0 nonmetals are all assigned values greater than 2. Only Cl (3.0), N (3.0), O (3.5) and F (4.0) have values 3.0 or greater.

Most active metals Most active non-metals

The electronegativity values are quite useful in evaluating bond type and what we will term “bond polarity,” which arises when electrons are shared unevenly. In summation: Metals: larger size, lower ionization potential, lower electronegativity; tend to form positive ions Non-Metals: smaller size, higher ionization potential, higher electronegativity; function as anions in ionic compounds.

Group Work: Ranking of Periodic Properties

Biggest Highest IP, en

SUMMARY, UNIT THREE A) Molarity, Solution stoichiometry, titration calculations B) Dilution Problems C) Energy, Frequency, Wavelength relationships D) Scientist Contributions E) Shells, subshells, orbitals F) Electronic, Ionic Configurations G) Magnetism, Periodic Trends