14.2 – Periodic Trends

Atomic size, Ionization energy, Ionic size, & Electronegativity

Atomic Radius

Atomic radius = ½ the distance between the nuclei of diatomic molecules

X-ray diffraction = a way to estimate the distance between nuclei of atoms to determine their relative size

DNA

Trends (general)

Group – size increases as you move down

Periodic – size decreases as you move left to right

Ionization EnergyThe energy required to remove an electron from a gaseous atom

•First ionization = removal of 1st outermost e-

•Second ionization = removal of outermost e- from 1+ ion

•And so on…

This concept is used to predict ionic charges.

Trends

Noble gases

Group IA

Why is it more difficult to ionize the noble gases?

Ionic Size

Ionic radii are the radii of the anions and cations in crystalline ionic compounds

•Radii generally decrease across the periods

•     Radii generally increase down the families

•     Cations are smaller and anions are larger than neutral atoms of the elements from which they are formed

•     Cations are generally smaller than anions

•     Effective nuclear charge increases across each period

Summary

ElectronegativityElectronegativity is a measure of the tendency of an atom in a molecule or aggregate of atoms to attract electrons.

Linus Pauling developed a scale of electronegativities so we use the Pauling as a unit

•Electronegativity increases across the periods

•Electronegativity decreases down the families

•Trends are less regular for transition elements than for representative elements

•Metals have low electronegativities

•Fluorine and Oxygen are the two most electronegative elements

•Ionic bonds form between atoms of electronegativity difference of 1.7 or greater

Summary:

Practice:

a. How does the radius of an anion compare to the radius of a neutral atom for that element?

b. Which element in each pair has a larger ionization energy: Na or K; Mg or P?

c. For which of these properties does lithium have a larger value than potassium? 1st ionization energy, atomic radius, electronegativity, ionic radius?

Answers

a. Anion is larger

b. Na and P

c. 1st ionization energy and electronegativity

Check out this site: http://www.webteach.addr.com/CHSChemistry/periodic_table.htm