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Periodic Trends:

Objectives:

    1.    Define the term periodic trend.

    2.    Identify four important periodic trends and explain how each reflects the electron configurations of the elements.

Key Terms:

    periodic trend    atomic radius    ionization energy    electron affinity    electronegativity

Web Resources:

 

Notes:

Periodic Trends:

Atomic Radius is the distance from the center of the atom to its outermost electron.  Although measurement of the atom is not exact because of our understanding of the uncertainty principle, you can make some relative predictions based on the arrangement of the periodic table.
Atoms get larger as they go down a group (column)
        They get larger because the principle energy level increases (1s, 2s, 3s, 4s, ...)
Atoms get smaller as you go left across a period (row) left to right
        Since the elements are in the same principle energy level as you go across the period, the size of the element decreases due to the increasing number of protons within the nucleus (think stronger magnet)

(www.wellesley.edu/.../inhallents/ atomic-radius-s.gif)

The ionic size of an element is different that the neutral size of the element due to the loss or gain in electrons.  Here is the general rule:

As atoms lose electrons (1A & 2A) the atom size becomes smaller
As atoms gain electrons (5A, 6A, & 7A) the atom size becomes larger

(wine1.sb.fsu.edu/.../Bonding/ IonSize/halogen_radii.jpg)

Ionization energy refers to the energy required to remove an electron from an atom.  It is the measure of how strongly an atom holds on to its outer (valence) electrons.  Here are some general rules:

The higher the ionization energy, the less likely the atom is to loose an electron (and vice-a-versa)
Ionization energies decrease as you move down a group (column)
Ionization energies increase as you across a period (row) from left to right
The pattern follows the electronic radius pattern of the periodic table

Diagram

(chemed.chem.purdue.edu/.../ bp/ch7/graphics/ch7_12.gif)

Graph

(chemed.chem.purdue.edu/.../ bp/ch7/graphics/ch7_12.gif)

Removing each successive electron requires an increased amount of ionization energy.  This  increase is not linear due in part to the decrease in electron-electron repulsion and increased attraction to its positive core.  Once it reaches its noble core, the energy to remove the the next electron greatly increases.

(www.chemistry.ohio-state.edu/ .../Slide6.JPG)

Electron affinity refers to the energy change that occurs when an atom gains an electron.  Many electron affinities are reported in negative numbers.  The reason for the negative numbers is that these elements due not require any additional energy to acquire an electron.  Electron affinities follow some general rules.

The greater the attraction for an electron, the greater the negative number
Nonmetals have a greater electron affinity that metals
    The exception in the noble gasses (all orbitals are filled)
Electron affinity decreases as more electrons are needed to fill the valence orbitals (establishes the octet rule)

(www.chemistry.ohio-state.edu/ .../Slide6.JPG)

Electronegativity refers to an atoms ability to attract electrons in a chemical bond.  There are no units involved nor is it a property but is a relative scale from 1-4.

The trend closely resembles electronegativity
Increases as you move left to right across a period

(coke.physics.ucla.edu/laptag/ mchs/electronegativity-s.jpg)

 

 

Last modified: October 16, 2002