Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2015

Lecture Notes 24: 27 March


Atomic Structure & Chemical Periodicity

The Periodic Table

Look at the Periodic Chart. The pattern arises due to a repetition or periodicity of chemical properties. The vertical columns of the charts are called groups, while the rows are referred to as periods.

Periodic Table of the Elements
   H  He
Li Be    B C N O F Ne
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
 Rb Sr  Y Zr Nb Mo Tc Ru Rh Pd  Ag  Cd  In Sn Sb Te I  Xe
 Cs  Ba Lu Hf Ta  W Re Os Ir  Pt Au  Hg Tl  Pb Bi Po At Rn

Note the numbering of the groups. The numbers from 1 - 18 are the internationally accepted numbers. We will also use the I - VIII "American" numbering system. Note that the "tallest" columns comprise what are referred to as the "representative elements" (IA - VIIIA).


You should know the terminology above.

Recall the introduction to Chemical Periodicity in Lecture 6, including hydrogen combining ratios (LIH, BeH2, BH3, CH4, H3N, H2O, HF) and acid/base properties of oxides (basic for metals, acidic for non-metals)

Let's look at some of the elements and see what their properties are like:

2 Na + 2 H2O right arrow 2 Na+ + 2 OH- + H2

2 Na + Cl2 right arrow 2 NaCl

Trends in Chemical Periodicity

(plots ©1994 Hanson, Harper, Paselk, & Russell)

Trends:Note the trends for:

atomic size: decreases going from left right arrow right and from bottom right arrow top.

Periodic Table with Atomic Radii Trend arrow

plot of nonpolar covalent atomic radii vs. atomic number demonstrating periodicity

Next we continue with the trend for first ionization energy (the energy needed to strip the outermost electron from a free atom) and electronegativity (an indication of how electrons are shared by atoms in bonded atoms).

First ionization energy: increases from left right arrow right and from bottom right arrow top.

Periodic Table with ionization energies Trend arrow

plot of first ionization energy vs. atomic number demonstrating periodicity


Electronegativity increases from left right arrow right and from bottom right arrow top.

Electronegativity is a measure of how electrons are shared between two interacting atoms. It is an empirical (experimental) measure, ranging in value from <1 (Cs) to 4.0 (F). Note that the truely inert Noble gases such as He and Ne will have NO electronegativty values since they don't bond

Group 1 elements have the least tendency to attract bonding electrons, while F has the greatest attraction for electrons in bonds.

Periodic Table with Electronegativity Trend arrow

plot of electronegativity vs. atomic number demonstrating periodicity

Note and memorize the electronegativities for H (2.1) and the elements of the second Period (Li {1.0}, Be {1.5}... F {4.0})

Highest Densities

periodic table with highest density elements indicated by period

Note, elements are near "center" of each Period. Due to a combination of nuclear mass, size and packing in crystals.

Highest Melting Points

periodic table with highest melting point elements by period

Due to multiple strong covalent bonds in Representative elements, and strong "covalent-metallic" bonding via unfilled d orbital electrons in Transition elements.

What is the basis of the periodicity of properties?

Electrons are held in shells.


Electronic Configurations & Periodicity

There are a number of different notation conventions for electronic configurations:

Spectroscopic notation



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© R A Paselk

Last modified 27 March 2015