Periodic Table Notes
Periodic Table
Dimitri Mendeleev-1869- developed the first version of the periodic table.
He expressed the regularities as a periodic function of the atomic mass.
Henry Moseley- revised Mendeleev periodic table by describing regularities in
physical and chemical properties as periodic functions of the atomic
number
Groups (family) – vertical column
Elements with similar valence electrons configurations
Group 1 – alkali metals – reactive
Group 2 - alkaline earth metals – reactive
Group 3-12 – transition metals – transition valence e-, can have mult. charges
Group 15 – nitrogen family – N & P essential to life
Group 16 – oxygen family – reactive
Group 17 – halogens – very reactive
Group 18 – noble gases – very stable
Periods – horizontal rows
Period number corresponds to the principal quantum number of valence electrons
Periodic Trends
1. Atomic Size
Increases – down a group
Decreases – across a period
Size of ions
Cation
Anion
Ca+2/Ca
S-2/S
Ca larger because Ca+2 lost 2 electrons
S-2 larger because S-2 gained 2 electrons
2. Ionization Energy – energy required to remove an electron from an individual atom in a
gas phase
M(g)  M+(g) + e• Metals lose electrons to non-metals so relatively low energy is needed
• High ionization energy means an electron is hard to remove
Decreases – down a group
Increases - across a period
3. Electron Affinity – Electron affinity is the energy involved when an electron is added to a
gaseous atom.
• Negative values of energy mean that energy was released during the process. Atoms
with negative values of electron affinity have a very strong attraction for electrons.
• Positive values of electron affinity have very little attraction for electrons.(energy
involved in negative ions)
Decreases – down a group
Increases - across a period
4. Electronegativity is the tendency of an atom to draw electrons to itself when in a
covalent bond. Consequently, the trends are the same as for electron affinity.
The atoms with the highest electronegativity are fluorine, then oxygen, then nitrogen. It is
also important to know that the electronegativity of hydrogen is slightly less than that of
carbon.
Decreases – down a group
Increases - across a period
5. Metallic Character
Increases – down a group
Decreases – across a period
Summary of Trends
Electronegativity
Electronegativity