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# Classifying the Elements: The Periodic Law and the Periodic Table

• ### Periodic Law

• When the elements are arranged in order of increasing atomic mass, certain sets of properties recur periodically.
• The law is based on atomic volume, or atomic mass divided by the density of its solid form.
• ### Atomic Volume

• The atomic volume increases as you go down in each growth, with the first group having the largest atomic volume.
• In group one, the melting point also decreases as you go further down, but that is limited to the first group only.

# Metals and Nonmetals and their Ions

• Metalloids are elements that look like metals, but they have some nonmetallic properties

# Ionization Energy

Ionization energy is the amount of energy a gaseous atom must absorb to be able to expel an electron.

Mg(g) -> Mg+(g) + e-     I1 = 738 kJ/mol
Mg+(g) -> Mg2+(g) + e-     I2 = 1451 kJ/mol

The reason there is an I 1 and and I 2 is that they are the ionization energies to release another electron to get it to that number. So, the first I is called the first ionization energy, and the second is called the second ionization energy, and so on.

RULE: Ionization energies decrease as atomic radii increase.

# Electron Affinity

Ionization is the energy an atom must absorb to expel an atom. Electron affinity is the measure of the energy change that occurs when a gaseous atom gains an electron.

When an atom gains an electron, it gives off energy, making it an exothermic reaction.

David Witten