### element

```Dmitri Mendeleev
• Arranged elements
by increasing
atomic mass
• Predicted
properties of
missing elements
Episode 401
Henry Moseley
• Discovered that each element has a
unique atomic number
• Arranged elements by increasing atomic
number
• Now all the elements fit into place
based on properties
Episode 401
Modern Periodic Law
• Properties of elements repeat when
elements are arranged by increasing
atomic number
• Periodic function or repeats
Episode 401
Questions about the extended
periodic table:
• Why do you think these elements get
pulled out?
– It makes the periodic table more compact.
• Which element is in a different column
than we are used to?
– He
• What happens to the number of energy
levels as you begin each new row?
– Increases by 1
• What is the similar characteristic in each
column?
– Electron distribution
Episode 401
Elements in the same column:
• Have similar electron distributions
• Have same number of valence electrons
– Electrons lost, gained, or shared
– Found in the s and p orbital of highest
energy level
• Have similar chemical properties
– Ne:
• 1s2 2s2 2p6
– Kr:
• 1s2 2s2 2p6 3s2 4s2 3d10 4p6
Episode 401
Noble Gases
• Have full outer
energy level
• Have 8 valence
electrons
• Are stable
– (inert or
unreactive)
Episode 401
Using the periodic table to find
electron distributions:
Episode 401
Noble Gas Configuration
• Shorter way to do electron configurations
using noble gases
• Find noble gas on the end of row before
the element and put its symbol in brackets.
• Use diagonal rule to write the electron
distribution for Sn (atomic # 50)
– 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p2
• Use the noble gas distribution to write
electron distribution for Sn
– [Kr] 5s2 4d10 5p2
Episode 401
Noble Gas Configuration
• Find noble gas on the end of row before
the element and put its symbol in
brackets.
• Use diagonal rule to write the electron
distribution for Fe (atomic # 26)
– 1s2 2s2 2p6 3s2 3p6 4s2 3d6
• Use the noble gas distribution to write
electron distribution for Fe
– [Ar] 4s2 3d6
Episode 401
```