The Periodic Table 16.3

Periodic table – a table where elements are organized by increasing atomic number (number of protons)

In the late 1800’s, Dmitri Mendeleev devised the first periodic table based on atomic mass. However, some elements were out of order.

In 1913, Henry G. J. Moseley arranged elements by atomic number In 1913, Henry G. J. Moseley arranged elements by atomic number. This is what we use today.

Elements are arranged: Vertically into Groups Horizontally Into Periods

Groups – vertical columns of elements with similar properties – groups are numbered 1 – 18 Elements in same group have the same number of electrons in their outer energy level

Group 2 atoms all have 2 electrons in their outer shells Be (Beryllium) Atom Mg (Magnesium) Atom

The number of outer or “valence” electrons in an atom affects the way an atom bonds. The way an atom bonds determines many properties of the element. This is why elements within a group usually have similar properties.

Periods – horizontal rows of elements that contain increasing numbers of protons and electrons Periods are numbered 1 – 7 Each row in a periodic table ends when an outer level is filled

Period 4 atoms each have 4 electron containing shells 4th Shell K (Potassium) Atom Kr (Krypton) Atom Fe (Iron) Atom

Each of the seven energy levels can hold a maximum number of electrons. Level 1 = 2 electrons Level 2 = 8 electrons Outer level can hold a max of 8 electrons – except H and He (only max 2)

Electron Dot Diagrams – use the element symbol and dots to represent outer level electrons