The Periodic Table

History of the Periodic Table Dmitri Mendeleev organized the 63 known elements in 1860 Based on increasing atomic mass and arranged in columns based on properties and reactivity Left gaps where he predicted undiscovered elements

Mendeleev’s prediction “Ekasilicon” (1872): Actual properties of Germanium (discovered in 1886) Atomic mass: 72 High melting point Density = 5.5 g/cm3 Dark gray metal Density when combined with oxygen: 4.7 g/cm3 Atomic mass: 72.61 Melting point: 938°C Density = 5.323 g/cm3 Gray metal Density when combined with oxygen: 4.23 g/cm3

Lanthanide and Actinide Series Horizontal rows are called periods “periodical” – repeating in a pattern Vertical columns are called groups or families Similar chemical properties Lanthanide and Actinide Series

Metals Properties of metals include: Solid at room temperature Malleable Ductile (can be drawn into wires) Good conductors of heat and electricity

Nonmetals Properties of nonmetals include: Mostly gases at room temperature Brittle Poor conductors of heat and electricity at room temperature Poor conductors are called insulators

Metalloids Metalloids have some metallic and some nonmetallic properties Some metalloids can be semi-conductors Under some conditions they are good conductors, and under other conditions they are insulators

Trend #1: Atomic Mass In general, atomic mass increases across a period

Trend #2: Atomic Radius Atomic radius generally decreases across a period More protons = stronger grip on the electrons in the energy level Atomic radius generally increases down a group This is because with each new group, a new S-orbital (new energy level) is added

Atomic Radius in the periodic table

Trend #3: Reactivity Elements in the same group generally have the same reactive properties This is because elements in the same group have the same number of valence electrons Valence electrons are the electrons in an atom’s outermost orbital

Alkali Metals Group 1 (minus hydrogen) Oxidation number +1 Highly reactive Tend to react with group 17 (or 7A) elements

Tends to react with group 16 (or 6A) Alkaline Earth Metals Group 2 Oxidation number +2 Very reactive Tends to react with group 16 (or 6A)

Halogens Elements in group 17 (or 7A) are called halogens 7 valence electrons Oxidation number -1 nonmetals They often react with the alkali metals

They have a full outer shell 2 for He 8 for the others Non-reactive Noble Gases Group 18 (8A) Oxidation number 0 They have a full outer shell 2 for He 8 for the others Non-reactive