Periodicity
CHEMISTRY 11
MS. MCGRATH
The First Periodic Law
• Dmitri Mendeleev discovered the periodic law in 1869
• First periodic law – when elements are arranged in order of
increasing atomic mass, elements with similar properties recur
at regular intervals
The Modern Periodic Law
• Henry Moseley determined the nuclear charge, aka the atomic
number
• He arranged the elements by order of atomic number
The Modern Periodic Law
When elements are arranged in order of increasing atomic
number, elements with similar properties occur at regular
intervals.
The physical and chemical properties of the elements are a
periodic function of the atomic number.
Features of the modern periodic table
Groups (aka Families)
• 18 vertical columns containing elements with similar chemical and
physical properties
• Group 1, 2, 13 – 17 constitute the main-group elements
• Group 1 – alkali metals
• Group 2 – alkaline earth metals
• Group 13 – earth metals
• Group 17 – halogens
• Group 18 – rare gases, noble gases, inert gases
• Groups 3 – 12 are called the transition metals
Features of the modern periodic table
Periods
• 7 horizontal rows
• the first period contains two elements – hydrogen and helium
• the bottom two rows represent two series – the lanthanide
series and the actinide series
Features of the modern periodic table
Metals
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more than ¾ of elements are metals
good conductors of heat and electricity
characteristic lustre (shine)
malleable (can be hammered into sheets)
ductile (can be drawn into wires)
high densities
high melting and boiling points
mercury is the only liquid metal at room temperature
Features of the modern periodic table
Non-metals
• do not have lustre
• brittle
• non-ductile, non-malleable
• poor conductors of heat and electricity
• iodine, phosphorus and carbon are solid non-metals
• bromine is the only non-metal that is a liquid at room temperature
• gases such as chlorine, oxygen and helium are non-metals
Features of the modern periodic table
Metalloids
• has properties of metals and non-metals
• boron, silicon, germanium, arsenic, antimony, tellurium,
polonium and astatine fall in this category
Properties Noble Gases
• all these gases are colorless, odorless, and chemically inert
• under proper conditions, these gases can combine with very
reactive elements such as fluorine
Properties Alkali Metals
• group 1
• chemically active
• they react vigorously with water to release hydrogen from the
water
• all react with chlorine to colorless compounds which crystalize
in cubic shapes and have similar formulas: LiCl, NaCl, KCl, RbCl,
CsCl, FrCl
Properties Halogen
• group 17
• the word halogen means “salt-former”
• they react with hydrogen to form compounds that dissolve in
water to make an acidic solution
• they react with metals to form salts
• in their natural state, these elements are diatomic: F2, Cl2, Br2,
I2
Properties Hydrogen
• it is often place in group 1of the periodic table, although it is not a
metal
• it can form compounds with halogens, just as the metals of group 1
• it could also be place in group 17 (the halogens) because it can also
react with metals, such as sodium, and non-metals, such as nitrogen
• because of its unique characteristics, hydrogen does not have a
definite position on the periodic table – although on most periodic
tables it is found in group 1
Metallic trends
• the most metallic elements are found in group 1
• the metallic character of the elements decreases from left to
right in a row and increases from top to bottom in a group
• francium is the most metallic element in the periodic table
Non-metallic trends
• the most non-metallic elements are in group 17
• there is a gradual transition from metallic to non-metallic
properties as one goes from left to right within a period
• fluorine is the least metallic element (or the most non-metallic
element) in the periodic table