Transcript Mendeleev`s Periodic Table

Chapter 3-2
Organizing the Elements
Looking for Patterns
In 1830 scientists knew of 55 elements, and the number was
growing.
Scientists looked for ways to organize known elements and
predict new elements.
•Mendeleev published his first periodic table in 1869.
He left question marks in some places. Based on the
properties of surrounding elements, he predicted that
new elements with specific characteristics would be
discovered.
Mendeleev’s Periodic Table:
1. Based upon pattern of increasing atomic mass.
-predicted location of three elements
-scandium (45), gallium (68), and
germanium (70) discovered 16
years later
2. Was not perfect.
-properties of some elements did not
follow patterns
atomic mass
– the average mass
of one atom of an
element.
– sum of protons
and neutrons
The Modern Periodic Table
•In 1915, 50 years later, Henry Moseley discovers atomic
numbers
•Atomic number is number of protons in nucleus of atom.
Modern Periodic Table
– elements arranged in order of atomic number,
– elements with similar properties are grouped in
same column.
Reading the Periodic Table
Organization of the Periodic Table
• groups
– elements in same column
– also called a family.
– elements have similar
characteristics.
-examples:
1. all of Group 1 are very
reactive with water.
2. Group 18 elements
rarely react with any
other elements.
•Periods
-horizontal row of elements.
-elements have different
properties.
-properties change from left to
right according to a pattern:
-increasing by 1 proton
-decreasing reactivity
Why the Periodic Table Work?
•Based on structure of the atom
•specifically the valence electrons
•valence electrons increase by 1 from left to right
•elements in same family/group all have same number
of valence electrons
Group 1 has 1 valence electron
Group 2 has 2 valence electrons
Group 18 has 8 valence electron, etc
•can be used to predict property of an element