Transcript Mendeleev`s Periodic Table

Section 2: Organizing the Elements
• Objectives:
 state key events in the
historical development of
the periodic table
 list the information in the
periodic table, and describe
how it is organized
 compare the valence
electrons in a period and in
a family, and explain how
they influence the
properties of 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
•Inside the square:
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