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 electronGroup 2 has 2 valence electronsGroup 18 has 8 valence electron, etc

•can be used to predict property of an element