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Periods and Groups

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					Periodic Table
• This table is a remarkable way to show the manifold relationships between differing kinds of elements • The modern table was devised in 1869 by Dimitri Mendeleyev • He arranged the elements by weight and by their chemical properties

Periodic Table
• "...if all the elements be arranged in order of their atomic weights a periodic repetition of properties is obtained." Mendeleyev

Periods and Groups
• Horizontal rows in the periodic table are called periods • Vertical columns are called groups • We will learn later why the elements can be arranged in this fashion based on the arrangements of the electrons outside the nucleus

Periods and Groups
• Sizes of the atoms decrease as we move from left to right across a period • This is due to the increasing number of protons in the nucleus, so the electrical attraction between the nucleus and the orbiting electrons gets stronger and pulls the electrons closer to the nucleus

Periods and Groups
• We can display the table to demonstrate other properties as well • As you move from left to right across a period, the ability of the atom to attract another electron increases • This property is called electronegativity

Periods and Groups
• All the elements in a column have remarkably similar chemical properties • Some of the columns have been given special names to distinguish them • The first group is called the alkali metals • The second group is the alkaline-earth metals

Periods and Groups
• Groups 3-12 are called the transition metals • Groups 13-15 are not named • Group 16 are the chalcogens • Group 17 are the halogens • Group 18 are the noble gases and are basically inert

Lots of Questions
• Why are there only two elements in the first period? • Why are there eight elements in the second and third periods? • Why do we jump to eighteen in the fourth period? • Why are the chemical properties of a group so similar?

Lots of Questions
• Next time we will begin our studies of models of the atom • Our goal will be to answer these questions • This will take us into the realm of quantum mechanics • We will see how the wave properties of the electrons provide the answers


				
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