1. 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

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

5. 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

7. 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

9. 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

11. 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

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

14. 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?

15. 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