The Periodic Table An Introduction

I CAN describe the organization of the periodic table and draw a Lewis Electron Dot Structure for an element.

Development of the Periodic Table By the 1800s, scientists starting looking for ways to organize the elements known at the time [about 68] into a meaningful arrangement. A number of scientists developed tables of the elements. Since most had shortfalls, none of them were very good.

Mendeleev’s Table In 1869, a RUSSIAN chemist by the name of DMITRI MENDELEEV made a successful table of the element [about 68 by this time]. Mendeleev made cards with all the known information about each element, including its atomic mass number. He placed elements in horizontal rows from lowest to highest atomic mass number [amn]. When an element appeared that had similar properties to one already in a row, Mendeleev placed that element BELOW the previous one: this formed VERTICAL COLUMNS of elements.

Increasing ATOMIC MASS NUMBER

As Mendeleev continued placing elements into his table, sometimes he had to leave BLANK SPACES in order to keep all elements with similar properties in the same vertical column. The blank spaces represented elements that were NOT YET DISCOVERED!!

He predicted where several other elements would fit. Within his lifetime, 3 other elements were discovered that fit where he had predicted: Scandium (Sc), Germanium (Ge) and Gallium (Ga)

Problems with Mendeleev’s Table After Mendeleev died, more new elements were discovered that did not fit into his table as he had predicted. Often, they were only off by a vertical column to the right or left of where he had predicted. This problem was not solved for nearly two decades.

Moseley Rescues Mendeleev’s Table A ENGLISH scientist named HENRY MOSELEY fixed the problems with Mendeleev’s table with a simple rearrangement. By Moseley’s time (1913), it was possible to determine the atomic number of an element.

He rearranged the table by INCREASING ATOMIC NUMBER rather than by AMN. When he did this…ALL the elements fit into the table! The modern PT is still organized in this way!

The Concept of PERIODICITY Mendeleev had discovered an important principle of chemistry: PERIODICITY

Periodicity simply means that a property or characteristic REPEATS by a pattern. In Mendeleev’s table, it turns out that the properties of the elements were repeated by every 8th element.

The Periodic Law Mendeleev’s discovery lead to the proposal of the PERIODIC LAW. The Periodic Law simply states that “when the elements are arranged by increasing ATOMIC NUMBER, their physical and chemical properties repeat at regular intervals”. This is where the name PERIODIC TABLE comes from!

Valence Electrons The periodic table arrangement is based on the arrangement of electrons in an atom’s electron cloud. The physical and chemical properties of an element are a result of the arrangement of electrons in the outermost energy level of the atom.

The outermost energy level is referred to as the VALENCE ENERGY LEVEL and the electrons in this outermost level are called VALENCE ELECTRONS, or Ve- for short! The number of Ve- an atom has can easily be determined from the GROUP NUMBER THE ELEMENT IS IN on the periodic table.

Group 1 1 V e- Group 2 2 V e- Group 3-12 TRANSITION ELEMENTS [skip for now] Group 13 3 Ve- Group 14 4 Ve- Group 15 5 Ve- Group 16 6 Ve- Group 17 7 Ve- Group 18 8 Ve-

Lewis Electron Dot Diagrams Valence electrons [Ve-] are represented by Lewis Electron Dot Diagrams, or Lewis Structures for short. A Lewis Structure consists of the element’s chemical symbol with dots (sometimes an x is used) placed around it to represent Ve-.

Writing a Lewis Structure Find the element on the PT. Go to the TOP of column to the group number and determine the number of Ve- the element has. Write the element’s CHEMICAL SYMBOL. Place DOTS (or Xs) around starting on the RIGHT SIDE and working COUNTERCLOCKWISE in a diamond pattern (side, top, side, bottom).

If you have MORE THAN 4 Ve-, make a second trip around, forming PAIRS OF Ve-. There will NEVER BE MORE THAN 8 Ve-. This is an atom’s MOST STABLE FORM and is known as an OCTET.

O Practice Problem Draw a Lewis Structure for Oxygen 1. Write the chemical symbol: O 2. Add dots to represent the Ve- Oxygen is in column 16, so it has 6 Ve-

More Practice Problems Draw a Lewis Structure for each of the following elements: 1. Calcium 2. Aluminum 3. Chlorine 4. Potassium 5. Carbon 6. Sulfur 7. Boron 8. Xenon 9. Magnesium 10. Nitrogen