THE PERIODIC TABLE Mendeleev and Moseley Dmitri Mendeleev

THE PERIODIC TABLE

Mendeleev and Moseley • Dmitri Mendeleev created the first Periodic Table by arranging the elements according to their atomic weight and physical properties. The only problem was that not all the elements “fit” into his table correctly. • Fifty years later, Henry Moseley found that the elements “fit” correctly when they were put in order by their atomic number. This is the Periodic Table that we use today!

The Periodic Table • Today, the Periodic Table is a system used all over the world for organizing the elements into categories. • Remember: We talked about how you can find information about each element in the periodic table.

How does each Element “look” in the Periodic Table? Atomic Number Symbol Name Atomic Mass (this is an average, always round)

Periods • The periodic table is structured by the atomic number in the atoms and also the arrangement of electrons. • A row of elements across the periodic table is called a period. • Ex: Period 2 contains Lithium (Li) and extends across to Neon (Ne). • Because the number of protons in an atom is equal to its number of electrons, it’s also true that the number of electrons increases one at a time across a period.

Groups or Families • Elements in the same column are called a group or family. • Ex: Nitrogen, Phosphorus, and Arsenic are all a part of Group 15. • What is the next element in the same group as Beryllium?

Noble Gases • The Group 18 elements are known as the noble gases. • Atoms of these elements have eight valence electrons (except helium – it has two). Because of this, these atoms are stable – they undergo few chemical reactions. • Since the noble gases are stable, they do not react easily with other elements.

Halogens • The elements in Group 17 are called halogens. • Atoms in the halogen family have seven valence electrons. A gain of just one more electron gives these atoms the stable number of eight electrons. • Since they have 7 valence electrons, they react easily with other elements whose atoms can give up or share electrons. • Near room temperature, the halogens span all of the physical states. They have distinctive unpleasant odors, will burn exposed flesh, and are toxic.

Alkali Metals • Group 1 elements are called the alkali metal family. • Atoms of the this family have one valence electron. Alkali metal atoms can become more stable by losing their one valence electron (Lithium is an exception). This property makes the alkali metals very reactive. • These metals have a these common properties: they have low melting and boiling points compared to most other metals, are very soft and can be cut easily with a knife, and they have low densities. • Hydrogen is located above the other elements in Group 1 because it is has one valence electron. It is above the other metals because it is considered to be a nonmetal. Even though it is a reactive element, its properties differ greatly from those of the alkali metals.

Alkaline Earth Metals • Group 2 elements are called alkaline earth metals. • Atoms of the alkaline earth metals family have 2 valence electrons. They can become more stable by losing these two electrons. This property causes them to be very reactive. • The alkaline earth metals are silver colored, soft metals. They are considered to be ductile (can bend easily and can be pulled into thin strands or wires) and malleable (able to be rolled into thin sheets or eaten into complex shapes).

Transition Metals • The transition metals are the metallic elements that serve as a bridge, or transition, between the two sides of the table. • You will learn more about these elements in the next few years.

Metals usually…. • • • Are silver-grey in color Are solids at room temperature (except Mercury) Reflect light when polished (luster) Can be bent or hammered flat (malleable) Can be drawn into wire (ductile) Are strong and resist bending and breaking Have high melting and boiling points Are good conductors of heat and electricity Have high densities Are attracted to a magnet

Nonmetals • The nonmetals are found in parts of Groups 14 – 17, the noble gas family, and hydrogen. There are only 18 elements in the periodic table that are considered nonmetals. • Nonmetals gain or share electrons to become stable.

Nonmetals usually…. • Are solids, liquids, or gases at room temperature • Don’t reflect light when polished (no luster) • Can’t be bent or hammered flat (not malleable) • Can’t be drawn into wire (not ductile) • Are soft and bend or breaking easily • Have low melting and boiling points • Are poor conductors of heat and electricity • Have low densities • Are not attracted to a magnet

Metalloids • Several elements known as metalloids lie along a zigzag line between the metals and nonmetals. • Depending on the conditions, these elements can behave as either metals or nonmetals. • The metalloids have from 3 -6 valence electrons and can either lose or share electrons when they combine with other elements.

Lanthanides and Actinides • The Lanthanides are found at the bottom of the periodic table – above the Actinides. • The Actinides are found at the bottom of the periodic table – below the Lanthanides. • These elements are part of the transition metals that you will study later.