ELEMENTS Remember: Atoms of one type form an element Elements are the simplest form of a substance Every element is unique and has its own chemical and physical properties

Dimitri Mendeleev Russian chemist who worked in the mid 1800’s Researched the 63 known elements Made cards for each element with all the facts he could find Developed the first periodic table by atomic mass Not perfect!!

Henry Moseley British scientist who determined atomic numbers for the elements When elements arranged by atomic number and properties every element fell in place perfectly Periodic Law: Properties of elements repeat in a predictable way when atomic numbers are used to arrange elements into groups.

Elements are arranged: Vertically into Groups Horizontally Into Periods

Why?

If you looked at one atom of every element in a group you would see…

Each atom has the same number of electrons in it’s outermost shell. An example…

The group 2 atoms all have 2 electrons in their outer shells Be (Beryllium) Atom Mg (Magnesium) Atom

The number of outer or “valence” electrons in an atom effects the way an atom joins with other atoms. The way atoms join determine many properties of the element. This is why elements within a group usually have similar properties.

If you looked at an atom from each element in a period you would see…

Each atom has the same number of electron holding shells. An example…

The period 4 atoms each have 4 electron containing shells K (Potassium) Atom Fe (Iron) Atom Kr (Krypton) Atom 4 th Shell

Each group has distinct properties The periodic Table is divided into several groups based on the properties of different atoms.

Alkali Metals Soft, silvery colored metals Very reactive especially with water Not found in pure form in nature edu/webdata/U sers/DMcDowel l/GenChem/alka lishow.html

Alkaline Earth Metals Silvery-White Metals Fairly reactive Many are found in rocks in the earth’s crust

Transition Metals Malleable (easily bent/hammered into wires or sheets) Most are good Conductors of electricity

How many things can you think of that have Transition Metals in them?

Metalloids lie on either side of the “stairstep” line They share properties with both metals and non-metals Si (Silicon) and Ge (Germanium) are very important “semi-conductors”

Where are semiconductors used?

Nonmetals Brittle Do not conduct electricity

Groups Group 13  Boron Group Group 14  Carbon Group Group 15  Nitrogen Group Group 16  Oxygen Group

Most are poisonous Fairly reactive Halogens

Chlorine Gas was used as a chemical weapon during World War I. It was used by the Nazis in World War II.

Non-reactive Gases at room temperature Very Stable Noble Gases

WHY STABLE? Noble gases have full valence levels!

Actinide Series Share properties of Group 3 transition metals Rare and some are radioactive Elements with atomic numbers greater than 92 (Uranium) are synthetic. Lanthanide Series