Chapter 3 The Periodic Table The how and why

History u Dmitri Mendeleev - Russian scientist taught chemistry in terms of properties. u Mid 1800’s - molar masses of elements were known. u Wrote down the elements in order of increasing mass. u Found a pattern of repeating properties.

Mendeleev’s Table u Grouped elements in columns by similar properties in order of increasing atomic mass. u Found some inconsistencies - felt that the properties were more important than the mass, so switched order. u Found some gaps. u Must be undiscovered elements. u Predicted their properties before they were found.

The modern table u Elements are still grouped by properties. u Similar properties are in the same column. u Order is in increasing atomic number. u A column of elements Mendeleev didn’t know about was added later. u The noble gases weren’t found because they didn’t react with anything.

Why? u The part of the atom another atom sees is the electron cloud. u More importantly the outside orbitals. u The orbitals fill up in a regular pattern. u The outside orbital electron configuration repeats. u Outside electrons are called valence electrons u The properties of atoms repeat.

Electron Arrangements repeat u The shape of the periodic table is a representation of this repetition. u When we get to the end of the row the outermost energy level is full. u Full energy level is the most stable Noble gases do not react because they are already stable

Determining Valence Electrons u Valence electrons determine how a chemical will react. u For group A elements, the group number is the number of valence electrons

Valence Electrons u All elements would like to have 8 electrons (except H and He) u 8 electrons means full outside shell u 8 electrons means more stable

Electron Dot diagrams u A way of keeping track of valence electrons. u How to write them: Write the symbol. Put one dot for each valence electron Don’t pair up until they have to X

The Electron Dot diagram for Nitrogen l Nitrogen has 5 valence electrons. l First we write the symbol. N l Then add 1 electron at a time to each side. l Until they are forced to pair up.

Write the electron dot diagram for u Na u Mg uCuC uOuO uFuF u Ne u He

Periodic Table Setup

u Horizontal rows are called periods u There are 7 periods

u Vertical columns are called groups. u Elements are placed in columns by similar properties. u Also called families

1A 2A3A4A5A6A 7A 8A 0 u The elements in the A groups are called the representative elements

The group B are called the transition elements u These are called the inner transition elements and they belong here

u Group 1A are the alkali metals u Group 2A are the alkaline earth metals

u Group 7A is called the Halogens u Group 8A are the noble gases

Transition Metals u Chromium, gold, silver, nickel, zinc, iron, etc. u Transition metals do not behave predictably u Their atomic structure is more complicated

Inner Transition Metals u Two rows “under” main periodic table u First row is lanthanides – rare earth metals u Second row is actinides – radioactive

Nonmetals u Don’t conduct electricity u Brittle as solids u Low melting points

Metalloids u Between metals and nonmetals u Semiconductors – conduct some electricity

Semiconductors u Electricity is the flow of electrons u Metals conduct electricity because their electrons are free to move u Nonmetals do not conduct because their electrons are locked in place

Semiconductors u Semiconductors work best when a small amount of another element is added u Called doping u Si (4 valence e - ) doped with P (5 valence e - ) gives an extra electron u Called n-type for negative charge

Semiconductors u P- type have one less electron and are positive u Can combine types to form pnp- or npn- type u Allow electronic devices to be small: laptop, hearing aids, cell phones