Periodic Table:HISTORY, GROUPS, AND CHARACTERISTICS

(History of) The Periodic Table

Mendeleev (Russian) – 1869, was the first person to arrange elements in order by weight (protons + neutrons, but he didn’t know that’s what gave them their mass), he put each of the known elements on a card with their properties, and distributed them as if they were playing cards. When put in order by mass he saw repeating patterns in their properties.

Moseley (English) – 1913, discovered that each element has an atomic number (protons), by which they should be organized (this improved on the patterns seen by Mendeleev).

Periodic Law – says that when elements are arranged in order by atomic number, their physical and chemical properties show a repeating pattern every 8 elements.

Basic Organization

Groups/Families – vertical column – same # of valence electrons, which dictates their behavior (groups behave similarly)

Period/Row – horizontal row – same outermost energy level, behavior changes predictably from left to right

Periodic Characteristics

1. Atomic size – distance from the center of the atom (nucleus) to the outer edge of it’s electron cloud (measured by measuring the distance between the nuclei of two bonded atoms and dividing by two).

2. ionic size – distance from the center to the outer edge of an ion. Cation (+) lost electrons smaller than it’s atom

Anion (-) gained electrons bigger than it’s atom

Periodic Characteristics (cont.)

3. Metallic properties

Luster – shiny

Conductivity – able to transfer heat or electrons

Malleability – can be rolled or hammered into sheets

Ductility – can be drawn (pulled) into a wire (like a specific version of malleable)

Explained by: bonding by sea of mobile electrons

Nonmetallic properties

Luster – varies

Poor conductor of heat and electricity

Brittle

Explained by: electrons are shared tightly in bonds

Periodic Characteristics (cont.)

4. Ionization Energy – energy needed to remove one of an atom’s electrons (1st ionization energy is required to remove the first electron, 2nd ionization energy is required to remove the second, etc.)

5. Electronegativity – the ability of an atom to attract electrons in a chemical bond (think of it as how tightly the electrons are held). Noble gases don’t have electronegativity values because they don’t participate in bonds.

Groups

Elements in the same group have more similarities than elements in the same period because they have the same number of valence electrons.

Alkali Metals (Group 1)

Soft, can be cut with a knife

Low density and melting points

React violently with water and quickly with the oxygen in the air

Never found uncombined in nature (always bonded to some other element)

When they bond, always give away 1 electrons (making +1 ions)

Alkaline Earth Metals (Group 2)

Soft

Higher density and melting points than Group 1

Very reactive but not as much as Group 1

Not found uncombined in nature (also always bonded to another element)

When they bond, always give away 2 electrons (making +2 ions)

Transition Metals

Metals with higher densities and boiling points

Variable properties across the group

In the d-block

Very flexible with their electrons (leading to their variable properties)

Still metals – tend to give away some number of electrons making (+) ions

Metalloids (say with a robot accent)

Can behave more like metals or nonmetals depending on the environment they are in

Touch the stairstep line on the periodic table: B, Si, Ge, As, Sb, Te

Diatomic Elements

HONClBrIF (or BrINClHOF) elements – found combined with self (Br2, I2, Cl2, etc)

Never appear as just one atom, if not combined with something else, they bond with another atom of the same element.

Halogens (Group 17)

Form salt compounds with metals

Exist as diatomic molecules

Highly reactive

Not free elements in nature

I2 is a solid at room temperature, Br2 is a liquid, and Cl2 and F2 are gases

Tend to gain one electron, making (-1) ions

Noble Gases (Group 18)

Least reactive of the elements

All have full valence shell (which is why they’re least reactive)

All gases at room temperature