C7 The Periodic Table

G8 Science

The Periodic Table

Of the 118 elements we know about, only 88 occur naturally. The rest are make in laboratories.

The periodic table is a display of the known elements and shows how they relate to each other.

What is the periodic table?

The periodic table is arranged by increasing atomic number from hydrogen to element 118.

What is the atomic number?

Who designed the periodic table?

Scientists spent a long time trying to find a logical way to organize the elements. The breakthrough came with Dmitir Mendelev in 1869.

What are the parts?

The horizontal rows are called periods. The properties of elements change gradually across the period.

The vertical columns are called groups. Elements in the same group have similar properties.

What are the main categories?

Metals are typically shiny, opaque, ductile, and good conductors of heat and electricity.

Nonmetals are poor conductors, brittle and dull (or gasses)

Metalloids have properties between the two.

What is usually on a table entry?

Entries on the periodic table will contain: Element name

(maybe) Element symbol –

Capital letter with maybe a lowercase letter after it

Atomic number Atomic mass (in

amu) What is an amu?

What else can be on it?

You may also find:• Stable mass numbers Count of electrons in each shell Picture of the element Link to element information

Groups on the periodic table

Alkali metals: group 1, soft, silvery, highly reactive. Combines 2:1 with oxygen Li2O

Alkaline Earth metals: group 2, combine 1:1 with oxygen CaO. Reactive, but less so that alkali metals.

Halogens: group 17, toxic, very reactive, seldom found in pure form, when combined with alkali metals form salts.

Noble Gases: group 18, unreactive, inert, almost always found in pure form.

Transition metals: center of table, good conductors

Energy levels and the periodic table

Period 1: only one shell (k), up to 2 electrons

Period 2: two shells (k, l), 2 in first, up to 8 in second

Period 3: three shells (k, l, m), 2 in first, 8 in second, up to 8 in third

Period 4: four shells (k, l, m, n) 2 in first, 8 in second, 8 – 18 in third, up to 8 in fourth.

Period 5: five shells (k, l, m, n, o)

Valence electrons are those in the outermost shell. For period 3 we only care about the m-shell.

Warning Warning Warning

The periodic table that we talk about is not the only periodic table of elements.

Physical and Chemical Properties

Physical properties are things you can see through direct observation, like density, boiling point, and melting point. The chair seat is a dull red, flexible, low-odor

flexible solid at room temperature. Water is a colorless, odorless substance that is

liquid at room temperature.

Physical properties are sometimes reversible and do not create new substances. If you cool water it becomes ice. I can heat it to

make a liquid again. If you heat water it becomes steam. Both are still H2O

If you heat plastic it gets softer. If you cool plastic it gets brittle. If I break it I have more than one piece of plastic, but, it is still plastic.

Chemical properties can be observed when an object is changed from one substance to another – when it undergoes a chemical reaction. If you send an electric current through water, you

get hydrogen gas and oxygen gas. If you burn plastic, you get a dark solid substance

and some gases.

Chemical changes take for effort to reverse – another chemical reaction. Changing H2 and O2 back into water is expensive

and delicate.

Room Temperature Most pure elements are solid a room temperature

11 of the 92 naturally occurring elements are gas: H, He, N, O, F, Ne, Cl, Ar, Kr, Xe, Rn

2 of the 92 are liquids: Br, and Hg

Elements are solid when intermolecular forces are stronger than thermal motion.

Noble gases (group 18) are gases because their valence shell is full, with 8 electrons, and so they have a weak intermolecular force

Periodic Properties

First elements have a low melting and boiling point. They increase until you reach the middle of the table, then decrease again.

Melting and boiling points reflect strong intermolecular forces.

Intermolecular forces are strongest when the valence shell is half full – because a half full valence shell has the most electrons to participate in bonding.

Thermal and Electrical Conductivity Good electrical conductors allow electrons to flow

easily through them. This is a property of metals.

Most metals are good thermal conductors

Elements to the right of the table are good insulators, i.e. poor conductors.

Metals and Metal Alloys

An alloy is a solid mixture of one or more elements, of which one is a metal.

Steel – iron and carbon (rusts but stronger than iron)

Stainless steel – iron, carbon, chromium, vanadium, manganese (does not rust)

Bronze – copper, tin

Brass – copper, zinc

Aluminum alloys

Titanium alloys

Carbon and Carbon-like Elements

< .01% of Earth’s crust

Most molecules that make up plants and animals are build around carbon (Organic Chemistry)

Pure carbon Diamond

Tetrahedral crystal, Hardest natural substance (diamond tip drill bits) Highest thermal conductivity

Graphite Sheet like Lubricant for locks and keys

Silicon Second most abundant

element in Earth’s crust (after Oxygen)

Sand, rocks, minerals Glass (SiO2)

Semiconductors (also make with Germanium)

Nitrogen, Oxygen, Phosphorus

Nitrogen Colorless, tasteless, odorless 78% of Earth’s atmosphere N2

Boils at -196°C Crucial to to life

Oxygen 21% of Earth’s atmosphere 46% of Earth’s crust Crucial to life

Phosphorus Key ingredient in DNA Glow in the dark