History and Trends of the Periodic Table!

Hello fellow students, today I will be informing you about the history of the periodic table and also the trends on the periodic table. 

So here is a picture of a typical periodic table. But believe it or not, it did not look like this when it was first created. (I know right! Shocking!)

Here are some facts you might find interesting!

• By 1817, there were 52 elements discovered and 62 by 1863! 
• The first attempt to make a periodic table was in the 1820's. 



1863-1866 John Newlands began ordering the known elements by their masses (Law of Octaves - every 8th element shared a common set of properties)

In 1857, William Odling separated the known elements into 13 different groups based on their physical & chemical properties.

1869 Russian Dimitri Mendeleev published a method if organizing the elements according to both their masses & properties (can I get a wootwoot for mr. Mendeleev?)

1914 Henry Moseley discovered the relationship between element's atomic number and frequency of its x-rays

Now in a modern periodic table you have a series of rows and columns organized by their atomic number and not their atomic massses. 

In the pictures above, it shows that a period is the set of all elements in a given row going across the table. Whereas a group (or aka family) is the set of all elements in a given column going down the table.

There are different chemical families that are present in the periodic table. There is the:

Now let's talk a little bit about metals, non-metals, and semiconductors.

Metals reflect light when its polished (like silverware). It becomes shiny and has a metallic luster. Metals are usually opaque and good conductors of heat and electricity. They are generally flexible in sheet form and are usually solid at room temperature (expect for mercury). Metals lose electrons whereas non-metals gain.

Now non-metals are the white boxes from the image above. They are gases, liquids, or brittle solids at room temperature. They are poor conductors of heat and electricity compared to metals and they are opaque to translucent and dull - lustrous in appearance. Non-metals can be divided into 2 different groups: very low electrical conductivity & fair to moderate conductivity.

Semiconductors are metalloids/semimetals that have properties which resemble metals more than non-metals. Metal conductivity decreases with increasing temperature where as electrical conductivity increases with increasing temperature.

WOOHOO! Now that we're done with the history, lets move on to the trends on the periodic table.

There are several trends such as:

1. Metallic Properties
2. Atomic Radius
3. Ionization Energy
4. Electronegativity
5. Reactivity
6. Ion Charge
7. Melting/Boiling Point
8. Density

Here are some notes on these trends. I'm only going to do a few so that you don't fall asleep.

Metallic Properties

- properties of the element change from metallic to non-metallic going from left to right across the table.

- elements become more metallic going down a group in the periodic table.

Atomic Radius

- atomic radii of an atom decreases going across a row & increases doing down a group.

- if you go from left --> right on a given period, the atomic number (protons) increases & the positive charge on the nucleus increases also.

- increase in atomic number = increase in the number of electrons.

- all electrons in a given shell can be assumed to have the same average distance from the nucleus.

- as the number of protons increase, there is a greater force of attraction for electrons in the shell & the distance between the electrons and the nucleus decreases.

Ionization Energy

- energy needed to completely remove an electron from an atom.

- increases going up and to the right.

- all noble gases have high ionization energy (highest = Helium; lowest = Francium).

- opposite trend from atomic radius.

- measured in kJ/mol.

- increases going across a row.

- decreases going down a row.

Electronegativity

- how much atoms want to gain electrons.

- same trend as ionization energy.

- tendency of an atom to attract (completely remove) electrons from a neighboring atom.

- atoms with high electronegativity strongly attract their own valence electrons (difficult to remove when atoms have high electronegativity)

- atoms with low electronegativity have little attraction to the electrons & little tendancy to remove an electron from a neighbour.

- top right corner of the periodic table has the highest electronegativity (excluding noble gases). (Flourine is the MOST electronegative element).

- electronegativity increases as you go across a period.

- electronegativity decreases as you go down a group.

Ion Charge

- the ion charge changes as you go across the period on a periodic table.

- it goes from +1 (Alkali Metals), +2 (Alkaline Earth Metals), variable charges (Transition Metals), +3 (group that starts with Boron), +/-4 (group that starts with Carbon), -3 (group that starts with Nitrogen), -2 (group that starts with Oxygen), -1 (Halogens), 0 (Noble Gases).

Melting/Boiling Point

- elements in the centre of the table have the highest melting point.

- noble gases have the lowest melting point.

- from L --> R, melting point increases (until the middle of the table).

Some main things to remember is:

• When you are going down the periodic table, it means going to the outer shells so the radius increases.
• When you are going across the periodic table, it mean adding electrons to the same shell & increasing the positive charge of the nucleus, so the radius decreases as the shell is pulled in.