Placing elements in order
How the Periodic Table was designed
Scientists have managed to place all the elements in order using the following system:
They are placed in order of increasing proton number.
Hydrogen is first with a proton number of 1.
Next the list of elements are picked out and placed into groups. Groups are defined by the number of electrons in the outer shell of the atom.
Example: Lithium has one electron in its outer shell so is placed into Group 1. Atoms with two outer shell electrons are placed in Group 2. There are eight groups in total.
Finally, the groups are placed next to one another to finally complete the Periodic Table:
The groups
As has already been stated there are eight groups in the Periodic Table plus, a block of elements called the Transition Metals.
Some of the groups have special names:
Group 1: the alkali metals.
Group 2: the alkaline earth metals.
Group 7: the halogens.
Group 0 or 8: the noble gases.
The zigzag line through the Periodic Table separates the Metals on the left with the non-metals to the right of the line.
The Periods
The horizontal rows are called Periods.
Period 3 contains Sodium(Na), Magnesium(Mg), Aluminium(Al), Silicon(Si), Phosphorus(P), Sulphur(S), Chlorine(Cl) and Argon(Ar).
The Periods represent the energy shell these atoms outer electrons are located within.
Period 3 elements all have their outer electrons in the third energy level/shell.
Period 2 elements have their outer electrons in the second energy shell, and so on.
These atoms have more complicated electronic configurations. They are found in the long block in the middle of the periodic table:
Properties:
Often used in industry as catalysts.
Useful in making alloys.
They form coloured compounds.
The noble gases and halogens
This group contains helium, neon, krypton, xenon and radon.
They are different to elements belonging to other groups due to their resistance to form compounds. The reason behind their unreactivity is their full outer shells that give stability to the atoms.
Properties:
Non-metals.
Gases.
Colourless.
Although they have similar properties they are not identical. For example, as you descend Group 0 the density of the gas increases as does the mass of a single atom.
Fluorine, chlorine, bromine and iodine make up the family of halogens. The last three being the most common. All the halogens exist as molecules, bonding covalently to their own atoms - adding stability as they complete their full outer shells.
Properties:
They all form coloured vapours:
Chlorine - green.
Bromine - red/brown.
Iodine - purple.
Melting points and boiling points are relatively low due to molecules been held together by weak inter-molecular forces. As you descend the group the melting and boiling point increases as the attraction between molecules gets larger.
As with the noble gases, the halogens do have similar properties but not exactly the same ones. For example, the reactivity of the element decreases as you descend the group.
This can be seen if we observe the reaction between iron wool and the different halogens.
So why are the halogens reactive?
The answer lies in the electronic configurations and specifically their outer shell electron configurations. The halogens need only gain 1 electron from another atom to gain more stability.
Fluorine is the most reactive since the electron it is attempting to attract is coming into a shell closest to the positive nucleus. Greater attraction means that it is easier to gain an extra electron - therefore it is the most reactive.
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