The Periodic Table


Development of the periodic table

The periodic table of elements was developed as chemists tried to classify the elements.

It is arranged in an order in which similar elements are grouped together.

It is called the periodic table because of the regularly repeating patterns in the properties of elements.

Mendeleev's table had gaps for the undiscovered elements which when discovered matched his predictions so the table was accepted by the scientific community. 

The number of electrons in an atoms outer shell determines its chemical properties.

The group number in the periodic table equals the number of electrons in the outermost shell.

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Electronic structures and the periodic table

The atomic (proton) number of an element determines its position in the periodic table. 

The atoms of metals tend to lose electrons, whereas those of non-metals tend to gain electrons.

The noble gases in group 0 are unreactive because of their stable electron arrangements. 

The boiling points of the noble gases increase going down group 0. 

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Group 1 -- the alkali metals

The elements in group 1 are called the alkali metals.

Their melting and boiling points decrease going down the group.

The metals all react with water to produce hydrogen and an alkaline solution containing metal hydroxide.

They form 1+ ions in reactions to make ionic compounds. These are generally white and dissolve in water giving colourless solutions.

The reactivity of the alkali metals increases going down the group.

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Group 7 -- the halogens

The halogens all form ions with a negative charge in their ionic compounds with metals.

The halogens form covalent compounds by sharing electrons with other non-metals.

A more reactive halogen can displace a less reactive halogen from a solution of one of its salts. 

The reactivity of the halogens decreases going down the group. 

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Explaining trends

You can explain trends in reactivity as you go down the groups in terms of the attraction between the electrons in the outermost shell and the nucleus.

This electrostatic attraction depends on:

- the distance between the outermost electron shell and the nucleus

- the number of occupied inner shells (energy levels) of electrons which provide a shielding effect

- the size of the positive charge on the nucleus (the nuclear charge) 

In deciding how easy it is for atoms to gain or lose electrons from their outermost shell, the above three factors must be taken into account. The increased nuclear charge caused by extra protons in the nucleus going down a group is outweighed by the other two factors.

Therefore electrons are easier for the larger atoms to lose going down a group and harder for them to gain going down a group.

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