C3.1 THE PERIODIC TABLE

a) Newlands, and then Mendeleev, attempted to classify the elements by arranging them in order of their atomic weights. The list can be arranged in a table so that elements with similar properties are in columns, known as groups. The table is called a periodic table because similiar properties occur at regular intervals.

b) The early periodic tables were incomplete and some elements were placed at inappropriate groups if the strict order of atomic weights was followed. Mendeleev overcame some of the problems by leaving gaps for elements that he thought had not been discovered.

a) When electrons, protons and neutrons were discoverd early in the 20th Century, the periodic table was arranged in order of atomic (proton) numbers. When this was done, all elements were placed in appropriate groups.

b) The modern periodic table can be seen as an arrangement of the elements in terms of their electronic structures. Elements in the same group have the same number of electrons in their highest occupied energy level (outer shell).

a) The elements in Group 1 of the periodic table (known as the alkali metals): are metals with low density (the first 3 elements in the group are less dense than water), react with non-metals to form ionic compounds in which the metal ion carries a charge of +1. The compounds are white solids that dissolve in water to form colourless solutions, react with water-releasing hydrogen, form hydroxides that dissolve in water to give alkaline solutions.

b) In Group 1, the further down the group an element is: the more reactive the element and the lower its melting and boiling point. 

c) Compared with the elements in Group 1, transition elements: have higher melting points (except mercury) and higher densities, are stronger and harder and are less reactive and so do not react as vigorously with water or oxygen. 

d) Many transition elements have ions with different charges, from coloured compounds and are useful as catalysts. 

e) The elements in Group 7 of the periodic table (known as the halogens) react with metals to form ionic compounds in which the halide ion carries a charge of -1. 

f) In Group 7, the further down the group an element is: the less reactive the element, the higher its melting point and boiling point. 

g) A more reactive halogen can displace a less reactive halogen from an aqueoussolution of its salt. 

h) The trends in reactivity within groups in the periodic table can be explained because the higher the energy level of the outer electrons:the more easily electrons are lost and the less easily electrons are gained.