Chemistry AQA GCSE -The Periodic Table

The periodic table

The modern periodic table has been developed from work begun by Newlands and Mendeleev. There are trends in chemical properties within the periodic table linked to how easily the element gains or loses electrons.

You should know to:

■ evaluate the work of Newlands and Mendeleev in terms of their contributions to the development of the modern periodic table

■ explain why scientists regarded a periodic table of the elements first as a curiosity, then as a useful tool and finally as an important summary of the structure of atoms.

The early 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 similar properties occur at regular intervals.

b) The early periodic tables were incomplete and some elements were placed in 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.

The modern periodic table

a) When electrons, protons and neutrons were discovered 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).

Trends within the periodic table

a) The elements in Group 1 of the periodic table (known as the alkali metals):

■ are metals with low density (the first three 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

■ the lower its melting point and boiling point.

c) Compared with the elements in Group 1, transition elements:

■ have higher melting points (except for mercury) and higher densities

■ are stronger and harder

■ are much less reactive and so do not react as vigorously with water or oxygen.

d) Many transition elements have ions with different charges; form 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.

Additional guidance:

The periodic table that will be used in the examinations is on the Data Sheet, with main groups numbered from 1 to 7 and the noble gases as Group 0.

You are not expected to know detailed electronic configurations for elements beyond calcium, but should understand that the number of electrons in the highest occupied energy level (outer shell) for elements in the main groups is equal to the group number.

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 aqueous solution 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

■ the less easily electrons are gained.

You need to know what are the steps of the following procedures:

■ demonstration of the combustion of reactions of sodium and potassium

■ demonstration of the reactions of sodium and potassium with chlorine

■ demonstration of the reactions of lithium, sodium and potassium with water

■ demonstration of the reactions of the halogens with iron wool

■ investigation of the displacement of halogens from solutions of their salts by more reactive halogens

■ heating transition metals in air (any of Ti, Cr, Co, Ni, Fe, Cu) to compare reactivity and melting points with Group 1

■ demonstration of the reaction of iron wool with steam

■ observation of as many salts of transition metals as possible (bottles with formulae clearly displayed)

■ demonstrations of transition metals and their salts as catalysts

■ investigation of the catalysis of hydrogen peroxide decomposition by different transition metals and their compounds.

You should only know the work of Newlands and Mendeleev on the history of the  periodic table. Examination questions would give information about other models so that comparisons can be made.