C2 - The Periodic Table

• The periodic table of elements developed as chemists tried to classify the elements. It arranges them in an order in which similar elements are grouped together
• The periodic table is so named because of the regularly repeating patterns in the properties of elements
• Mendeleev's periodic table left gaps for the unknown elements, which when discovered matched his predictions, and so his table was accepted by the scientific community
• The atomic (proton) number of an element determines its position in the periodic table
• The number of electrons in the outermost shell (highest energy level) of an atom determines its chemical properties
• The group number in the periodic table equals the number of electrons in the putermost shell
• The atoms of metals tend to lose electrons, whereas those of non-metal tend to gain electrons
• The noble gases in Group 0 are unreactive because of their very stable electron arrangements
• The elements in Group 1 of the periodic table are called the alkali metals
• Alkali metal melting points and boiling points decrease going down the group
• The metals all react with water to produce hydrogen and an alkaline solution containing the 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
• The halogens all form ions with a single negative charge in their ionic compound 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 tha halogens decreases going down the group
• You can explainntrends in reactivity as you go down a group in terms of the attraction between electrons in the outermostvshell and the nucleus
• This electrostatic attraction depends on:

  • The distance between the outermost electrons and the nucleus
  • The number of occupied inner shells (energy levels) of electrons, whixh provide a shielding effect
  • The size of the positive charge on the nucleus (called the nuclear charge)
• In deciding how easy it is for atoms to lose or gain electrins from their outermost shell, these 3 factors must be taken into account. The increased nuclear charge, due to extra protons in the nucleus, going down a group is outweighed by the other 2 factors
• Electrons are easier for the larger atoms to lose going down a group, and harder for them to gain going down a group
• Compared with the alkali metals, transition elements have much higher melting points and densities. They are also stringer and harder but are much less reactive
• The transition elements do not react vigorously with oxygen or water
• A transition element can form ions with different charges, in compounds that are often colour3d
• Transition elements and their compounds are important industrial catalysts