C2

  • Development of the Periodic Table

    • 19th-century scientists aimed to find patterns in element behavior
    • Initially arranged by atomic weight; Newlands proposed law of octaves in 1864, suggesting every eighth element had similar properties
    • New elements did not fit Newlands' table; his ideas were not widely accepted
    • In 1869, Mendeleev arranged elements by atomic weight, altering order for similar properties, and left gaps for undiscovered elements
    • Used his table to predict properties of undiscovered elements; eventually accepted after discovery disproved some predictions
    • Knowledge of isotopes later clarified heavier atomic weights

  • Electronic Structures and the Periodic Table

    • Elements arranged by atomic numbers in groups with similar properties
    • Group number correlates to electrons in outer shell (highest occupied energy level)
    • Metals (left/center of table) lose electrons, forming positive ions; nonmetals (right/top) gain electrons, forming negative ions
    • Noble gases (Group 0) are stable with full outer shells (8 electrons, except helium with 2)

  • Group One: Alkali Metals

    • Alkali metals are highly reactive, soft solids at room temp with low melting/boiling points that decrease down the group
    • Low densities; lithium, sodium, potassium float on water
    • Quick reactions with oxygen (oxidizing layer) and water (producing hydrogen gas and metal hydroxide)
    • Alkali metal hydroxides soluble in water, yielding high pH colorless solutions
    • All have one electron in their outer shell; react to form ionic compounds with +1 charge, e.g., Na+ with halogens to make colorless salts
    • Reactivity increases down the group

  • Group Seven: Halogens

    • Nonmetallic elements, small molecules (e.g., Cl2) with single covalent bonds, low melting/boiling points that rise down the group
    • Physical states: fluorine (gas), chlorine (gas), bromine (liquid), iodine (solid, can vaporize)
    • Poor conductors of electricity and heat, all have seven electrons in their outer shell
    • Form ionic compounds with metals (halide ions -1 charge) and covalent bonds with nonmetals
    • Reactivity decreases down the group; more reactive halogens displace less reactive ones

  • Reactivity Trends in Groups

    • Reactivity depends on total number of electrons; larger atoms have outer electrons less attached to the nucleus due to shielding from inner electrons
    • For metals, reactivity increases down the group as they lose electrons; for nonmetals, it decreases because they gain electrons
    • Group 1: increased reactivity due to weaker nucleus-electron attraction
    • Group 7: decreased reactivity as outer electrons are less attracted to the nucleus

  • Transition Elements

    • Found in central block of periodic table (between groups 2 and 3), all metals
    • Good conductors of electricity and heat, hard/strong with high densities, except mercury, which has a low melting point
    • Less reactive than group one metals, slow reactions with oxygen and water
    • Form positive ions with various charges, compounds often colorful, many act as catalysts in reactions.