2.2 Trends in Groups within the Periodic Table

• Alkali metals include: lithium, sodium, potassium, rubidium, cesium, and francium
• Alkali metals are relatively soft with low densities and low melting points
• Alkali metals are more reactive than other metals and react vigorously with water, oxygen, and group 7 elements like chlorine
• As you go down the group of alkali metals, they become even more reactive, with cesium being so reactive that it can ignite spontaneously at room temperature
• As you go down the group of alkali metals, the melting points and boiling points decrease
• Explanation for the reactivity of alkali metals:

  • All alkali metals are in group 1 and have one electron in the outermost shell
  • To be stable, they need to lose that one electron, which is easy because there's only one electron to lose
  • By losing one negative charge, they become a one positive ion
• Trend of increasing reactivity as you go down the group:

  • As you go down the group, the atom's radius gets bigger, and the outermost shell gets further away from the positive nucleus
  • The increased distance weakens the attractive force between the positive nucleus and the outer negative electron, making it easier for the electron to be lost, increasing reactivity
• Alkali metals almost always form ionic compounds with non-metals because they can lose their outer electrons easily

  • Ionic compounds are formed when a metal atom donates an electron to a non-metal to achieve full outer shells
  • The oppositely charged ions are attracted to each other by electrostatic forces, forming an ionic bond
• When alkali metals react with water:

  • They react vigorously to produce a metal hydroxide and hydrogen gas
  • The reactions release more energy as you go down the group, igniting hydrogen gas from potassium onwards
• When alkali metals react with chlorine:

  • They form white metal chloride salts
  • Reactions become more vigorous as you go down the group
• When alkali metals react with oxygen:

  • They form metal oxides
  • The type of oxide formed depends on the particular metal
  • Examples include lithium oxide (Li2O), sodium oxide (Na2O), sodium peroxide (Na2O2), potassium peroxide (K2O2), and potassium superoxide (KO2)
• In a group on the periodic table, the atomic number and mass number generally increase as you descend the group, indicating an increase in the size of the atoms.
• Correspondingly, the melting points of elements within a group tend to decrease as you move down the group, implying that atoms become less tightly bound to each other.
• For instance, considering the trend, the next element in the group, rubidium, is expected to have a lower melting point than the preceding element, which has a melting point of 63°C.
• As we move down Group 1, the first ionization energies decrease, suggesting that it becomes easier for these atoms to lose one electron.
• This can be attributed to the fact that the outermost electrons are further away from the nucleus, making them less strongly attracted by the positive charge of the nucleus.
• Halogens:

  • Fluorine is a poisonous yellow gas and very reactive
  • Chlorine is a less reactive poisonous green gas
  • Bromine is a red-brown volatile liquid and poisonous
  • Iodine is a dark gray solid that can form poisonous purple vapors, also used as an antiseptic
  • All halogens can exist as diatomic molecules by sharing electrons and forming covalent bonds
  • Halogens can form covalent bonds with non-metals to form compounds like hydrogen fluoride or carbon tetrachloride
• Trends of Halogens:

  • Melting and boiling points increase as you go down the group
  • Reactivity decreases as you go down the group due to the outermost shell getting further away from the positive nucleus
• Ionic Bonds with Metals:

  • When a halogen gains an electron to form a one minus ion, it's called a halide
  • Halogens can form ionic bonds with metals, often seen with alkali metals in group 1 like sodium chloride
• Displacement Reactions:

  • More reactive halogens displace less reactive ones
  • For example, chlorine can displace bromine in a solution of potassium bromide to form potassium chloride
• Noble Gases:

  • Exist as colorless gases
  • Inert, meaning they don't react with other atoms
  • Exist as single atoms
  • Non-flammable
  • Boiling point increases as you go down the group