HS Chemistry - Periodic Table Trends

Unit Summary

General Periodicity

Periodicity is the periodic trends - the trends of the elements' properties that can be seen on the Periodic Table.

• The atomic radius increases as you go further down a Group $column$ in the Periodic Table, and increases as you go further to the left of the Table.

• The ionic radius increases as you do further down a Group in the Periodic Table.

  • For the metals, the ionic radius decreases as you go to the right of the Periodic Table.
  • Since non-metals form anions instead of cations, they don't lose a valence shell in ionization, and therefore non-metals have a larger ionic radius than metals that are in the same Period $row$. The ionic radius of non-metals still decreases as you go the right of the Periodic Table, but because they don't lose a valence shell in ionization, non-metals in a certain Period of the Periodic Table have a larger ionic radius than metals in the same Period.

• Elements that are metals can conduct electricity, while elements that are not metals cannot conduct electricity.

• Elements that are metals have higher melting points than elements that are non-metals, but elements with giant covalent structures $e.g. Carbon, Silicon$ have even higher melting points than non-metals.

• Within metals and elements with giant covalent structures, elements higher up in the Group will have higher melting points, and within the same Period, elements to the right of the Table will have higher melting points. However, in non-metals, the opposite is true.

• Metals will almost always have higher melting points than non-metals, and elements with giant covalent structures will generally have higher melting points than metals.

• Elements in the 3^rd Period of the Periodic Table will form oxides when they react with Oxygen. When these oxides react with water, either a base will be formed $if the oxide's pH is greater than 7$ or an acid will be formed $if the oxide's pH is less than 7$.

  • Oxides with a metal $in this case, Na~2~O, MgO, and Al~2~O~3~$ have a pH greater than 7, and oxides with a metalloid or non-metal (in this case, SiO₂, P₂O₅, SO₂, and Cl₂O) have a pH less than 7.

• Elements in the 3^rd period of the Periodic Table will form chlorides when they react with Chlorine. When these chlorides react with water, HCl will likely be a product.

  • The Chlorides are NaCl, MgCl₂, AlCl₃, SiCl₄, PCl₅, SCl₂, and Cl₂

Group 2 Elements' Properties

The elements in Group $column$ 2 of the Periodic Table $aka the Beryllium Group$ exhibit specific properties and trends due to their similar electron configuration.

• Group 2 elements have larger atomic and ionic radii the further down the group they are.

• Group 2 elements can all conduct electricity.

• Group 2 elements further down the Group have lower melting and boiling points than those higher up in the Group - the only exception to this is Magnesium, which has a lower melting point than Calcium even though Calcium is further down the Group than Magnesium.

• The density of the Group 2 elements decreases from Beryllium to Calcium, then starts increasing.

• Each of the Group 2 elements have the same electron configuration as the previous noble gas element, plus two electrons in the next shell's s subshell. For example, Beryllium $Be$ has the same electron configuration as Helium, plus 2 2s electrons. It's electron configuration is [He]2s². Each of the Group 2 elements has 2 valence electrons.

• The first ionization energy decreases for Group 2 elements as you go down the group.

• The electronegativity for Group 2 elements also decreases for Group 2 elements as you go down the group.

Group 2 Elements' Reactions

The elements in Group $column$ 2 of the Periodic Table $aka the Beryllium Group$ exhibit specific properties and trends due to their similar electron configuration. They also react in similar ways, and trends can be seen in these reactions.

• The reactivity of Group 2 elements increases down the Group.

• Reactions with Oxygen:

  • The general formula for a Group 2 element's reaction with Oxygen $we'll use Beryllium, but Beryllium can be substituted for any Group 2 element$ is 2Be $s$ + O₂ $g$ → 2BeO $s$
  • A metal oxide is formed, which is solid at room temperature $20°C$, white in color, and insoluble in water.
  • As the reactivity of Group 2 elements increases down the Group, these reactions become more energetic down the Group.

  • This reaction is exothermic, and the heat it releases is typically seen in the form of a flame.

    • If Magnesium is used, a bright white flame will be seen.
    • If Calcium is used, an orange-red flame will be seen.
    • If Strontium is used, a solid red flame will be seen.
    • If Barium is used, a solid green flame will be seen.

• Reactions with Water:

  • The general formula for a Group 2 element's reaction with water $we'll use Beryllium, but Beryllium can be substituted for any Group 2 element$ is Be $s$ + 2H₂O $l$ → Be$OH$₂ $aq$ + H₂ $g$

    • A metal hydroxide and Hydrogen gas are formed. As the reactivity of Group 2 elements increases down the Group, these reactions become more energetic down the Group, and the metal hydroxide becomes more soluble.

  • However, when steam is used instead of liquid water, the reaction becomes Be $s$ + H₂O $g$ → BeO $s$ + H₂ $g$

    • Hydrogen gas is still formed, but instead of a metal hydroxide forming, a metal oxide forms. As the reactivity of Group 2 elements increases down the Group, these reactions become more energetic down the Group.

• Reactions with Acids:

  • The general formula for a Group 2 element's reaction with hydrochloric acid $we'll use Beryllium, but Beryllium can be substituted for any Group 2 element$ is Be $s$ + 2HCl $aq$ → BeCl₂ $aq$ + H₂ $g$

    • A metal chloride and Hydrogen gas is formed. As the reactivity of Group 2 elements increases down the Group, these reactions become more energetic down the Group, and the metal hydroxide becomes more soluble.

  • The general formula for a Group 2 element's reaction with sulfuric acid $we'll use Beryllium, but Beryllium can be substituted for any Group 2 element$ is Be $s$ + H₂SO₄ $aq$ → BeSO₄ $aq$ + H₂ $g$

    • A metal sulfate and Hydrogen gas is formed. As the reactivity of Group 2 elements increases down the Group, these reactions become more energetic down the Group, and the metal sulfate becomes less soluble $i.e. MgSO~4~ is less soluble than BeSO~4~, CaSO~4~ is less soluble than MgSO~4~, etc.$.

  • Group 2 reactions with nitric acid are more complicated, as the products of the reaction depend on the concentration of the nitric acid. The general formula for a Group 2 element's reaction with water $we'll use Calcium, but Calcium can be substituted for any Group 2 element other than Beryllium, as it is not known for sure whether Beryllium reacts with nitric acid$ depends on the concentration of nitric acid:

    • Dilute Concentration of Nitric Acid:

      Ca $s$ + 2HNO₃ $aq$ → Ca$NO~3~$₂ $aq$ + H₂ $g$

    • Moderate Concentration of Nitric Acid:

      3Ca $s$ + 8HNO₃ $aq$ → 3Ca$NO~3~$₂ $aq$ + 2NO $g$ + 4H₂O $l$

      Although NO is formed, it quickly reacts in the air to form NO₂

    • High Concentration of Nitric Acid:

      Ca $s$ + 4HNO₃ $aq$ → Ca$NO~3~$₂ $aq$ + 2NO₂ $g$ + 2H₂O $l$

      When the concentration of nitric acid is high, NO₂ is directly formed, instead of NO being formed and then quickly reacting with the air to form NO₂

• Group 2 Ionic Compounds:

  • Group 2 hydroxides all have roughly the same formula $Beryllium is used for this example, but any Group 2 element can be used in Beryllium's place$ of Be$OH$₂
  • The vast majority of hydroxides are insoluble, but Group 2 hydroxides are an exception. Except for Beryllium, all Group 2 hydroxides are soluble, and their solubility increases as you go further down the group.
  • The more soluble a metal hydroxide is, the more hydroxide $OH^\-^$ ions are dissolved in water, and the more basic $higher pH$ the resulting solution becomes.
  • Group 2 metals react with carbonate ions $CO~3~^2\-^$ to form carbonate compounds. Group 2 carbonates all have roughly the same formula $Beryllium is used for this example, but any Group 2 element can be used in Beryllium's place$ of BeCO₃
  • Almost all carbonates are insoluble, and this applies to Group 2 carbonates as well. They do, however, react with acids. This reaction will form a metal salt, CO₂, and H₂O.

• Thermal Decomposition:

  • When Group 2 carbonates are heated, they break down to form a Group 2 metal oxide and CO₂. For example, BeCO₃ → BeO + CO₂
  • When Group 2 nitrates are heated, they break down to form a Group 2 metal oxide, nitrogen dioxide, and oxygen gas. For example, 2Be$NO~3~$₂ → 2BeO + 4NO₂ + O₂

Group 17 Elements' Properties

The elements in Group $column$ 17 of the Periodic Table $aka the Fluorine group, or the **halogens**$ exhibit specific properties and trends due to their similar electron configuration.

• Halogens have larger atomic and ionic radii the further down the group they are.

• Halogens are incapable of conducting electricity, unless they're in an ionic compound that's either in the liquid state or dissolved in solution.

• Halogens further down the Group have higher melting and boiling points than those higher up in the Group.

  • Fluorine and Chlorine are gases at room temperature and pressure (RTP)
    • Fluorine is a yellow-green gas.
    • Chlorine is also a yellow-green gas, albeit a darker one.
  • Bromine is a liquid at RTP
    • Bromine is a brownish-red liquid.
  • Iodine and Astatine are solids at RTP
    • Iodine is a purple solid. When heated, it becomes a purple gas.
    • Astatine is a dark-colored solid.
  • In general, halogens further down the group have a darker color than those higher up in the group.

• The first ionization energy decreases for Group 17 elements as you go down the group.

• The electronegativity for Group 17 elements also decreases as you go down the group.

• The reactivity of the halogens also decreases down the group.

• Ionic compounds with Group 17 elements further down the Group have lower melting and boiling points than ionic compounds that contain Group 17 elements higher up in the Group.

• When ionic compounds that contain Chloride, Bromide or Iodide ions react with sulfuric acid $H~2~SO~4~$, they form poisonous gases. The color of the gases is different depending on which halogen is used - therefore, to determine which halogen is present in an ionic compound, we can observe their reaction with concentrated sulfuric acid.

  • If an ionic compound with Chloride ions is used, the products will be:

    • HCl in the gas state. This will be seen as a white gas.
    • A type of bisulfate, in the solid state

  • If an ionic compound with Bromide ions is used, the products will be:

    • Br₂ in the gas state. This will be seen as a red-brown gas, and its temperature is above 20°C
    • A type of bisulfate, in the solid state
    • Water $H~2~O$, in the liquid state
    • Sulfur dioxide $SO~2~$, in the gas state

  • If an ionic compound with Iodide ions is used, the products will be:

    • I₂ in the gas state. This will be seen as a dark purple gas, although it may be so dark that it appears black $in reality it is still purple$. It's temperature is above 20°C
    • A type of bisulfate, in the solid state
    • Water $H~2~O$, in the liquid state
    • Sulfur $S$, present as a yellow solid.
    • Hydrogen Sulfide $H~2~S$, in the gas state.
    • Sulfur Dioxide $SO~2~$, in the gas state

Nitrogen & Sulfur Compounds

• Nitrogen makes up 78% of the Earth's atmosphere. The vast majority of it is present as N₂ gas.

• In an N₂ molecule, the two Nitrogen atoms will share 3 valence electrons, forming a triple bond between the atoms that has a total bond energy of almost 1000 kJ/mol. For this reason, N₂ gas is incredibly unreactive.

• In a thunderstorm, the heat from a lightning strike will cause some of the N₂ gas in the air to react with O₂ gas to form nitrogen monoxide $NO$ gas.

  • Afterwards, NO gas quickly reacts with O₂ gas to form nitrogen dioxide $NO~2~$ gas.
  • NO₂ gas will then react with water vapor $H~2~O$ to form nitric acid $HNO~3~$, dissolved in raindrops.

• Ammonia, which is industrially created through the Haber Process, is a weak base.

• Ammonium, being a polyatomic cation, can react and form ionic compounds with anions, called ammonium compounds. Ammonia can also react with acids to form ammonium compounds.

• Under a constant supply of heat, ammonium compounds react with bases to yield ammonia and some other products.

• Nitrogen pollution, often caused by using agricultural fertilizers, can harm the environment by boosting faster-growing plants, allowing nitrogen to be leached into water bodies, and allowing eutrophication.

• Sulfuric acid in the atmosphere is the primary cause of acid rain:

  • Sulfur dioxide $SO~2~$, an air pollutant released by burning high-sulfur fuels, reacts with O₂ gas in the atmosphere to produce sulfur trioxide $SO~3~$. Sulfur trioxide then reacts with water vapor $H~2~O$ to form sulfuric acid $H~2~SO~4~$.

  • Sulfur trioxide is also formed when sulfur dioxide reacts with nitrogen dioxide $NO~2~$.

    • The main source of man-made nitrogen monoxide is combustion processes from nitrogen compounds in fuels, such as exhaust gases from automobiles. Nitrogen monoxide then reacts with O₂ gas to form NO₂
    • Car exhaust systems and catalytic converters react carbon monoxide $CO$ released by cars with nitrogen monoxide $NO$ released by cars to form carbon dioxide $CO~2~$ and nitrogen $N~2~$ gas. This prevents NO₂ from forming in the atmosphere, helping to decrease the amount of sulfuric acid and acid rain that is produced in the atmosphere.

Period 3 Compounds

• Elements in the 3^rd Period of the Periodic Table will form oxides when they react with Oxygen. When these oxides react with water, either a base will be formed $if the oxide's pH is greater than 7$ or an acid will be formed $if the oxide's pH is less than 7$.

  • Oxides with a metal $in this case, Na~2~O, MgO, and Al~2~O~3~$ have a pH greater than 7, and oxides with a metalloid or non-metal (in this case, SiO₂, P₂O₅, SO₂, and Cl₂O) have a pH less than 7.

• Elements in the 3^rd period of the Periodic Table will form chlorides when they react with Chlorine. When these chlorides react with water, HCl will likely be a product.

  • The Chlorides are NaCl, MgCl₂, AlCl₃, SiCl₄, PCl₅, SCl₂, and Cl₂