Periodic Table - Notes on Classification, Development and Trends

6.1 Classifying Elements

• Elements are classified into metals, non-metals, and semi-metals based on their physical properties.
• Metals are typically shiny and silvery-white, with exceptions like copper and gold. Non-metals often have a dull appearance and come in various colors (e.g., sulfur is yellow, phosphorus can be red or yellow, and graphite is black).
• Metals are generally solids at room temperature (except for mercury), while non-metals can be gases or solids (except for bromine).
• Metals usually have high melting and boiling points, are hard and strong, malleable, ductile, and have high density. They are good conductors of heat and electricity.
• Non-metals typically have low melting and boiling points (with exceptions like diamond and graphite), are brittle, not malleable or ductile, have low density, and are poor conductors of heat and electricity (except for graphite).
• Exceptions:
  • Sodium is a metal that is soft and can be cut with a knife, has a low melting point (below $100°C$), and low density, causing it to float on water.
  • Graphite is a non-metal but is an electrical conductor with a shiny appearance and a very high melting point ($3730°C$).
• To classify an element, consider its physical state at room temperature and pressure:
  • Gaseous elements are non-metals.
  • Most solid elements are metals, with some being non-metals.
  • Bromine is the only liquid non-metal.
  • Mercury is the only liquid metal.
• Semi-metals (or metalloids) have properties of both metals and non-metals, such as boron, silicon, and germanium.
• Semi-metals are brittle and shiny solids that do not conduct electricity well at room temperature but do at elevated temperatures.
• Silicon is used in the electronics industry for making transistors and computer chips.

6.2 Development of the Periodic Table

• In 1869, Dmitri Mendeleev arranged elements in a table called the Periodic Table based on their properties.
• The modern Periodic Table arranges elements in order of increasing atomic number.
• Periods are horizontal rows in the Periodic Table, numbered 1 to 7. Period number corresponds to the number of occupied electron shells in an atom of that element.
• Groups are vertical columns in the Periodic Table, with eight main groups numbered I, II, III, IV, V, VI, VII, and 0. Group number corresponds to the number of electrons in the outermost shell of an atom of that element.
• Exceptions:
  • Hydrogen does not belong to any specific group.
  • Helium (Group 0) has two electrons in its outermost shell, while other Group 0 elements have eight.
• Elements between Group II and Group III are called transition elements (or transition metals).

6.3 Patterns in the Periodic Table

• Metals and non-metals can be separated by a staircase-like dividing line in the Periodic Table. Metals are on the left, and non-metals are on the right.
• Across a period:
  • Elements transition from metals to semi-metals, and then to non-metals.
  • For example, in Period 3, sodium, magnesium, and aluminum are metals; silicon is a semi-metal; and phosphorus, sulfur, chlorine, and argon are non-metals.
• Elements within the same group have similar chemical properties due to having the same number of outermost shell electrons.
• There is a gradual change in physical and chemical properties down a group, known as the group trend.
• Isotopes of an element have the same number of outermost shell electrons, and thus similar chemical properties.

6.4 Group I: The Alkali Metals

• Group I elements (alkali metals) include lithium, sodium, potassium, rubidium, cesium, and francium. They all have one outermost shell electron.
• Similarities:
  • Soft metals that can be cut with a knife.
  • Low densities; lithium, sodium, and potassium are less dense than water and float on water.
  • Reactive metals that react readily with air and are stored in paraffin oil.
  • React with water to form an alkaline solution and hydrogen gas.
    $sodium + water \rightarrow sodium hydroxide + hydrogen$
  • React with non-metals to form ionic compounds.
    $sodium + chlorine \rightarrow sodium chloride$
• Difference in reactivity:
  • Reactivity increases down the group: Lithium < Sodium < Potassium < Rubidium < Cesium < Francium.

6.5 Group II: The Alkaline Earth Metals

• Group II elements (alkaline earth metals) include beryllium, magnesium, calcium, strontium, barium, and radium. They all have two outermost shell electrons.
• Similarities:
  • Low densities, but denser than Group I elements in the same period.
  • Less reactive than alkali metals.
  • React with water (except beryllium) and dilute hydrochloric acid to form hydrogen.
    $magnesium + hydrochloric acid \rightarrow magnesium chloride + hydrogen$
  • React with non-metals (except beryllium) to form ionic compounds.
    $magnesium + chlorine \rightarrow magnesium chloride$
• Difference in reactivity:
  • Reactivity increases down the group: Beryllium < Magnesium < Calcium < Strontium < Barium < Radium.

6.6 Group VII: The Halogens

• Group VII elements (halogens) include fluorine, chlorine, bromine, iodine, and astatine. They all have seven outermost shell electrons.
• The melting and boiling points increase down the group, and the physical state changes from gas to liquid to solid.
• Similarities:
  • Have colors that become darker down the group.
  • React with metals to form ionic compounds.
    $sodium + bromine \rightarrow sodium bromide$
  • React with non-metals to form covalent compounds.
    $hydrogen + chlorine \rightarrow hydrogen chloride$
• Difference in reactivity:
  • Reactivity decreases down the group: Fluorine > Chlorine > Bromine > Iodine > Astatine.

6.7 Group 0: The Noble Gases

• Group 0 elements (noble gases) include helium, neon, argon, krypton, xenon, and radon.
• Similarities:
  • Colorless gases with very low melting and boiling points. Very unreactive.
• Stability:
  • All noble gases (except helium) have 8 outermost shell electrons (octet).
  • Helium has 2 electrons in its only occupied shell (duplet).
  • Atoms attain stability by achieving an octet or duplet electronic arrangement.
• Octet rule states that atoms tend to attain the stable electronic arrangement of a noble gas.

6.8 Predicting Properties

• The properties of an unfamiliar element can be predicted from its position in a group in the Periodic Table.
• Example: Francium (Group I) is predicted to be a silvery solid that can be easily cut with a knife and reacts explosively with water, even more so than cesium.
• Example: Astatine (Group VII) is predicted to be a black solid that reacts slowly or not at all with iron wool when heated.