Periodic Table: Element Classification, Trends, and Group Properties

Posted on May 11, 2025 in Chemistry

Periodic Table Structure and Properties

The Periodic Table is a method of classifying elements. They are arranged in order of increasing atomic number. The columns are called groups. The rows are called periods. Metals are generally found to the left and non-metals to the right. Elements in the same group typically have similar chemical properties. For a specific main group, the group number indicates the number of valence electrons, and the period number indicates the number of electron shells occupied by electrons. Depending on whether an element is a metal or a non-metal, this information can help us predict its properties.

Understanding Periodic Trends

Non-metals are located on the right side of the Periodic Table, while metals are on the left. When moving down a group of metals (excluding transition metals), they generally become more reactive. This is because there are more electron shells; since metals react by losing electrons, the more electron shells an atom has, the weaker the attraction is between the positive nucleus and the outermost valence electrons, making them more easily lost. Conversely, when moving down a group of non-metals, they generally become less reactive. The reason is essentially the inverse of that for metals: the increased distance from the nucleus and greater shielding by inner electrons make it harder for the nucleus to attract an additional electron to complete its valence shell.

Properties of Element Groups

Alkali Metals (Group 1)

(For IGCSE, focus is typically on Lithium, Sodium, and Potassium)

Physical Properties
  • Good conductors of heat and electricity.
  • Relatively soft compared to other metals.
  • They have low densities (for metals).
  • They have low melting and boiling points compared to most metals.
Chemical Properties
  • Form ionic compounds.
  • React vigorously, sometimes violently, with chlorine.
  • Burst into flames when heated in oxygen, producing characteristic flame colors:
    • Lithium: Red flame
    • Sodium: Yellow flame
    • Potassium: Lilac flame
  • Form soluble white compounds when reacting with oxygen.
  • React with cold water to produce hydrogen gas and a metal hydroxide.
Reactivity Patterns

Reactivity, density, and softness generally increase further down the group (e.g., Caesium is more reactive than Lithium). Melting and boiling points generally decrease down the group. Reactivity increases down the group because as the number of electron shells increases, the valence electron is further from the nucleus and experiences more shielding, resulting in a weaker attraction to the nucleus, so it is lost more easily.

Halogens (Group 7)

Physical Properties
  • Form colored substances (states and colors at room temperature: Fluorine is a pale yellow gas, Chlorine is a greenish-yellow gas, Bromine is a reddish-brown liquid, and Iodine is a black solid that sublimes to a purple vapor).
  • Are poisonous.
  • Are brittle and crumbly in their solid form.
  • Do not conduct electricity.
  • Exist as diatomic molecules (e.g., F2, Cl2).
Reactivity Patterns

Down the group: atomic size, mass, and density increase, and the color of the elements becomes darker. Reactivity decreases down the group. This is because these elements react by gaining an electron. Atoms with fewer electron shells (higher up the group) can attract an incoming electron more strongly due to its closer proximity to the positive nucleus and less electron shielding from inner electrons. Thus, they are more reactive.

Characteristics of Transition Elements

Transition elements are a collection of metals typically exhibiting the following properties:

  • Hard, tough, and strong.
  • High melting points (with mercury being a notable exception as it is liquid at room temperature).
  • Malleable (can be hammered into different shapes) and ductile (can be drawn out into wires).
  • Good conductors of heat and electricity (silver is one of the best).
  • High density (typically greater than water’s density).
  • Generally much less reactive than Group 1 metals, although some, like iron, can corrode (rust) easily.
  • Do not show a simple, uniform trend in reactivity across the periods or down the groups.
  • Often act as catalysts in chemical reactions.
  • Mostly form colored compounds.
  • Can form complex ions.
  • Can form ions with different charges (exhibit variable valency); therefore, they can form more than one compound with another element (e.g., iron can form FeO and Fe2O3).

Noble Gases: Unreactivity and Uses

Noble gases (Group 0 or Group 18) are characterized by their general unreactivity. They are also non-metals, colorless gases, and exist as monatomic (single atom) gases.

Common Uses
  • Helium (He): Used for filling balloons and airships because it is much lighter than air and is non-flammable.
  • Argon (Ar): Used to provide an inert atmosphere in (tungsten) filament light bulbs, preventing the filament from reacting with oxygen and burning out.
  • Neon (Ne): Used in advertising signs and lasers because it glows with a distinct reddish-orange light when an electric current is passed through it.