The Periodic Table and Group Trends: notes

Arrangement of the Table

• Atomic number order: elements are arranged by increasing proton number. Mendeleev first arranged them by relative atomic mass and left gaps for undiscovered elements.
• Periods: the horizontal rows. Elements in the same period have the same number of electron shells. Sodium, magnesium and chlorine are all in Period 3.
• Groups: the vertical columns. Elements in the same group have the same number of outer electrons, so they have similar chemical properties.

Group I: Alkali Metals

• Softness and density: soft metals that can be cut with a knife. They are less dense than water, so lithium floats on it.
• Reactivity increases down the group: potassium reacts more violently with water than lithium does. Lithium + water gives lithium hydroxide + hydrogen.

Group VII: Halogens

• Diatomic molecules: halogens exist as pairs of atoms, such as Cl2, Br2 and I2.
• Colour darkens down the group: chlorine is pale green, bromine is red-brown and iodine is a grey-black solid.
• Reactivity decreases down the group: chlorine displaces bromine from potassium bromide solution. Chlorine + potassium bromide gives potassium chloride + bromine.

Noble Gases

• Group 0: helium, neon and argon. They have full outer shells, so they are very unreactive.
• Uses: helium fills balloons, and neon glows red in advertising signs.

Transition Elements

• Position: the block between Group II and Group III, such as iron, copper and zinc.
• Properties: high melting points, dense metals, and often coloured compounds. Iron acts as a catalyst in the Haber process.

Metallic to Nonmetallic Across a Period

• Trend: across Period 3, elements change from metals (sodium, magnesium) to a metalloid (silicon) to nonmetals (phosphorus, sulfur, chlorine).
• Reason: the number of outer electrons increases, so atoms find it easier to gain electrons than to lose them.