The Periodic Table

The One Thing

The periodic table arranges elements by increasing proton number, and an element’s position indicates its electron arrangement. This arrangement explains periodic trends, chemical reactivity, bonding, and many physical properties.

Reactions, Processes and Experiments

Key Terms

• Periodic table: An arrangement of elements in order of increasing proton number, organized into periods and groups.
• Proton number: The number of protons in the nucleus of an atom; it identifies the element and is also called atomic number.
• Period: A horizontal row in the periodic table. The period number shows the number of occupied electron shells in an atom of an element.
• Group: A vertical column in the periodic table. Elements in the same group have the same number of outer-shell electrons and similar chemical properties.
• Valence electrons: Electrons in the outer shell of an atom. They are mainly responsible for an element’s chemical reactions and bonding.
• Electronic configuration: The arrangement of electrons in shells around the nucleus, such as 2,8,1 for sodium.
• Periodic trend: A regular change in a property of elements across a period or down a group.
• Atomic radius: A measure of atomic size. It generally decreases across a period and increases down a group.
• Metal: An element that usually conducts heat and electricity, is malleable, and tends to lose electrons to form positive ions.
• Non-metal: An element that generally does not conduct electricity well and tends to gain or share electrons.
• Group 0 noble gases: Helium, neon, argon, and other Group 0 elements. They have full outer electron shells and are very unreactive.
• Halogens: Group 7 elements, such as fluorine, chlorine, bromine, and iodine. They are reactive non-metals that gain one electron to form 1− ions.
• Displacement reaction: A reaction in which a more reactive halogen displaces a less reactive halogen from a solution of its compound.
• Transition elements: Elements in the central block of the periodic table that form coloured compounds, have variable oxidation states, and often act as catalysts.
• Catalyst: A substance that increases the rate of a reaction without being used up chemically.
• Oxidation state: A number showing the apparent charge of an atom in a compound. Transition elements commonly have more than one oxidation state.
• Ion: A charged particle formed when an atom or group of atoms loses or gains electrons.

Easily Confused

• Proton number and relative atomic mass: The modern periodic table is arranged by increasing proton number, not relative atomic mass.
• Period and group: A period is a horizontal row and indicates the number of occupied electron shells; a group is a vertical column and indicates the number of outer-shell electrons.
• Atomic radius across a period and down a group: Atomic radius generally decreases across a period but increases down a group.
• Group 1 and Group 7 reactivity: Group 1 metals become more reactive down the group because they lose an outer electron more easily; Group 7 halogens become less reactive because gaining an electron becomes more difficult.
• Noble gases and halogens: Noble gases have complete outer shells and are very unreactive; halogens need one additional electron and are reactive.
• Metals and non-metals: Metals are mainly on the left and centre of the periodic table and tend to lose electrons; non-metals are mainly on the right and tend to gain or share electrons.
• Transition elements and main-group elements: Transition elements commonly form coloured compounds, have variable oxidation states, and act as catalysts; these properties are not generally characteristic of main-group elements.
• Catalyst and reactant: A catalyst increases the rate of a reaction without being used up chemically, whereas a reactant is chemically changed.
• Group 0 electron shells: Helium is complete with 2 electrons in its first shell, whereas the other noble gases usually have 8 outer-shell electrons.
• Halogen colour and halogen reactivity: Halogen colours generally become darker down the group, while halogen reactivity decreases down the group.

What Gets Asked

• Explain why elements in the same group have similar chemical properties. The required point is that they have the same number of outer-shell electrons.
• Explain the decrease in atomic radius across a period. Marks depend on stating that proton number and nuclear charge increase while electrons are added to the same main shell, so attraction becomes stronger and shielding remains similar.
• Explain the increase in atomic radius and the change in reactivity down a group. The relevant factors are the addition of occupied electron shells and increased shielding.
• Describe and explain the Group 1 and Group 7 reactivity trends. Do not reverse them: Group 1 reactivity increases down the group, whereas Group 7 reactivity decreases.
• Identify observations and products in a halogen displacement reaction, such as Cl2 + 2KBr → 2KCl + Br2. The reaction must show a more reactive halogen displacing a less reactive halide.
• Describe the properties of Group 0 and transition elements. Answers should include complete outer shells and low reactivity for Group 0, and coloured compounds, variable oxidation states, catalytic activity, density, high melting points, conductivity, and tensile strength for transition elements where relevant.