The Periodic Table

The One Thing

The periodic table arranges elements by increasing atomic number, and their electronic configurations determine their positions and chemical properties. Regular changes in nuclear charge, electron shells, shielding, and valence electrons produce predictable periodic trends across periods and down groups.

Reactions, Processes and Experiments

Key Terms

• Periodic table: A systematic arrangement of elements in rows and columns according to increasing atomic number and similar chemical properties.
• Modern periodic law: The physical and chemical properties of elements are periodic functions of their atomic numbers.
• Atomic number: The number of protons in the nucleus of an atom; it identifies the element and determines its position in the modern periodic table.
• Mass number: The total number of protons and neutrons present in the nucleus of an atom.
• Period: A horizontal row in the periodic table. The period number indicates the number of occupied electron shells in an atom.
• Group: A vertical column in the periodic table. Elements in the same group generally have the same number of valence electrons and similar chemical properties.
• Valence electrons: Electrons present in the outermost shell of an atom. They take part in chemical bonding and largely determine valency and reactivity.
• Valency: The combining capacity of an element, usually related to the number of electrons lost, gained, or shared to achieve a stable outer shell.
• Noble gases: Group 18 elements with completely filled outermost shells, making them generally unreactive.
• Alkali metals: Group 1 elements, except hydrogen, which are soft, highly reactive metals and usually form ions with a charge of +1.
• Alkaline earth metals: Group 2 elements that generally form ions with a charge of +2 and are less reactive than Group 1 metals.
• Halogens: Group 17 elements that are reactive non-metals and commonly form ions with a charge of -1.
• Metals: Elements that generally conduct heat and electricity, are malleable and ductile, and tend to lose electrons to form positive ions.
• Non-metals: Elements that generally do not conduct heat or electricity well and tend to gain or share electrons.
• Metalloids: Elements with properties intermediate between metals and non-metals, such as silicon and boron.
• Atomic radius: A measure of the size of an atom, often considered as the distance from the nucleus to the outermost occupied shell.
• Ionisation energy: The energy required to remove the most loosely held electron from an isolated gaseous atom.
• Electron affinity: The energy change when an isolated gaseous atom gains an electron.
• Electronegativity: The tendency of an atom in a molecule to attract the shared pair of electrons towards itself.
• Periodic trend: A regular change in an element's property as one moves across a period or down a group.
• Dobereiner's triads: Early groups of three elements with similar properties in which the atomic mass of the middle element was approximately the average of the other two.
• Newlands' law of octaves: An early classification stating that every eighth element had properties similar to the first when elements were arranged by increasing atomic mass.
• Mendeleev's periodic law: The properties of elements are periodic functions of their atomic masses. Mendeleev left gaps for undiscovered elements and predicted their properties.
• Modern periodic table: The present arrangement of elements based on increasing atomic number, with seven periods and eighteen groups.

Easily Confused

• Atomic number and mass number: Atomic number is the number of protons; mass number is the total number of protons and neutrons.
• Period and group: A period is a horizontal row and indicates occupied electron shells; a group is a vertical column and generally reflects valence electrons.
• Atomic radius and ionisation energy: Atomic radius measures atomic size; ionisation energy is the energy required to remove an electron.
• Electron affinity and electronegativity: Electron affinity is the energy change when an isolated gaseous atom gains an electron; electronegativity is the attraction for shared electrons in a molecule.
• Metallic and non-metallic character: Metallic character generally decreases across a period and increases down a group, whereas non-metallic character follows the opposite trend.
• Mendeleev's periodic law and modern periodic law: Mendeleev's law is based on atomic mass; the modern law is based on atomic number.
• Hydrogen and Group 1 elements: Hydrogen has one valence electron but has properties different from both Group 1 metals and Group 17 elements, so its position is special.
• Valence electrons and valency: Valence electrons are outer-shell electrons; valency is the combining capacity associated with losing, gaining, or sharing electrons.
• Metals and metalloids: Metals generally have strong conducting and malleable properties, whereas metalloids, such as silicon and boron, have intermediate properties.
• Alkali metals and alkaline earth metals: Alkali metals are Group 1 and generally form +1 ions; alkaline earth metals are Group 2 and generally form +2 ions.

What Gets Asked

• Identifying an element's position from its electronic configuration: Use the number of occupied shells to determine the period and the number of valence electrons to help determine the group. The common error is confusing the number of shells with the number of valence electrons.
• Calculating subatomic particles: For mass number $A$ and atomic number $Z$, state protons $=Z$, electrons in a neutral atom $=Z$, and neutrons $=A-Z$. The common error is using $A$ as the number of neutrons.
• Explaining periodic trends: Atomic radius generally decreases across a period and increases down a group; ionisation energy and electronegativity show the opposite directional pattern. The common error is reversing the across-period and down-group trends.
• Determining valency and ion charge: Group 1, Group 2, and Group 18 generally have valencies 1, 2, and 0, while Group 17 elements commonly form $-1$ ions. The common error is treating valency and ionic charge as interchangeable in every context.
• Comparing metallic and non-metallic character: Metallic character decreases from left to right and increases down a group, whereas non-metallic character increases from left to right and decreases down a group. The common error is applying the metallic trend to non-metallic character.
• Comparing periodic classification systems: Dobereiner's triads, Newlands' law of octaves, Mendeleev's periodic law, and the modern periodic law must be distinguished by their organising principle. The common error is attributing atomic-number ordering to Mendeleev, whose arrangement was mainly based on atomic mass.