Periodic Classification of Elements

The One Thing

The periodic table arranges elements in increasing atomic number so that recurring patterns in electronic configuration and chemical properties become apparent. Periodic trends are explained mainly by changes in valence electrons, occupied electron shells, atomic size, nuclear charge, and attraction between the nucleus and outer electrons.

Reactions, Processes and Experiments

Key Terms

• Element: A pure substance made of only one kind of atom and represented by a chemical symbol.
• Atomic number: The number of protons in the nucleus of an atom; represented by $Z$, it determines the identity of the element.
• Modern periodic law: The physical and chemical properties of elements are periodic functions of their atomic numbers.
• Periodic table: A systematic arrangement of elements in increasing order of atomic number, showing repeating patterns in their properties.
• 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 a group generally have the same number of valence electrons and similar chemical properties.
• Valence electrons: Electrons in the outermost shell of an atom; they mainly determine chemical reactivity and bonding.
• Valency: The combining capacity of an atom, usually determined by the number of electrons lost, gained, or shared to achieve a stable outer shell.
• Atomic size: The approximate distance from the nucleus to the outermost electron shell of an atom.
• Metallic character: The tendency of an element to lose electrons and show metallic properties.
• Non-metallic character: The tendency of an element to gain or share electrons and show non-metallic properties.
• Noble gases: Group 18 elements with completely filled outermost shells, making them generally very unreactive.
• Mendeleev’s periodic table: An earlier arrangement of elements based mainly on increasing atomic mass, with elements of similar properties placed together.
• Isotopes: Atoms of the same element having the same atomic number but different mass numbers because they contain different numbers of neutrons.
• Atomic mass: The quantity used in Mendeleev’s arrangement of elements; it is distinct from the atomic number used in the modern periodic table.
• Mass number: The total number of protons and neutrons in an atom, given by $A=Z+$ number of neutrons.
• Nuclear charge: The positive charge associated with the number of protons in the nucleus; it increases with atomic number.
• Metalloids: Elements near the zigzag boundary of the periodic table that show properties of both metals and non-metals.
• Alkali metals: Group 1 elements.
• Alkaline earth metals: Group 2 elements.
• Halogens: Group 17 elements.

Easily Confused

• Atomic number and mass number: Atomic number $Z$ is the number of protons, whereas mass number $A$ is the number of protons plus neutrons.
• Modern periodic law and Mendeleev’s arrangement: The modern periodic table is based on increasing atomic number, whereas Mendeleev arranged elements mainly by increasing atomic mass.
• Periods and groups: Periods are horizontal rows and indicate occupied electron shells; groups are vertical columns and generally contain elements with the same number of valence electrons.
• Atomic size across a period and down a group: Atomic size generally decreases across a period but increases down a group.
• Metallic and non-metallic character: Metallic character generally decreases across a period and increases down a group; non-metallic character shows the opposite trends.
• Valence electrons and valency: Valence electrons are electrons in the outermost shell, whereas valency is the combining capacity determined by electron loss, gain, or sharing.
• Isotopes and different elements: Isotopes have the same atomic number but different mass numbers; different elements have different atomic numbers.
• Noble gases and halogens: Noble gases are Group 18 elements with completely filled outermost shells and are generally very unreactive; halogens are Group 17 elements with seven valence electrons.
• Group number and valence-electron number: For Groups 1 and 2, the group number generally gives the number of valence electrons; for Groups 13 to 18, the number of valence electrons is usually group number minus 10.
• Number of shells and shell capacity: The period number gives the number of occupied shells, whereas the maximum capacity of a shell is represented by $2n^2$.

What Gets Asked

• Questions may ask for the position of an element from its electronic configuration. For example, an element with configuration 2,8,1 is in Period 3 and Group 1; confusing the number of shells with the group number costs marks.
• Questions may require comparison of atomic size, metallic character, or non-metallic character across a period and down a group. The relevant direction must be stated correctly.
• Questions may ask why elements in the same group have similar properties. The required explanation is their similar number of valence electrons and related outer electronic configurations.
• Questions may test the distinction between atomic number and mass number. Use $Z=$ number of protons and $A=Z+$ number of neutrons; substituting atomic mass for atomic number is incorrect.
• Questions may compare the historical classification systems of Dobereiner, Newlands, and Mendeleev with the modern periodic table. The specific limitations and bases of arrangement must be identified: triads for Dobereiner, octaves mainly for lighter elements in Newlands’ system, increasing atomic mass and predicted gaps for Mendeleev, and atomic number for the modern table.
• Questions may ask about special groups or positions, including alkali metals, alkaline earth metals, halogens, noble gases, hydrogen, metalloids, and isotopes. In particular, isotopes generally occupy the same position because they have the same atomic number, while hydrogen has similarities with both Group 1 metals and Group 17 non-metals.