Decoding the Periodic Table and Periodic Trends

Biographical Information: Dmitri Mendeleev ( $1834$ – $1907$ ) was a Russian chemist who laid the foundation for the modern periodic table.
The $1869$ Periodic Table: Mendeleev produced one of the first practical periodic tables in $1869$ .
Scientific Innovation: * Before Mendeleev, chemists grouped elements in two distinct ways: by common chemical properties (e.g., gases or highly reactive metals) or by atomic mass. * Mendeleev saw a way to combine these approaches into a single table. * Most significantly, Mendeleev’s table left blank spaces for elements that had not yet been discovered.
Characteristics of Mendeleev’s Table: * Elements were arranged according to increasing atomic mass. * It used columns for groups and rows for periods. * It omitted noble gases, as they had not yet been discovered.

Total Elements: There are $118$ elements in the modern periodic table.
Organizational Principle: Elements are arranged according to increasing atomic number. Each box contains exactly one element.
Periods (Horizontal Rows): * There are $7$ periods in the modern table. * Inner Transition Metals: These are the two rows located below the main body of the periodic table. * Lanthanides: Collectively located in the first of the two separate rows; often referred to as ‘rare earth elements.’ * Actinides: Located in the bottom separate row; these are mostly artificial elements, with only a few exceptions.
Groups (Vertical Columns): * There are $18$ groups in total. * Labeling Systems: * The numbering $1$ - $18$ is the standard established by IUPAC. * Conventional numbering uses Roman numerals and letters (e.g., Group $1$ is labeled Group $IA$ ). * Representative Elements: Elements in Groups $1$ , $2$ , and $13$ - $18$ belong to ‘Group A’ and are known as representative elements. * Transition Metals: Groups $3$ - $12$ are labeled as ‘Group B’ and are often referred to as the ‘ $d$ -block.’

Metals: * Characteristic properties include being good conductors of thermal energy and electricity. * They possess high density and high melting points. * They are malleable (can be hammered into thin sheets).
Non-metals: * These are poor conductors of thermal energy and electricity. * They generally have low density and low melting points. * In solid form, they are brittle (break easily when stressed).
Metalloids: * These elements possess properties similar to both metals and non-metals. * While they appear more like non-metals, some can conduct electricity. * Semiconductors: Specific metalloids such as Silicon ( $Si$ ), Germanium ( $Ge$ ), and Antimony ( $Sb$ ) are critical as semiconductors in electrical devices. * Location: They are found in a step-wise pattern on the periodic table, beginning with Boron ( $B$ ).

Definition: Periodic trends are repeating patterns observed in the periodic table when moving across a period or down a group.
Effective Nuclear Charge: * Definition: A measure of the ability of protons in the nucleus to attract the electrons orbiting at the edge of the atom. * Trend Across a Period: The effective nuclear charge increases because the number of protons increases. * Trend Down a Group: The effective nuclear charge decreases due to the increasing number of electron shells.
Atomic Size (Atomic Radius): * Trend Across a Period: Atomic size generally decreases. This occurs because the increasing nuclear charge pulls the electron shells closer to the nucleus. * Trend Down a Group: Atomic size increases because the number of electron shells increases, creating a larger distance between the nucleus and the outermost orbital.
Reactivity Patterns: * Group $1$ : Reactivity increases drastically as you descend the group. * Group $18$ (Noble Gases): These are extremely stable and unlikely to react because they have filled valence shells.

Q: Predict if Chlorine ( $Cl$ ) is a metal or non-metal. * A: Chlorine is a non-metal, found in Group $17$ .
Q: Fluorine ( $F$ ) is highly reactive. Prediction for Iodine ( $I$ ) found below it? * A: Iodine would be less reactive than fluorine.
Q: Helium ( $He$ ) is at the rightmost part of the table. What is its atomic size? * A: Helium has the smallest atomic size.
Q: What happens when Francium ( $Fr$ ) is added to water? * A: Because reactivity increases down Group $1$ , Francium (at the base) will react explosively.
Knowledge Check Questions: * Halogens: These elements are located in Group $17$ , one group before the noble gases. * Silicon ( $Si$ ): Classified as a metalloid because it exhibits properties of both metals and non-metals. * Non-characteristic of Metals: Brittleness is not a characteristic of metals (it is a property of non-metals). * Groups and Properties: Elements in the same group possess similar chemical properties.
Extension Task: Gallium ( $Ga$ ): Gallium was discovered six years after Mendeleev published his table. Its discovery helped the table's acceptance because its properties matched the predictions Mendeleev had made for the empty spaces he left.