Who is credited with arranging the periodic table? How are the elements arranged in the modern periodic table?

The foundation of today's understanding of the chemical elements stems from the brilliant mind of Dmitri Mendeleev. This Russian chemist and inventor is the one to whom we owe our gratitude for the initial arrangement of the periodic table.

Recognizing patterns where others saw random chaos, Mendeleev, in 1869, masterminded an early version of the table that grouped elements according to their atomic mass. His intuition led him to leave spaces for undiscovered elements, predicting their properties with astonishing accuracy. This foresight cemented his legacy, as his predictions were later confirmed with the discovery of gallium, scandium, and germanium.

At the core of the modern periodic table lies the concept of atomic number, a fundamental aspect that governs the table's structure. The atomic number, defined as the number of protons in the nucleus of an atom, serves as the organizing principle for the elements.

Unlike Mendeleev's original table arranged by atomic mass, the atomic number provides a clear and unambiguous identifier for each element. Each element has a unique atomic number, making it the defining characteristic that determines its place on the table, ensuring the periodicity of elements' chemical and physical properties align flawlessly within this system.

The Periodic Law is the cornerstone of the periodic table, presenting the recurrence of elements' properties. This law states that when elements are arranged in order of increasing atomic number, they exhibit a pattern of periodicity in their physical and chemical properties.

Mendeleev's Periodic Law initially based on atomic mass has evolved with the modern understanding that atomic number is a more accurate sorting criterion. This periodicity arises from the configuration of electrons as they fill various atomic orbitals, a behavior that underpins the table’s structure and the elements' behaviors.

The periodic table is meticulously organized into groups and periods, facilitating the understanding of elements’ relationships and reactivities. Groups, or families, are columns of the table that comprise elements sharing the same number of valence electrons. These valence electrons dictate an element's chemical behavior, so elements in the same group exhibit similar properties and react in similar ways.

On the other hand, periods are horizontal rows on the table. Each period signifies a new principal energy level being filled with electrons. As you move from left to right across a period, the atomic number increases and the elements display a graduation in properties, from metals to metalloids to nonmetals.

Valence electrons are the electrons that reside in the outermost shell of an atom and are paramount in determining an element's chemical properties. These electrons are primarily responsible for an atom's ability to form bonds with other atoms.

Since elements in the same group on the periodic table have identical valence electron configurations, they tend to display similar bonding characteristics and reactivity. For example, the alkali metals found in group 1 all possess one valence electron, making them highly reactive and prone to form similar bonds in chemical reactions.