Periodic Classification of Elements
The periodic classification of elements is the arrangement of chemical elements in a tabular form based on their properties. It was developed to organize and categorize the increasing number of elements in a systematic manner.
1. Mendeleev's Periodic Table
Dmitri Mendeleev, a Russian chemist, is credited with the discovery of the first periodic table in the mid-19th century. He arranged the known elements in increasing atomic mass and observed that elements with similar properties occurred at regular intervals, forming groups and periods.
Mendeleev left gaps for undiscovered elements and made predictions about their properties, which later proved to be accurate. His periodic table became the foundation for modern periodic tables.
2. Modern Periodic Table
The modern periodic table is based on the atomic number of elements, which is the number of protons in their nuclei. The elements are arranged in increasing atomic number, and they are organized into periods (rows) and groups (columns).
The modern periodic table is divided into s, p, d, and f blocks based on the filling of electron orbitals. Each block represents a different type of electron configuration.
3. Groups and Periods
Groups are the columns of the periodic table, and elements in the same group have similar chemical properties due to the same number of valence electrons. There are 18 groups in the modern periodic table.
Periods are the rows of the periodic table, and they represent the energy levels or shells in which electrons are present. There are 7 periods in the modern periodic table.
Numerical Example: Group and Period
Identify the group and period of chlorine (Cl) in the periodic table.
Solution:
Chlorine (Cl) has the atomic number 17, so it belongs to Group 17 (also known as Group VIIA) in the periodic table.
Chlorine (Cl) is in Period 3 in the periodic table.
Therefore, chlorine (Cl) is located in Group 17 and Period 3 of the periodic table.
4. Representative Elements and Transition Elements
Representative elements are the elements in Groups 1, 2, and 13 to 18. They have completely filled s and p orbitals in their outermost electron shells.
Transition elements are the elements in the d-block (Groups 3 to 12). They have partially filled d orbitals in their electron configuration.
5. Noble Gases and Halogens
Noble gases are the elements in Group 18 (also known as Group VIIIA). They have completely filled outermost electron shells, making them stable and unreactive.
Halogens are the elements in Group 17 (also known as Group VIIA). They are highly reactive nonmetals and tend to form halide ions by gaining one electron to achieve a stable electron configuration.
Properties of the Periodic Table
The periodic table is a systematic arrangement of chemical elements based on their properties. It displays elements in a tabular form, providing essential information about each element. Here are some of the key properties of the periodic table:
1. Periods and Groups
The periodic table is divided into periods (rows) and groups (columns). Elements in the same group have similar chemical properties due to the same number of valence electrons. There are 18 groups in the modern periodic table. Elements in the same period have the same number of electron shells.
2. Atomic Number
The atomic number of an element is the number of protons in its nucleus. Elements in the periodic table are arranged in increasing atomic number, from left to right and top to bottom.
3. Atomic Mass
The atomic mass of an element is the average mass of its atoms, considering all the isotopes. It is usually expressed in atomic mass units (amu) or unified atomic mass units (u). The atomic mass of an element can be found below its symbol in the periodic table.
4. Classification into Blocks
The periodic table is divided into s, p, d, and f blocks based on the filling of electron orbitals. Each block represents a different type of electron configuration. The s-block includes Groups 1 and 2, the p-block includes Groups 13 to 18, the d-block includes Groups 3 to 12, and the f-block includes the inner transition metals.
5. Periodic Trends
The periodic table displays various trends across periods and groups:
- Atomic Size: Atomic size generally increases down a group and decreases across a period.
- Ionization Energy: Ionization energy generally decreases down a group and increases across a period.
- Electronegativity: Electronegativity generally decreases down a group and increases across a period.
- Electron Affinity: Electron affinity generally decreases down a group and increases across a period.
6. Representative and Transition Elements
Representative elements are the elements in Groups 1, 2, and 13 to 18. They have completely filled s and p orbitals in their outermost electron shells. Transition elements are the elements in the d-block (Groups 3 to 12). They have partially filled d orbitals in their electron configuration.