Mendeleev's Periodic Table: Structure and Element Properties

Mendeleev's Criteria for Sorting Elements

Mendeleev sought a system linking elements based on the properties of their simple substances. He ranked them in increasing order of their atomic weights, from hydrogen to uranium. The arrangement ensured that elements with similar properties remained in the same column.

Mendeleev's Prediction and Gaps

Mendeleev noted that some known elements did not fit his structure, so he left empty spaces ('holes') to maintain correct positioning based on properties. Convinced his system was correct, he assumed these gaps would be filled by undiscovered elements.

Acceptance of Mendeleev's Solution

In 1869, the scientific world was skeptical of his predictions, but Mendeleev did not wait long for his work to be recognized.

Location of Elements in the Current Periodic Table

The current periodic table orders elements by increasing atomic number, ensuring elements with similar properties are located in the same column. The atomic number of each element differs from the previous one.

Periodic Properties of Elements

These are properties that vary periodically across the table.

Atomic Radius

This is the distance from the center of the nucleus to the outermost energy level. To calculate it, the experimental distance between two adjacent, identical atomic nuclei is determined.

• In each group, the atomic radius increases from top to bottom.
• In each period, the atomic radius decreases from left to right.

Ionization Energy (Ei)

Ionization energy is the energy required to form an ion, specifically a cation. It is the energy needed to 'rip' an electron from a single atom in its ground state.

• Ei decreases from top to bottom in each group.
• Ei increases from left to right across a period, reaching its maximum value for each noble gas.

Electronegativity

This property measures an atom's tendency to attract electrons to itself.

• In a group, it decreases going down.
• In a period, it increases going to the right.

Types of Chemical Bonds

Ionic Bonds

Ionic bonds form between elements with a marked difference in electronegativity, generally occurring between a metal and a nonmetal. As a rule of thumb, if the difference in electronegativity values between two elements is greater than 1.7, the bond will be ionic; if lower, it will be covalent.

Covalent Union

When the differences between electronegativities are not as pronounced as between metals and nonmetals, a covalent bonding type exists.

Metallic Union

This union is present in all metals. They form very stable metal nets where cations are arranged in layers surrounded by a sea of circulating electrons. Metals are:

• Ductile (can be molded into thin threads or wires).
• Malleable (can form thin sheets or plates).
• Excellent conductors of electricity and heat.