Periodic trends

In chemistry, the periodic table has periodic trends. These are patterns in the periodic table that show how some elements change across the periods (rows) and groups (columns). They were discovered by Dmitri Mendeleev in 1863. There are many different periodic trends. These trends can be atomic radius, ionization energy, electron affinity, electronegativity, nucleophilicity, electrophilicity, valency, nuclear charge, and metallic character.

Mendeleev made the periodic table according to atomic weight. He put empty spaces where newer elements would be, once they were discovered. His periodic table let him predict how 3 unknown elements would act. These elements would later be discovered to be gallium, scandium, and germanium. Mendeleev organized the table by atomic weight. However, Henry Moseley discovered that organizing the table by atomic number would better show the properties of the periodic table.

 Summary of trends
Periodic propertyAcross the periodDown the groupAtomic radiusDecreasesIncreasesNucleophilicityMetallic characterNuclear chargeIncreasesEffective nuclear chargeDecreasesIonization energyElectron affinityElectronegativityNonmetallic characterValence electronsConstantValencyFirst increases then decreases

 Atomic radius 

The atomic radius is the distance from the atomic nucleus to the farest electron orbital. However, electron orbitals don't have a set size because of quantum mechanics. Therefore, atomic radius is found by putting two of the same atom next to each other. Then, half the distance between the two atomic nuclei is found. This is the atomic radius.

In general, the atomic radius becomes bigger when one goes down a group (column). As one moves down the group, there are more electron shells. This new shell makes the electron bigger.

In general, the atomic radius becomes smaller as one goes from left to right in a period (row). As one goes from left to right, the atomic number becomes bigger. As one goes from left to right, there are also more valence electrons in the furthest shell. More valence electrons creates a bigger effective nuclear charge. A bigger effective nuclear charge makes the atomic radius of an element smaller.

 Related pages 
 Periodic table

 References 

 Further reading 

 Periodic Table Of Elements (IUPAC)

Category:Chemical elements
Category:Periodic table