What is required for a chemical reaction to occur?

Explanation:

The conditions for a chemical reaction to occur are explained by the collision theory.

The collision must provide enough energy to overcome the energy barrier called activation energy.

If the energy of the collision is less than the activation energy then the bonds of the reactants will not be broken and the reactants will not be transformed into other substances (products).

Also, to brake the  bonds of the reactants and form the new ones of the transition state (intermediate compound known as activated complex), it is needed that the atoms of one molecule hit the right atoms of the other molecule, this is the collision must have the appropriate orientation to yield the products.

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.