A covalent bond, which is the strongest bond, takes action when two atoms valence overlap. For example, there are two suns and the light that shines from the two solid spheres forms many rays making a radiant cloud around each of the two. If these suns move closer together the radiant shine would be the valence. The closer they get the ray of light overlaps and they share light.
If an atom can share valence electrons it can form a bond. Two hydrogen atoms make hydrogen molecule, two oxygen atoms make an oxygen molecule, and two hydrogen atoms and one oxygen atom makes a water molecule. When these molecules are made they share electrons.
Electronegativity is a strong factor of a bond. If the electrons are shared equally a non- polar covalent bond forms. If electrons are shared unequally a polar covalent bond is formed. Electronegativity makes atoms differ in polarity. For example, a water molecule is polar, and hydrogen is non-polar.
Ionic bonds happen when a atom takes an electron regardless of the unequal charge, sodium chloride is an exampleof a psitive atom (cation) and a negative atom (anion). Ionic bond do not need the transfer of electrons, in this bond an atom takes what it wants.
There are weak chemical bonds that are found mostly in organisms, hydrogen and van der Waals interaction. An example of a hydrogen bond is the air we breathe, an oxygen molecule. Van der Waals occur when atoms and molecules are contuinually changing from negative to postive, that's why a gecko's wet toes stick to a wall, it's part of a protein and molecule.