Out of electrovalent and covalent bonds, which one is the strongest?
The electrovalent bond, or ionic bond, tends to be the stronger of the two when choosing between ionic and covalent. The ionic bond is a bond formed between two ions that have opposing polarities, one a positive cation (+) and one a negative anion (-). The larger the electronegative difference between the two ions, the stronger the force of attraction. A good example would be the formation of a sodium ion (Na+) and a chlorine ion (Cl-). The difference between these two ions is large, sodium being from group 1 on the periodic table, while chlorine is all the way across the periodic chart in group 17. These two form a very solid ionic bond to form one molecule of sodium chloride, table salt.
It should be noted there are no "pure" ionic bonds; they all have a degree of "covalence", which means they share the required electrons between them. A good example of a covalent bond would be any of the diatomic elements, such as oxygen, O2. Oxygen has an oxidation number of -2, meaning it has room for two electrons in it's outer electron orbital. It shares two single electrons with another oxygen atom and forms a relationship between them best descibed as "codependent", hence the term "covalent".