Acovalent chemical bond is one in which a covalent chemical bond is one in which outer-shell electrons of two atoms are shared so as to satisfactorily fill their respective orbitals electrons are removed from one atom and transferred to another atom so that the two atoms become oppositely charged protons and neutrons are shared by two atoms so as to satisfy the requirements of both atoms electrons from the same atom, but opposite spins, are paired outer-shell electrons of one atom are transferred to fill the inner electron shell of another atom

            A covalent chemical bond is one in which outer-shell electrons of two atoms are shared so as to satisfactorily fill their respective orbitals.

Explanation:

                   Covalent bonds are formed between two atoms having their electronegativity difference less than 1.7. In this type of bonding the valence electrons of one atoms forms molecular bond with the valence electrons of another atom. The electrons are mutually shared.

                   Covalent bond can be non-polar as for example formed between hydrogen and carbon atoms.

                   Also, covalent bond can be polar in nature as that formed between hydrogen and chlorine atoms because the chlorine atom is more electronegative and hence attracts the electrons more towards itself making density of electrons less on hydrogen atom.

a chemical reaction happens

explanation:

assuming that your periodic table has eighteen columns, you should highlight   the four elements in the third column from the left of the table.

neutral atoms of elements after calcium contain d electrons. the energy of d orbitals of a particular main shell lies between the energy of the s and p orbitals of the next main shell. for example, electrons in the have energy higher than those in the orbital while lower than those of the orbital.

by the aufbau principle, electrons fill orbitals one at a time, starting from those of the lowest energy. the orbital is therefore filled right after orbitals and before orbitals.  

potassium has electron configuration and calcium . the s orbital of a main shell holds a maximum of two electrons. the next element would thus have the electron configuration and satisfy the requirements.

a neutral atom of the element contains three electrons on top of the most recent noble gas element. it should thus be placed in the third column of the periodic table. both scandium and yttrium would satisfy the conditions. an aufbau diagram indicates that the 4f orbital is filled before the 5d. however, it appears that exceptions exists for both lanthanum and actinium. neutral atoms of the two d-block elements also demonstrate the and should be highlighted as well.