Covalent bonds are defined as the bond in which sharing of electrons takes place between atoms. The sharing of electrons is in equal number so that it form a stable balance of attraction and repulsion between atoms. In the given example of CO2 (first image) oxygen is sharing equal number of electrons with carbon to form a stable bond called covalent bond.
Ionic bonds are formed when valence electrons are transferred to other atoms and form oppositely charged ions. In ionic bond formation, the atoms that gain electrons become negatively charged and the atoms that loses electrons become positively charged. In the given example of Ca Cl2, Ca is also giving its 2 valence electrons to each Cl and there is no stable balance of attraction and repulsion between atoms.
Pressure decreases with increasing altitude. At higher elevations, there are fewer air molecules above a given surface than a similar surface at lower levels.
