•n2 • ccl4 • sio2 • alcl3 why are they covalent

To answer this, one needs to understand the difference between ionic bonding and covalent bonding. An ionic bond is a bond between an electronegative atom and an electropositive atom in which one atom supplies the valence electrons for the bond and the other uses those electrons to become stable. A covalent bond is a bond between two atoms of similar electrical charge in which both atoms supply a valence electron to form the bond and the electron travels between each atom in the bond.

An easy way to remember these is that an ionic bond is a bond between an atom on the upper right side of the periodic table with an atom on the left side of the periodic table. A covalent bond is a bond between an atom on the upper right side of the periodic table with another atom from the upper right side of the table.

Atoms on the left side don’t really bond with other left-side atoms because they don’t have the negative pull that right-side atoms do. Atoms want to become stable. In order to become stable, they need to have a full valence shell of electrons (that’s 8 electrons). An atom like chlorine has 7 valence electrons, so it REALLY want to bond with something more positive that wouldn’t mind giving up an electron (hence why we have ionic compounds like salt (NaCl)). Because the elements on the left side of the table are electropositive, they don’t have that drive to fill their valence shell, so they won’t bond with other electropositive atoms.

So these molecules that you listed above are covalent because each atom donates an electron to create a bond between atoms. You could also say that they are all elements of the right side of the table, so they can only be covalent.

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

explanation

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.