Why atoms can form bonds

Atoms can bond to one another because attractions between the atoms are stronger than the repulsions.

 

Origins of repulsions between atoms..

Repulsions: Repulsions exist because particles with the same sign charge repel, electrons repel electrons and plus charged nuclei repel plus charged nuclei .Repulsions are BAD for bonding because they increase the energy of the particles and force them apart. You have seen this with small magnets when the north poles of two magnets are brought together.

Origins of attractions between atoms.

Attractions: Attractions exist between appositely charged particles. Electrons are attracted to their own positively charged nucleus and the positively charged nucleus of a nearby atom. Attractions are good for bonding because they lower the energy of the particles and permit them to come close together. The nucleus of an atom is attracted to its own electrons and electrons of nearby atoms.

The repulsions are produced between negatively electrons in one atom and the negatively electrons in the other. There are also repulsion forces between the plus charged nucleus of an atom and the nucleus of a nearby atom.

In order for a pair of atoms to form a bond, the total of all repulsions must be less than the attractions. The electron-electron and nucleus-nucleus repulsions must be offset. The repulsions increase energy, the attractions release and lower the energy.

The energy released by all of all the attractions have to be more than the energy increase from all of the repulsions for a bond to exist. The attractions do not always release enough energy to offset the repulsions. Helium doesn't form a bond with another helium to make He₂ because the repulsions are bigger than the attractions. Mercifully you do not need to figure out the energy values to predict whether or not a bond will form. All you need to do is look at a nonmetal atom to see if it has a completed valence shell. The hydrogen has one electron in the "1s" so the valence shell still can accept one electron. Hydrogen can form one bond. When an atom has a filled valence shell it normally cannot form a bond. Helium has two electrons in the "1s" shell so it has a filled valence shell and doesn't form a bond with any other atom, no way no how.

Exercise: Let's try to predict whether a pair of atoms will form a bond. What do you predict will happen when a neon atom and another neon atom are brought close together? Will they form a bond? What justifies your answer?

Click here for answer

The octet rule is useful for predicting bonding. You look to see if the nonmetal atom has a complete octet of valence electrons.

Exercise:

Let's look at a combination of chlorine, Cl, and hydrogen, H.  Will they form a bond?

 

First check the group numbers to determine the number of valence electrons on each atom.

 

Chlorine is in group 7A, it has seven valence electrons. Hydrogen is in group 1A, it has one valence electron. The chlorine can accept one more electron to complete its valence shell. The hydrogen can accept one more elctron to complete its valence shell. The two atoms are predicted to form a bond. Guess what, HCl is a common compound. Hydrogen chloride is found dissolved low concentrations in stomach fluid.

Exercise:  Do you predict H and S will form a covalent bond? How many hydrogens will combine with the sulfur, S?

First thing to do is check the group numbers again. Hydrogen is still in group 1A. It can provide only one electron.

Sulfur is in group 6A. It has six valence electrons.

Some math tells us that the sulfur can accept two electrons to complete the valence shell octet.

This means that the H and S are predicted to form a covalent bond. The sulfur "needs" two electrons so it must form two bonds with two different hydrogen atoms to complete an octet. The formula for the combination will be H₂S.

Exercise:

What do you predict will happen when oxygen and hydrogen are brought together? Will they form a bond? How many hydrogens will bond to an atom of oxygen? Click for answer

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