Elements can form different types of chemical bonds to reach noble-gas configuration. Episode 4 recaps three types of chemical bonds.
Elements can form different types of chemical bonds to reach noble-gas configuration. Episode 4 recaps three types of chemical bonds. The foundation is laid by the octet rule (1:06). “How” an element reaches the octet is determined by the difference in electronegativity between elements. Electronegativity is the ability of an atom to attract a shared pair of electrons (2:45). This leads to three types of chemical bonding: covalent bonding (4:30), ionic bonding,(5:40) and metallic bonding (7:15).
Question: You have two white substances, one ionic, one molecular. Which of the following experimental procedures lets you identify the type of bonding?
A. color
B. grain size
C. electrical conductivity
D. solubility in water
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap Chemical Bonding.
Today’s episode has it all: It’s a story of giving and receiving, of sharing and attraction, but also of war, stealing and taking, of indifference and even repulsion! It is a story that has a powerful message and a happy conclusion: Everyone's a winner! Okay, okay, I know this is not Hollywood, but Chemistry, but all these elements play a role in today’s episode since we are recapping chemical bonding.
Let’s zoom in!
“Eight” seems to be the magic number in chemistry - it even has a rule named after it: the octet rule. The octet rule summarizes the observation that atoms tend to bond with other atoms in a way that each atom has eight valence electrons. One group in the periodic table already has eight valence electrons: the noble gases. So let’s start there. Noble gases have a complete valence shell, which leads to a state of low potential energy. In nature, there is a natural strive towards low potential energy, which results in unreactivity - hence the name noble gases: they don’t interact and react with other atoms (Chemistry is the “Science of exceptions, so that’s not the full picture, but works for now). The low potential energy means that scientists would have to put in A LOT of energy to get them to react. It is a bit like a teenager lying on the couch after a loooong day of school, you have low potential energy. Your siblings might stop by, try talking to you, but you are just too tired to react. Only when they start poking you (or threaten to use your gaming console) are you reacting - sigh, siblings, cannot live with them and cannot live without them!
So how do atoms that are not noble gases and do not have eight valence electrons reach that state of low potential energy? They form chemical bonds! Generally speaking, there are three approaches to get noble gas configuration: gaining electrons, losing electrons and sharing electrons. This is determined by the concept of electronegativity - also known as: The element’s aptitude in a game of “tug of war”.
Electronegativity is the ability of an atom to attract a shared pair of electrons. Or, in our tug of war analogy: with how much force the atom can pull on the rope. This force is mainly determined by two factors: how many protons the atom has and how far away the valence electrons are. The more protons - or the number of people on your team - the stronger the pull on the shared valence electrons; the further away, the weaker the pull on shared valence electrons. Elements that are small and have a relatively high number of protons are in the top right corner of the periodic table: fluorine, chlorine, and oxygen. These have a high electronegativity. Elements with a low electronegativity are in the bottom left corner of the periodic table: Cesium, Francium, Radium. Electronegativity is a periodic trend: it increases from left to right, since the number of protons increases and it decreases from top to bottom, because the distance between the nucleus and the valence electrons gets larger. Following that trend, generally speaking, metals have a low electronegativity and nonmetals have a relatively high electronegativity.
Now, how does electronegativity affect how atoms reach noble gas configuration? Let’s continue with our tug of war analogy: In the beginning, the marker starts between the two teams. Now, depending on the difference in skill and strength, the marker will either stay somewhat in the center or, over a certain amount of time, one team will pull it all the way to its side. This results in two different scenarios: Either the teams are in an eternal tug of war, because both teams pull with relatively similar strength, or one team is so much stronger that it wins the tug of war - don’t worry, as mentioned before, everyone's a winner today!
It is similar for bonded atoms: if the two atoms have a “decent” electronegativity and the difference in electronegativity is relatively small, the electrons are shared more or less equally between the two atoms because they are attracted by both nuclei: This results in a molecule with a covalent bond. These shared electrons are counted towards both atoms. Let’s look at an example: Fluorine has seven valence electrons. If two fluorine atoms share one of their valence electrons with one another, the difference in electronegativity is 0 - we have the eternal tug of war. At the same time, each fluorine has its six remaining valence electrons, the one that it shares and the one that is being shared with it by the other fluorine - 6+1+1 is eight, at least according to Adam Riese (German mathematician - give it a google!)
As a rule of thumb, if two nonmetals are bonded with one another, due to a similar high electronegativity, they form a covalent bond. Examples are all around us: sugar, water, plastic, and so many more substances! Covalently bonded substances have usually low melting points, are poor conductors of heat and electricity and their solid state is brittle - like charcoal.
What if we combine an element with a low electronegativity - a metal, like sodium - and an element with a high electronegativity - a nonmetal like chlorine? The “tug of war” is over very quickly! Due to its high electronegativity chlorine will steal sodium’s valence electron. Since sodium has lost an electron with a negative charge, it now has a 1+ charge and chlorine which gained an electron now has a charge of 1-. I mentioned before that everyone’s a winner, even if the tug of war is lost, so let’s look at the valence shells of these two elements: Prior to the tug of war, sodium had one valence electron, which is now gone; the shell below becomes the new valence shell and that one has the configuration of neon - a noble gas! YEEEAH! And chlorine - it had seven valence electrons and now has eight - the same electron configuration as Argon! Everyone’s a winner!
So what does that mean in terms of chemical bonding? Due to the positive charge on the metal, in our example sodium, and the negative charge on the nonmetal, in our case chlorine, the two ions are attracted to one another and form an ionic bond. This attraction between ions is stronger than the attraction between shared electrons and the nuclei. Therefore, ionic compounds, like table salt, are brittle solids with high melting and boiling points. They also conduct electrical current - but only when dissolved in water, not as solids.
Alright, last, but not least, what if we combine metal atoms? Metals are like “tug of war? Not for me!” - they are low in electronegativity. One could say they are somewhat indifferent towards valence electrons. The electrons are delocalized, which means they are moving freely around the nuclei of metal atoms. This is often called a “sea of electrons” that surrounds the metal atoms which are packed in a crystal lattice. It is a bit like a commune - everything belongs to everyone! The result of the attraction between the atomic nuclei and the sea of electrons is metallic bonding and gives metals their unique properties: they are ductile - meaning they can be drawn into wires and they are malleable - they can be hammered into thin sheets, like aluminum foil. Due to the free floating electrons, they also conduct electrical current and thermal energy as solids.
To recap……
To reach noble gas configuration, which is a low potential energy state, atoms can, depending on their electronegativity, share, gain or lose valence electrons. As a rule of thumb, nonmetals form covalent bonds, nonmetals and metals form ionic bonds and metals are held together by metallic bonding. The type of chemical bond determines the properties of the substance.
Coming up next on the Apsolute RecAP Chemistry Edition: Chemical Formulas and Naming
Today’s Question of the day is about Chemical Bonding.
Question: You have two white substances, one ionic, one molecular. Which of the following experimental procedures let’s you identify the type of bonding?
A. color
B. grain size
C. electrical conductivity
D. solubility in water