Sitting in your room, you might have salted peanuts as a snack, a pencil, tea with sugar and you listen to this episode with earphones in.
Sitting in your room, you might have salted peanuts as a snack, a pencil, tea with sugar and you listen to this episode with earphones in (0:31). Sodium chloride, sugar, graphite and copper represent four types of solids. Sodium chloride is an ionic solid with low vapor pressure, high melting points and high boiling points, because of the strong attractive forces. Sugar is a molecular compound with low melting points due to weak IMFs (1:22). Graphite is an example of a network covalent solid (5:29), which generally have high melting points. Copper is representative of a metallic solid, which are good conductors of heat (6:44), ductile and malleable (7:06). Homogenous mixtures of metals are alloys (7:21).
Which of the following could be the identity of a solid that exhibits the following properties: it melts at 2973°C; it doesn't conduct electricity as a solid nor as a liquid.
A. ionic B. covalent C. covalent network D. metallic
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap types of solids and their properties.
Lets Zoom out:
Unit 3 - Intermolecular Forces and Properties
Topic - 3.2 Properties of Solids
Big idea - Structure and Properties
Introduction:
I don’t know where you currently are while listening to our episode. But I’d like to think you are sitting with a nice cup of tea, in a ceramic mug, because it tastes so much better. You might have a snack with you: some salted peanuts, and probably your listening study guide in front of you, a pencil to take notes and maybe your headphones in. While I am imagining you sitting there in front of my inner eye, I can also see a lot of solids around you, with very different properties: The copper wire in your headphones conducts electricity while the sugar in your tea definitely doesn’t. The salt on your peanuts would if you throw it in the tea - but don’t do that, that’s gross.
Let’s zoom in:
Sodium chloride, sugar, graphite and copper are all solids at room temperature. But their different structures and differences in intermolecular forces lead to different properties. Sodium chloride is our poster child for an ionic compound. Ionic compounds form between elements with low electronegativity, metals, and high electronegativity, nonmetals. The difference in electronegativity leads to the formation of ions, as we’ve discussed in Units 1 and 2. The strong attractive forces between cation and anions determine the properties of sodium chloride and all ionic compounds: They have low vapor pressure, high melting points and high boiling points. Why is that? In order to boil, so to go from liquid to gas, the attractive forces between cation and anions have to be completely broken - and that takes a lot of energy! It is a bit more subtle for melting points, because the particles only have to be rearranged, but you can definitely see a trend towards high melting points. Ionic compounds are also brittle. Imagine you are hitting the ionic lattice with a tiny, tiny, tiny hammer so that you shift one layer of ions exactly next to another. That will result in two like charges to be next to each other. And as we all know, like charges repel - and the pieces break apart with a smooth edge. Ionic compounds as solids do not conduct electricity, because there are no free-floating charged particles, BUT, you will have mobile ions when you dissolve the ionic compound in water or another polar solvent! They also conduct electricity as liquids. Be careful with your vocab: liquid means your ionic compound melted; aqueous means you dissolved the compound in water.
Let’s move from salty to sweet and take a closer look at molecular solids like sugar. As you know, in comparison with ionic compounds, the chemical formula of a molecular compound tells us the exact number of nonmetal atoms that are bonded covalently. Therefore, we have distinct, individual units which have weak intermolecular forces. Because of these weak intermolecular forces, molecular solids have low melting points in comparison with other types of solids. You can make your own caramel by heating sugar in a pan, but you won’t be able to melt sodium chloride. Be careful though: too much heat on the carmel and it quickly turns into a black, gooey, smoking mass that is more or less impossible to clean and will most likely mean the end of your pan or pot. Ugh. Neither liquids or aqueous solutions of molecular solids conduct electricity, because there are no free floating or mobile charged particles. The valence electrons are either used in the covalent bond or they are lone pairs that stick close to home, uhm, to the atom.
Let’s take a closer look at your pencil. Graphite is an allotrope of carbon. It consists of carbon atoms only. These are arranged in a two-dimensional network that are layered. Graphite’s sparkling friend is a diamond. Diamonds are also pure carbon, formed under high pressure. The carbon atoms form a three-dimensional network, where each carbon has four carbon neighbors in a tetrahedral shape. But why is diamond so hard and rigid, scoring a 10 out of 10 on the Mohs mineral hardness scale but graphite is so soft that we can even write with it? Well, graphite’s layers can slide past each other. This structure also explains why graphite conducts electrical current and diamond does not: graphite has delocalized electrons that can move between layers.
Aside from diamond and graphite, which are elements, we can also have binary compounds that form covalent networks. Two examples include silicon dioxide, aka sand, and silicon carbide. Generally speaking, these covalent network solids have high melting points because of strong covalent interactions.
We’ve discussed compounds formed by nonmetals and metals, we’ve discussed compounds formed by nonmetals only, which leaves us with solids that are made of metal atoms - metals. The metallic bonding allows for the metal to be good conductors of heat - we’ve all been there, when touching a hot pan or pot - as well as electricity, because we have our sea of mobile electrons. Metals are also ductile, they can be drawn into wire, as well as malleable, which means they can be hammered into sheets; both without breaking. This is due to the ease with which the metal cores can rearrange. Homogenous mixtures of metals or metals with nonmetals are called alloys. We distinguish between two types of alloys: interstitial alloys and substitutional alloys. In interstitial alloys, like steel, smaller nonmetal atoms, like carbon, are mixed in the spaces between iron atoms. This makes the alloy more rigid and decreases the malleability and ductility. Steel, however, is also an example for a substitutional alloy, when nickel or chromium atoms replace iron atoms.
To recap:
We distinguish between four types of solids: ionic, molecular, covalent network and metallic solids, with different properties that can be explained by their structure. Ionic solids have high melting and boiling points, low vapor pressure, are brittle and conduct electricity as liquid or in aqueous solutions. Molecular solids are composed of distinct, individual units with weak intermolecular forces. Therefore, they have low melting points. They do not conduct electricity. Covalent networks, like diamond or silicon dioxide have high melting points and are rigid and oftentimes hard. They form two or three dimensional networks. Metallic solids are good conductors of heat and electricity due to their sea of electrons. They are malleable and ductile and can form alloys.
Coming up next on the APsolute RecAP Chemistry Edition: Deviations from the Ideal Gas Law
Today’s Question of the day is about identifying the type of solid.
Which of the following could be the identity of a solid that exhibits the following properties: it melts at 2973°C; it doesn't conduct electricity as a solid nor as a liquid.
A. ionic
B. covalent
C. covalent network
D. metallic