Back To CourseAP Chemistry: Exam Prep
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Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.
We've all heard of salt, right? Those white crystals we put on our fries are the kind of salt we are most familiar with, but what do we really mean in chemistry when we refer to something as a salt?
In chemistry, a salt is an ionic compound which is made up of two groups of oppositely charged ions. The ion with a positive charge is called a cation, and the one with a negative charge is called an anion. How many of each type of ion the salt has is important because the compound must have an overall electrical charge of zero - that is, an equal balance between positive charge and negative charge.
We'll cover more of this later.
Salts can be easily identified since they usually consist of positive ions from a metal with negative ions from a non metal.
The salt we put on our fries is actually sodium chloride and is made up of a Na1+ (that's our metal) and a Cl1- (our non-metal). Often you will see this written as Na+ and Cl- (the 1 is dropped), or simply NaCl.
Before we can understand the glue that bonds ions together, we need to learn why certain atoms become ions at all. Ions are formed one of two ways.
First, an atom can lose an electron to become a cation. Remember that electrons are negatively charged, so if an atom loses negative charge, it becomes positive.
An atom can also gain an electron to become an anion. This happens because of the atom's electron distribution and the magic number eight. Chemists often refer to the octet rule, which, put simply, just means that an atom will achieve stability when it can get eight valence, or outer, electrons. The atom will be similar to its closest noble gas in the periodic table. One way this can be achieved is by losing or gaining electrons to form an ion.
We have already learned that metals form cations by losing electrons. For example, sodium is a group 1 metal, and like all group 1 metals, it has one valence electron. This electron is not held very tightly by the atom and is easily lost, which forms the Na1+ cation.
Chlorine is a nonmetal and is found in group 17 (or 7A) on the periodic table. It has seven valence electrons, and it needs just one more electron to achieve the magic eight for stability! Getting that extra electron will form the Cl1- anion. Notice that both ions (the Na1+ and the Cl1-) have the same number charge, just opposite signs.
So, what happens when a metal sodium atom meets a gaseous chlorine atom? A vigorous reaction occurs as both atoms form ions. The sodium transfers its extra electron to the chlorine - that gives the chlorine its magic eight valence electrons and gives the sodium a positive charge. These ions are immediately attracted to each other, and the salt sodium chloride is formed. The ions are glued together by ionic bonds, which are the electrostatic attraction between opposite charged ions, which are the electrostatic attraction between opposite charged ions.
The physical and chemical properties of materials are closely linked to how they are bonded together. We now know that ions in a salt are strongly attracted to each other, forming strong ionic bonds. It takes a lot of energy to break apart an ionic bond, and the stronger the attraction, the stronger the bond. This attraction between ions means that a compound with ionic bonds will have a strong, ordered structure.
Salts often form a crystal structure or crystal lattice, a highly ordered formation of molecules. This is why we get crystals of salt on our fries.
This ordered structure and strong ionic bonding leads many salts to have some special properties. First, they tend to be crystalline solids with crystal structures. The solids also tend to be hard and brittle due to strong ionic bonding throughout the crystal. Salts also have high boiling and melting points because it takes a lot of energy to break those bonds and change the salt's matter state. Finally, salts are electrolytes, meaning they dissolve in water to create free moving ions, which are able to conduct electricity.
Keep in mind, though, while molten salts also conduct electricity, solid salts do not; ions must be free to conduct electricity.
So far, we have only talked about sodium chloride, abbreviated NaCl, but all combinations of metals and nonmetals form salts. Examples include magnesium iodide, abbreviated MgI2, and aluminum oxide, Al2O3. Remember, we always write the cation first, followed by the anion. The number following the atom tells us how many of that atom type are contained in that compound; where there is no number, there is just one atom.
For aluminum oxide, there are two aluminum atoms and three oxygen atoms, but why that number? Why not just one of each, or one aluminum and two oxygens? Because the overall salt is always electrically neutral; in other words, the positive charge must equal the negative charge, so they cancel each other out. With different charges for each atom, some salts will demand different numbers of atoms for each element.
The periodic table is the chemist's best tool for figuring out charges of ions and predicting the chemical formula of the salts they form.
The periodic table organizes elements into vertical groups. Main group metals - those in groups 1 and 2, plus aluminum, form just one positive ion. Nonmetals will only ever form one negative ion. Here's a summary of the different charges groups of the periodic table will have.
|Group Number||Ion Charge||Examples|
|1||1+||Li+, Na+, K+|
|17||1-||F-, Cl-, Br-, I-|
Unfortunately, life isn't always that simple for those metals in the middle of the table - the so-called transition metals. These metals are able to form more than one charged cation, so we can't tell just by looking at their placement in the periodic table. Instead, chemists use Roman numerals in conjunction with element names to indicate the atom's charge. For example, let's take an atom of iron (III) chloride. The Roman numeral after iron - III - tells us that the iron cation's charge is 3+.
Once we know the charge of the two ions in the salt, we can write the chemical formula with the correct number of atoms to make sure the two a zero change overall. A particularly easy way of doing this is to use the crisscross method: here the charge on one ion becomes the number following the other. When the number is 1, we just drop it.
Let's look at a few examples, starting with lithium bromide, a salt composed of Li1+ and Br1-. Since both charges are 1, we drop both numbers, and we see that the salt is composed of one lithium atom and one bromine atom.
Let's compare that with aluminum sulfide, a salt composed of Al3+ and S2-. Using the crisscross method, we take the numbers of each element and switch them to reveal the number of atoms in the compound. What do we get? Two aluminum atoms and three sulfur atoms, which means that aluminum sulfide will be abbreviated Al2S3.
Let's look at one more example. Calcium oxide consists of Ca2+ and O2-. The crisscross method would give us a result of two atoms each, Ca2O2; but we can divide by the common denominator of two for a result of just one atom each, and a compound symbol CaO.
A salt is a compound composed of two ion - a positively charged ion and a negatively charged ion. The attraction between the two ions forms strong ionic bonds, giving salts a hard and brittle crystalline structure. Salts have other specific properties due to these ionic bonds, including high melting and boiling points, as well as the ability to conduct electricity both in the molten form and when dissolved in water.
Chemical formulas of salts can often be predicted by finding the charge on one ion from its position in the periodic table, then making sure the overall charge on the salt is zero. When figuring out how many atoms will be needed of each element, just remember the crisscross formula - the charge of each element gives you the number of atoms required of the other element!
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Back To CourseAP Chemistry: Exam Prep
16 chapters | 163 lessons