In this next section of the class I'm interested in exploring chemical compounds in more detail. My goal is going to be to develop an idea of the three-dimensional shape of compounds. Shape is an important feature in organic chemistry (carbon compounds) an area we are going to discuss in a week or so. So how to get started, the best way is to reveiw some things we have already talked about. The Periodic Table!

The Periodic Table lists all of the known elements. Chemists separate the elements a variety of different ways. One way breaks the periodic table into three groups.

One group of elements is called the metals. The metallic elements are characterized by their solid phase (although there is one metal which is a liquid). They are shiny, malleable (can be easily hammered into a shape), ductile (can be easily drawn into a wire), have a high melting point (that is why they are solids!) and are able to conduct electricity.

Another group of elements is called the nonmetals. The nonmetallic elements are characterized by their range of phases. Nonmetallic elements can be solid, liquid or gas phase. Nonmetallic elements do not conduct electricity very well (although one of the nonmetallic elements can). Nonmetallic elements are not shiny.

The last group of elements are intermediate in their properties, some of their properties suggest metallic character, other properties suggest nonmetallic character. Needless to say these elements can not make up their minds! Interestingly enough the metalloids are found exactly between the metallic elements and the nonmetallic elements.

Finally, if an element belongs to the metallic group it can not be in any of the other groups.

Here is a periodic table with the metals in blue, nonmetals in yellow and the metalloids in..what is that color..between the metals and the nonmetals.

Elements in the periodic table combine to form compounds. Chemists have classify compounds into two general groups, again based on their physical properties. The two groups of compounds are called ionic compounds and covalent compounds.

Ionic compounds are characterized by the fact that they are always solids (there are some interesting exceptions to this rule that have recently been discovered) and have high melting points. They are called ionic compounds because they are composed of ions. Ions are charged atoms or molecules. Ions that have more electrons than they have protons are called anions, while ions with fewer electrons than they have protons are called cations. Cations and anions get together to form an ionic compound such that the total positive charge on the cation(s) equals the total negative charge on the anion(s). Chemists have discovered that ionic compounds almost always contain an element from a metallic group and at least one element from the nonmetallic group.

Here is a model of an ionic compound like sodium chloride.

 

A simple model to be used to imagine what ions (cations or anions) look like at the atomic level, is a sphere. For simple monoatomic ions and most polyatomic ions we can think of as charged spheres of different sizes.

In the state the spheres (ions) are arranged so that any particular ion is surrounded by the oppositely charged ion. What ever pattern which arises depends on several factors which we will not be discussing at this point in our class. Below is a picture of a model of a simple ionic compound. Notice the spheres (ions) of different sizes and how each sphere (ion) is surrounded by the other spheres (ion).

So now we know what an ionic compound 'looks' like at the atomic level, how do ions form? What causes an element to lose or gain electrons? And how many elecrons are gained or lost by an element? The real gory details are beyond the scope of this class so we'll limit our discussion to the trends that are observed. We started this discussion with the periodic table, now it is time to return.

When a metallic element and a nonmetallic element react to form an ionic compound the metal loses electron(s) and nonmetal gain electron(s). So how do we know how many electrons are lost or gained? This is a little more interesting. Metals and nonmetals tend to lose or gain electrons to attain the same number of electrons as the nearest noble gas element. Noble gases are very interesting, why?, because they do not react with very many other elements. In fact no known compounds containing helium or neon have ever been formed! What is so special about the noble gas elements? They appear to have a special number of electrons. Helium has 2, neon, 10, argon 18, etc. These numbers of electrons appear to lend extra stability, i.e., the element does not like to react with other elements.

Although we have talked about the number of electrons in an atom we have not done much beyond that. The electrons are located outside the nucleus. We know protons and neutrons are make up the nucleus, but electrons are located outside the nucleus. The electrons occupy regions of space which surround the nucleus. The regions of space ae 3-dimensional in shape. When an element reacts, it loses or gains electrons. The electrons lost are always the electrons furthest away from the nucleus. That makes sense because the electrons furthest from the nucleus do not feel the attraction that electrons close to the nucleus feel. The electrons furthest from the nucleus are also called 'valence' electrons. Every atom has a certain number of valence electrons. How do we know what the valence electrons are? We simply look at the number at the top of the column that the particular element is located in to find the number of valence electrons.

So how many valence electrons in the element Li?

Looking at the periodic table at the top of the page, lithium has a number and a letter at the top of the column, 1A. We'll just concern ourselves with the number for the moment. Lithium has one valence electron. We know that a neutral lithium atom has three electrons, but one of those electrons is a valence electron and the other two electrons are called 'inner-core' electrons. The inner-core electrons are close to the nucleus and are not as available to react as the valence electron.

So how many valence electrons in the element N?

Looking at the periodic table at the top of the page, nitrogen has five valence electrons. (In fact, all of the elements below nitrogen in the same column have five valence electrons.)

So how many valence electrons in the element Ne?

Looking at the periodic table at the top of the page, neon has eight valence electrons.(All of the elements below neon in the same column have eight valence electrons.)

So how many valence electrons in the element Ni?

This is a more difficult question. We'll now turn our attention to the letter next to the number at the top of each column. If the letter is an 'A', the number next to it is the number of valence electrons, if the letter is a 'B', the number next to it may be the numbner of valence electrons and it may not be! Nickel is a transition metal. The elements in the 'B' columns are called transition elements. The outer most electrons in these elements can be lost more easily and as a result the transition metals will lose different numbers of electrons under certain circumstances. For our purpose we must remember that the transition metal elements have two or three valence electrons. They actually have more than this but for now we just will remember two or three. So for nickel the answer is two or three.

So how many valence electrons in the element Fe?

Two or three.

Lets return to the number of valence electrons in the noble gas elements. That is the column with VIIIA at the top. This group has eight valence electrons and the elements int he group are not very reactive. It turns out that when a metal reacts with a nonmetal, both elements lose or gain electrons to have eight valence electrons. (Note: hydrogen is an exception to this rule, in that it likes to lose or gain one electron.)

So lets consider some examples,

How many electrons does a chlorine atom gain?

Well since chlorine is in group VIIA, it has seven valence electrons. To have eight valence electrons it will gain one electron. If a chlorine atom gains one electron, it would now have eight valence electrons, but a total of 18 electrons. A chlorine atom only has 17 protons, so there is one additional electron. How do we communicate the extra electron? We write the symbol as Cl-, and we change its name to chloride.

How many electrons does a oxygen atom gain?

Oxygene is in group VIA, it has six valence electrons. To have eight valence electrons it will gain two electrons. By gaining two electrons, it now has eight valence electrons, and a total of 10 electrons. An oxygen atom only has 8 protons, so there are two additional electrons. How do we communicate these extra electrons? We write the symbol as O2-, and we change its name to oxide.

How many electrons does a sodium atom lose?

Sodium is in group IA, it has one valence electron. Now you might be concerned becasue you may be asking how does a sodium atom with one valence electron lose electrons to get eight valence electrons?!! Actually it is very easy. If we remove one electron, the only valence electron it has, sodium will now have ten electrons! That is the same number of electrons as neon has! By losing one electron, sodium now has the same number of valence electrons as neon. Sodium would never want to lose two electrons. How do we communicate the loss of electrons? We write the symbol as Na+, but we still call it sodium, we just say sodium ion now.

How many electrons does a calcium atom lose?

Calcium is in group IIA, it has two valence electron. By losing two electrons, calcium now has the same number of valence electrons as argon. How do we communicate the loss of electrons? We write the symbol as Ca2+, but we still call it calcium, we just say calcium ion now.

Armed with this information we are now able to actually determine FORMULAS OF SIMPLE BINARY IONIC COMPOUNDS!!! Lets try a few for practice to see how it works.

What is the formula of a compound which contains sodium and chlorine?

We HAVE TO REMEMBER metals always lose electrons and nonmetals alway gain electrons when forming compounds. Sodium will lose its single valence electron to form Na+, and chlorine will gain one electron to form Cl-. Writing a formula means we need to have same amount of positive charge as negative charge, so the formula is neutral. And we alway select the simplest ratio of atoms. Since both ions have charges of one, the formula must be NaCl...sodium chloride. Isn't this cool!!

What is the formula of a compound which contains calcium and oxygen?

Calcium in in Group IIA and will lose two valence electrons to form Ca2+. Oxygen will gain two electrons to form O2-. Writing a formula means we need to have same amount of positive charge as negative charge, so the formula is neutral. And we alway select the simplest ratio of atoms. Since both ions have charges of two, the formula must be CaO.. calcium oxide. Isn't this cool!!

How do we name these binary ionic compounds?

The correct chemical name is composed of the name of the metallic element followed by the name of the nonmetal which has been modified to the identifying stem plus an -ide ending.

 

Symbol

Element

Stem

ending

B

boron

bor

boride

Br

bromine

brom

bromide

Cl

chlorine

chlor

chloride

F

fluorine

fluor

fluoride

H

hydrogen

hydr

hydride

I

iodine

iod

iodide

N

nitrogen

nitr

nitride

O

oxygen

ox

oxide

P

phosphorus

phosph

phosphide

S

sulfur

sulf

sulfide

 

So how would we name some compounds?

Example #1: NaCl

To name this compound we name the metal ion first, sodium and Cl is called chloride...sodium chloride.

Example #2: PbI2

To name this compound we name the metal ion first, lead and I is called iodide...lead iodide.

Example #3: Al2O3

Try naming this one yourself. Click here for an answer.

Example #4: BaS

Try naming this one yourself. Click here for an answer.

 

There are also a group of molecules that are charged ions, these are called polyatomic anions. Here is a table of the important polyatomic anions that I want you to memorize.

Name

Formula

Nitrate

NO3-

Sulfate

SO42-

Carbonate

CO32-

Bicarbonate

HCO3-

Hydroxide

OH-

Cyanide

CN-

Phosphate

PO43-

Nitrite

NO2-

We can write formulas of ionic compounds using metals and these polyatomic anions.

What is the formula of a compound which contains sodium and nitrate?

We HAVE TO REMEMBER metals always lose electrons when forming compounds. Sodium will lose its single valence electron to form Na+. Nitrate has the formula, NO3-. The charge on nitrate is a -1. Writing a formula means we need to have same amount of positive charge as negative charge, so the formula is neutral. And we alway select the simplest ratio of atoms. Since both ions have charges of one, the formula must be NaNO3...sodium nitrate.

What is the formula of a compound which contains calcium and nitrate?

Calcium in in Group IIA and will lose two valence electrons to form Ca2+. Nitrate has the formula, NO3-. The charge on nitrate is a -1. Writing a formula means we need to have same amount of positive charge as negative charge, so the formula is neutral. And we alway select the simplest ratio of atoms. Since calcium has a 2+ charge and nitrate has a 1– charge we need two nitrates for every calcium. To write the formula we need to be careful to properly indicate two nitrates. To do this we must use parentheses around the polyatomic anion....Ca(NO3)2. So two nitrate ions are communicated by using the subscripe two.

What is the formula of a compound which contains potassium and sulfate?

Potassium in in Group IA and will lose one valence electron to form K+. sulfate has the formula, SO42–. The charge on sulfate is a 2–. Writing a formula means we need to have same amount of positive charge as negative charge, so the formula is neutral. And we alway select the simplest ratio of atoms. Since potassium has a 1+ charge and sulfate has a 2– charge we need two potassium ions for every sulfate. Since potassium is not a polyatomic ion we do not need to use parenthese. So the correct formula is....K2SO4. The name of this compound is potassium sulfate. We do not use parenthese around the polyatomic anion when there is only one in the formula.

Ready to try a few on your own?

Example #5: Write the formula of a compound which contains barium and carbonate. Click here for an answer.

Example #6: Write the formula of a compound which contains magnesium and phosphate. Click here for an answer.

 

From this new group of polyatomic anions we can now generate a whole new list of ionic compounds to name.

Example #7:What is the name of the compound with the formula, Na2SO4?

We use the same apporach to name this compound as we used naming the binary ionic compounds. Name the metal then the polyatomic anion. The name of Na is sodium and the polyatomic anion is SO42–, sulfate. When memorizing the formulas of the polyatomic anions you need to be comfortable recognizing the patner of the formula without the charge. So seeing SO4 in Na2SO4 means sulfate, SO42–.

Example #8:What is the name of the compound with the formula, LiNO2?

We use the same apporach to name this compound as we used naming the binary ionic compounds. Name the metal then the polyatomic anion. The name of Li is lithium and the polyatomic anion is NO2–, nitrite. The correct name is lithium nitrite.

Example #9:What is the name of the compound with the formula, NaHCO3? Click here for an answer.

Example #10:What is the name of the compound with the formula, Mg(NO3)2? Click here for an answer.

Covalent compounds are characterized by the fact that they may be gases, liquids, or solids. So the melting point of covalent compounds is low. Covalent compounds are not composed of ions. When a covalent compound is formed the atoms are attracted together by the sharing of electrons (more about this later). Chemists have found that almost all covalent compounds contain only nonmetallic elements.

Here is a list of formulas of some ionic and some covalent compounds. Based on the information above indicate whether the compound is ionic or covalent.