October 21, 2011
CH-111
Chapter 2 - Overview
Atoms, Molecules, and Ions
I'm going to backtrack a little to Chapter Two. I'm doing this because I feel like I need a more solid understanding and foundation of charges, etc. I'm not going to go by section, but rather, I'm going to summarize the entire chapter in one long burst of information. Here goes.
Atomic Numbers, Mass Number, and Isotopes
An atom is individual from all other atoms based on its number of protons. This is how we define an atom as Carbon, or Aluminum, or Arsenic. Each atom has a characteristic number of protons. This number is called the element's atomic number. Carbon's atomic number is 6, Aluminum 13, etc. This is because Carbon has 6 protons in its nucleus. The number of protons and neutrons in the nucleus of an atoms is its mass number. Carbon has a mass number of 12, because it has 6 protons and 6 neutrons. Although all atoms of a particular element have the same number of protons, and that is characteristic of all atoms of their kind, the number of neutrons their nuclei contain can vary. These variations are called isotopes. Carbon-13, for example, has 6 protons and 7 neutrons. Because this variation is possible, atoms may also vary in mass. The average of all the weights of all the isotopes of a particular type of elements is considered the atomic weight of that element. For Carbon, that weight is 12.0107, often times rounded simply to 12.01, or sometimes even just 12 for rough calculations (I prefer 12.01).
Atomic Mass Scale
We determine the masses of individual atoms with a high degree of accuracy. To make this more convenient, we use the atomic mass unit (amu) when dealing with extremely small masses, as we see in subatomic particles.
1 amu = 1.66053 x 10^-24g, that's 000000000000000000000000.166053g, unimaginably small.
1g = 6.02214 x 10^23amu, again, unimaginably small.
The amu is used to prevent calculation mistakes when working with so many zeros and simplify making calculations (as much as possible).
Atomic Weight
I breifly discussed atomic weight above as being an average of the masses of all isotopes of a particular element.
Atomic weight = Σ [(isotope mass) x (fractional isotope abundance)] / all isotopes of the element
I'm proud of myself for hunting down that symbol for "sum".
Here's a rundown of that equation, using the mother of all elements, Carbon. Go figure.
Carbon in its naturally occuring state, is composed of 98.93% Carbon-12 and 1.07% Carbon-13. The masses of these isotopes are 12amu exactly and 13.00335amu, therefore:
(0.9893)(12amu) + (0.0107)(13.00335amu) = 12.01amu
Periodic Table
I am not going to summarize what a periodic table is, how to use it, or really, anything about it at all. I'm going to note the important groups and the type of elements they contain as a reference.
Group 1A contains the Alkali metals: Li, Na, K, Rb, Cs, Fr
Group 2A contains the Alkaline Earth Metals: Be, Mg, Ca, Sr, Ba, Ra
Group 6A contains the Chalcogens: O, S, Se, Te, Po
Group 7A contains the Halogens: F, Cl, Br, I, At
and Group 8A contains the Noble Gases: He, Ne, Ar, Kr, Xe, Rn
Ions and Ionic Compounds:
Ion - a charged particle formed as the result of electrons being removed or added to an atom.
Cation - An ion with a positive charge.
Anion - A negatively charged ion.
When an atom loses an electron, it becomes positively charged by 1, therefore +1. If it loses two electrons, it becomes +2, etc. When an atom gains an electron it becomes negatively charged by 1 (-1), and etc. These net charges are represented by a superscript. For +1 or -1 superscripts, we just put + or -. It is important to remember that a positive charge actually means that an atom has LOST electrons, and a negatively charged ion has actually GAINED electrons. This is because a proton is positively charge, an electron is negatively charged, and when they are in equal amounts they form a neutral atom. When electrons are distributed or taken away, the result is negative or positively charged ions. It is solely dependent on electron, proton amounts always remain the same.
A chlorine atom has 17+ protons and 17- electrons, making a neutral atom.
When an electron is gained (as with chlorine it always is, we will elaborate later...), the atom becomes an ion, as it has gained an extra negative charge from the negatively charged election, it thus becomes Cl-.
In general metal atoms tend to lose electrons to form cations and nonmetal atoms tend to gain electrons to become anions. Thus, ionic compounds tend to be composed of metals bonded to nonmetals.
Polyatomic ions consist of atoms joined as in a molecule, but they have a net positive or negative charge. SO4-2 (sulfate ion) and NH4+ (ammonium ion) are examples of polyatomic ions.
It is important to remember that ions are not the same as the atoms they are formed from. They behave differently than their associated atoms.
Predicting Ionic Charges
Helpful, helpful, helpful.
All elements will gain or lose electrons to attain as stable an electron configuration as possible. The noble gases already have stable electron configurations, so we can say that all elements will gain or lose electrons to resemble the electron configuration of the noble gas nearest to them. So all we need to do is look at the periodic table to determine whether it's a gain, a loss, and what noble gas is the nearest to determine how many electrons we're gaining or losing.
For example, we'll go back to Chlorine, with it's gain of one electron to become Cl-, we see that chlorine always gains one electron to stabilize its electron configuration because on the periodic table it sits next to the noble gas Argon. Argon has 18 electrons, so chlorine with its 17 will gain one.
Conversely, we can look at potassium (K), which is also one away from Argon. Potassium, however, has 19 electrons and therefore loses an electron and becomes positively charged. Atoms will always go the easiest route to attain stabilized electron configurations. Ie: Potassium, being nearest in number of electrons to Argon, will not lose 9 electrons to resemble Neon, it will lose one to be closest to Argon. It will not gain 17 electrons to become Krypton. For this reason, we can arrange the periodic table (at least the metals and nonmetals) according to how many electrons they lose/gain by group:
Group 1A - Will always lose 1 electron - Li, Na, K, Rb, Cs, Fr
Group 2A - Will always lose 2 electrons - Be, Mg, Ca, Sr, Ba, Ra
Group 3A - Will always lose 3 electrons - B, Al, Ga, In, Tl
Group 4A - Can gain or lose 4 electrons - C, Si, Ge, Sn, Pb
Group 5A - Will always gain 3 electrons - N, P, As, Sb, Bi
Group 6A - Will always gain 2 electrons - O, S, Se, Te, Po
Group 7A - Will always gain 1 electron - F, Cl, Br, I, At
This is just a general guideline.
Ionic Compounds
Much of what constitutes chemical activity involves electron transfer. Elemental sodium, allowed to react with elemental chloride, will form the ionic compound Na+Cl-. This happens because an electron is transferred from the sodium atom to the chlorine atom. Ionic compounds are generally combinations of metals and nonmetals. Molecular compounds are usually non-metals only.
Ions in an ionic compound always occur in such a ration that the total positive charge is equal to the total negative charge, as in NaCl. Na +1, Cl -1, the charges are equal. Thus, when determining ionic compounds, we can reverse charges to find the numbers of each element necessary to balance the ion.
For example; How many of each atom in MgN to have equal positive and negative charges?
Mg is in group 2A so we know that Mg loses 2 electrons, and N is in group 5A so we know that N gains 3 electrons- Mg+2, N-3. Simply reverse the charges: Mg3N2. 3 Mg atoms give up 2 electrons each = 6, 2 N atoms take 3 electrons each = 6. 6 electrons lost, 6 electrons gained.
Nomenclature
Naming Inorganic Compounds
CATIONS
a) Cations formed from metal atoms have the same name as the metal:
Na+ sodium ion
Zn+2 zinc ion
Al+3 aluminum ion
b) If a metal can form cations with different charges, the positive charge is indicated by a Roman numeral in parentheses following the name of the metal:
Fe+2 iron(II) ion
Fe+3 iron(III) ion
Cu+ copper(I) ion
Cu+2 copper(II) ion
Most metals that occur with different charges are transition metals, see periodic table group 3B-2B. Ag+ and Zn+2 are exceptions, only having one possible charge. When in doubt with transition metals, use Roman numerals.
c) Cations formed from nonmetal atoms that have names ending in -ium:
NH4+ ammonium ion
H3O+ hydronium ion
These two are the most frequently encountered ions ending in -ium.
Common Cations:
Charge of +1
H+ hydrogen ion
Li+ lithium ion
Na+ sodium ion
K+ potassium ion
Cs+ cesium ion
Ag+ silver ion
Charge of +2
Mg+2 magnesium ion
Ca+2 calcium ion
Sr+2 strontium ion
Ba+2 barium ion
Zn+2 zince ion
Cd+2 cadmium ion
Charge of +3
Al+3 aluminum ion
Other ions
NH4+ ammonium ion
H3O+ hydronium ion
Ions with Roman numerals
Co+2 cobalt(II) ion
Cu+ copper(I) ion
Cu+2 copper(II) ion
Fe+2 iron(II) ion
Fe+3 iron(III) ion
Mn+2 manganese(II) ion
Hg2+2 mercury(I) ion
Hg+d mercury (II) ion
Ni+2 nickel(II) ion
Pb+2 lead(II) ion
Sn+2 tin(II) ion
Cr+3 chromium(III) ion
ANIONS
a) The names of monatomic anions are formed by replacing the ending of the name of the element with -ide:
b) Polyatomic anions containing oxygen have names ending in either -ate or -ite, they are called oxyanions. The -ite used for the oxyanion of the same element but with fewer oxygen atoms. The -ate suffix is use for the most common or representative oxyanion of an element, while the 0ite is used for an oxyanion that has the same charge but one O atom less.
Prefixes are used when the series of oxyanions of an element extends to four members (as with the halogens).
Prefixes:
Per- indicates one more O atom than the oxyanion ending in -ate, ex: ClO4- is perchlorate, because it has one more O than ClO3-, which is chlorate.
Hypo- indicates one O atom fewer than the oxyanion ending in -ite, ex: ClO2- is chlorite, so hypochlorite would be ClO-.
c) Anions derived by adding H+ to an oxyanion are named by adding as a prefix the word hydrogen, or dihydrogen, as appropriate. ex: H2PO4- dihydrogen phosphate ion.
Note that each H+ added reduces the negative charge of the parent anion by one.
Remember bicarbonate? It's old, outdated even. We don't roll like that anymore. Now we're more into calling it "hydrogen carbonate ion". But no, people refuse to conform. So it's necessary to know it as BOTH names, common in chemistry.
How about a little list action, eh?
Charge -1
H- hydride ion
F- flouride ion
Cl- chloride ion
Br- bromide ion
I- iodide ion
CN- cyanide ion
OH- hydroxide ion
CH3COO- acetate ion
C2H3O2- acetate ion
ClO3- chlorate ion
ClO4- perchlorate ion
NO3- nitrate ion
MnO4- permanganate ion
Charge -2
O^2- oxide ion
O2^2- peroxide ion
S^2- sulfide ion
CO3^2- carbonate ion
CrO4^2- chromate ion
Cr2O7^2- dichromate ion
SO4^2- sulfate ion
Charge -3
N^3- nitride ion
PO4^3- phosphate ion
My favorite is permanganate. I fear I will have a very unlucky child someday who bears this name. When she asks why I did that to her, I may scar her forever by telling her she was a permanent stain on my life. She may or may not need serious psychological help after this occurrence. Hey, look at that. I spelt occurrence right on the first shot for once. Back to chem.................
Names and formulas for acids
Acid: a substance whose molecules yield hydrogen ions (H+) when dissolved in water. An acid is composed of an anion connected to enough H+ ions to neutralize, or balance, the anion's charge. For example, one of the most commonly known acids is HCl, which is the corresponding acid to Cl-. The hydrogen added on has a positive charge of +1, therefore it is capable of neutralizing chloride's -1 charge. Similarly, Sulfide (S^2-) will need two hydrogen atoms to neutralize its -2 charge, and therefore the corresponding acid to sulfide would be H2S, or hydrosulfuric acid.
Let's talk prefixes.
1. Acids containing anions whose names end in -ide are named by changing the -ide ending to -ic, adding the prefix hydro- to the anion name, and then following it all up with the word acid. We can see this process for naming acids clearly in both of our earlier examples, HCl and H2S. "Hydro-chlor-ic acid", we add the "hydro-" prefix, change "chloride" to "chloric" and end it with "acid". It's written: hydrochloric acid. And it's awesome. Sorry, I'm a bit over-tired today.........
2. Acids containing anions whose names end in -ate or -ite are named by changing -ate to -ic and -ite to -ous and then adding the word acid.
ClO4- (perchlorate) HClO4 (perchloric acid)
ClO3- (chlorate) HClO3 (chloric acid)
ClO2- (chlorite) HClO2 (chlorous acid)
ClO- (hypochlorite) HClO (hypochlorous acid)
Names and Formulas of Binary Molecular Compounds
1. The name of the element father to the left in the periodic table is usually written first, exceptions include compounds containing oxygen and
Chlorine
Bromine
Iodine
in this cases, oxygen is written last.
2. If both elements are in the same group, the lower one is named first.
3. The name of the second element is given an -ide ending.
4. Greek prefixes are used to indicated the number of atoms of each element. The word "mono" is never used for the first element. When the prefix ends in a or o and the name of the second element begins with a vowel, the a or o of the prefix is often dropped, as in dichlorine monoxide (Cl2O).
Cl2O - dichlorine monoxide
N2O4 - dinitrogen tetraoxide
NF3 - nitrogen triflouride
P4S10 - tetraphosporus decasulfide
Next, I look at Chapter 3 : Stoichiometry, and decide if I really want to torture myself by overviewing it. Hrmmmmmm.