Chapter 2 Atoms, Molecules and Ions

Dalton's Atomic Theory

Element — fundamental substance that can not be broken down chemically into simpler units.

Atomic Symbol — representation of elements in one or two-letter symbol

All are listed in the Periodic Table — Remember period and group from Chapter 1.

Dalton proposed:

Element of chemistry = atoms

Atoms in elements have the same mass and the same property

Compounds (molecules) are made of different elements of simple numerical ratio

In chemical rxn, no atom is created nor destroyed

Conservation of Mass

Matter can neither be created nor destroyed during a chem rxn. It also means that the mass of all reactants add up to the mass of all products.

Priestly (1774) Isolation of O₂ by Heating HgO

2HgO → 2Hg + O₂

Lavoisier (1774) Oxygen is involved in combustion.

Law of Definite Proportions

Compounds contain — same proportions of elements by mass

For example: Benzene has carbon and hydrogen and always 12g(C)/1g(H) ratio.

methane also contains C and H: 12g(C)/4g(H) = 3g(C)/ 1g(H)

Law of Multiple Proportions

Mass of compounds == Integral multiples of two or more elements

Elements A and B can form compounds in: AB, AB₂, A₂B, AB₃, etc.

Example: If there is 17g of Ammonia, then there must be 14g of Nitrogen and 3g of Hydrogen. In ammonia, the ratio of H:N is always 1:4.632. Always.

Example: In a closed vessel, 0.455g of Mg is allowed to burn in O₂ with its mass 2.315g. The unreacted O₂ was found t be 2.015g. What is the mass of MgO₂ produced?

Using Conservation of Mass, we know that:

Before Rxn: Mg(0.455g) + O₂ → Total mass = 2.770g

After Rxn: O₂(2.015g) + MgO( ? g) → Total mass = 2.770g

Therefore, m(MgO) = 2.770g - 2.015g = 0.775g MgO.

The Structure of the Atom

Electron

First component of atom discovered -- Electron

_e-

_p+

The yellow sponge cake portion is the smeared positive charges

The electrons are embedded as chunks of plums, which oscillate back and forth.

... but this is a wrong picutre!

Radioactivity

X ray → Pentrating radiation from Radioactive substance
Subsequently,

α particle

β particle

γ ray

Radioactivity = spontaneous emission of particles or radiation

Proton and Nucleus

Rutherford, Geiger, Marsden (1909) -- nuclear atom

Applied α particles (from radioactive decay) to tink gold foil

From this and other experiments, they concluded:

Atom -- mostly empty space

Magnitude of positive charge -- different for different atoms, approximately 1/2 of atomic mass

The charge of nucleus = # of electrons -- and that atom is electrically neutral.

They proposed a "planetary model of the atom".

.... but this is also wrong!

Neutron

H structure was solved by Rutherford. But problem remained.
Helium mass was 4 times as much as Hydrogen, for where Rutherford knew there is one proton. Helium is known for having 2 protons, then why 4 time as much mass?

Chadwick (1932, long after Rutherford's experiment of proton) found Neutron

Today's View

This is how we view today. more details in chapters 5 and 6.

Nuclear Components:

How atom should look

Proton discovered by Rutherford in 1919

Neutron discovered by Chudwick in 1932
Each of them is made up of 3 quarks.

It's just a fuzzy ball.

Today, this is how we view atom. A fuzzy ball. The blue halo is fast-moving electron cloud, and lighter the blue more electrons are found in that region. The halo effect comes from the fact that we can not pin point where the electrons are since they are in constant motion.

Pictorial representation of atom

This picture is much more familiar, but perhaps not very accurate. In the center, there is a nucleus, containing number of protons and nuetrons and surrounding the nucleus we have electrons moving.

Atomic Number, Mass Number, and Isotopes

In an atom

Define Mass # → A
Define # of protons → Z
Define # of neutrons → N

Then,

Z → differentiate atoms

If atoms share the same Z, but different N → Isotopes

A = Z + N

# of electrons = Z → therefore atom is electrically neutral

If # of electron in excess or deficient → Ion

and

If # of electrons > Z → Anion

If # of electrons < Z → Cation

For smaller atoms, the number of protons is the same as the number of neutrons. However, for larger atoms, the number of neutrons exceeds that of the protons.

Size Comparison: Baseball proton and baseball Sun Let's compare the size of Sun-Earth system with that of a simplest atom hydrogen, consisting only one proton and one electron. We're going to visualize them by making a baseball to either Sun or proton. The electron and Earth are represented by a smallest bead I could possibly find in my wife's hobby box! The following table gives the pertinent distances.

	Diameter	Distance		Diameter
Sun	1.39 x 10⁶ km	1.5 x 10⁸ km	Earth	1.28 x 10⁴ km
Proton	1 fm	0.53 Å	Electron	a point
Baseball	7.5 cm		Small bead	0.8 mm

The ratio between diameters of Earth to Sun and small bead to a baseball is about the same. In both cases, the smaller one is about 1 % of the larger. Since electron is a point particle (to a good approximation), so there is no size. But, to visualize the system, we need to make electron a particle with finite size. The distance between the two parts of the systems are also listed.

From these we can calculate the distance from Sun to Earth if the size of the Sun is shrunk to that of a baseball. $? m_{b a s e b a l l} = 1.5 \times 10^{8} k m (\frac{7.5 c m_{b a s e b a l l}}{1.39 \times 10^{6} k m}) (\frac{1 m}{100 c m}) = 8.1 m = 8 m$

To check the consistency of my baseball-bead hydrogen atom model, we use size of Earth (size of the bead) as our reference. If the following calculation agrees with the one we calculated with baseball (Sun), our model should be reliable.

$? m_{b e a d} = 1.5 \times 10^{8} k m (\frac{0.8 m m_{b e a d}}{1.28 \times 10^{4} k m}) (\frac{1 m}{10^{3} m m}) = 9.4 m = 9 m$

1 m or so difference. Not too bad. Two calculations are just consistent. Now, calculate for baseball hydrogen.

$? m_{b a s e b a l l} = 0.53 Å (\frac{10^{- 10} m}{1 Å}) (\frac{7.5 c m_{b a s e b a l l}}{10^{- 15} m}) (\frac{1 m}{100 c m}) = 4000 m = 4 k m$ Wow!! So, if the baseball was our Sun, Earth would be orbiting only 8~9m away, but if the baseball was a proton in hydrogen atom, it would be zipping around near South Ferry in Manhattan!! (For those of you who are outside of LIU, we are located near Manhattan Bridge in Brooklyn, New York.)

Atomic Symbol

There are about 100 naturally occuring elements, listed on the Periodic Table.

Each element, represented by an atomic symbol (one- or two-letter symbol)

Nuclear charge (aka Atomic Number), Z = # of protons in nucleus

Average mass in amu (atomic mass unit)

Representation on the left is from the Periodic Table and on the right is the isotopic notation.

In the above example, Xenon atom, with atomic symbol Xe, is given here. The information we can obtain from these are,

It has 54 protons — from both
Its A = 131 — from isotopic notation
The average mass <m> = 131.30 — from Perodic Table notation

From these information we can calculate:

N = A - Z = 131 - 54 = 77 neutrons
# of electron = 54 = Z

Example: From 2.100. How many protons, neutrons, and electrons are in each of the following: a. ${}_{7}^{15}N$ b. ${}_{27}^{60}C o$ c. ${}_{53}^{131}I$ d. ${}_{58}^{148}C e$

a. In this notation, the subscripted number, that is Z, is 7. It means that we have 7 protons. The superscripted number, A, is 15. It means that we have combined nuclear particles (protons and neutrons) counts 15. Then, the number of neutrons is N = A - Z and there are 8 neutrons. The number of electrons are the same as the number of protons in an atom, because atom is electrically neutral.

b. Similarly, Z = 27, N = 60 - 27 = 33, e^- = 27

c. Z = 53, N = 131 - 53 = 78 , e^- = 53

d. Z = 58, N = 148 - 58 = 90 , e^- = 58

Atomic Mass

Atomic mass is represented by atomic mass unit (amu), and is defined by $1 a m u = \frac{m (C- 12)}{12} = 1.660539 \times 10^{- 24} g$

where m(C-12) is the mass of carbon-12 isotope.

Using the above Xe example, hence, the average mass of Xe is 131.30 amu.

Example: How many atoms are there in 10.0 g of Na?

As we've seen in Chapter 1, we can use dimensional analysis to solve this problem. As you read the question, you can write the equation down as,

?_Na = 10.0g_Na

should suffice. The conversion of 1 amu is 1.660539 x 10^-24 g.

$?_{N a} = 10.0 g_{N a} (\frac{1 a m u}{1.660539 \times 10^{- 24} g}) (\frac{1_{N a}}{22.99 a m u}) = 0.2619 \times 10^{24} N a$

That's a huge number!

The Periodic Table

Over 90 different naturally occuring elements listed in the Periodic Table of Elements

Group = elements in the same column
Period = elements in the same row

Each group has similar chemical properties

For example, Carbon belongs to group 14 so as silicon. Both can form up to 4 bonds.

Majority of elements are metals, i.e. conducts electricity and has shiny colors

Group 1	Alkali metals
Group 2	Alkali Earth metals
Groups 3 - 12	Transition metals
Groups 13 - 18	Main group elements
Elements 57 - 71	Lanthanides
Elements 89 - 103	Actinides
Group 15	Pnictogens
Group 16	Chalcogens
Group 17	Halogens
Group 18	Noble gas

Molecules and Ions

Molecule

Molecule — one unit of chemical compound

Consisting of two or more elements in a definite arrangement

Atoms in molecule are held by chemical bond

Molecules are represented by chemical formula. Water's chemical formula is H₂O, representing 2 hydrogens andn 1 oxygen.

Ions

Ion — Charged chemical compound.

Cation

Anion

Since atoms are electrically neutral, in comparison with the parent atom

cation — missing electrons

anion — extra electrons

Example: How many electrons are in the following atoms and ions? H, H⁺, Cl, Cl^-, Mg²⁺, O^2-

Remember that atoms are electrically neutral. It means that the number of protons, Z, is the same as the number of electrons. So, for H, the number of electron is 1. For Cl, it is 17. For cations, the number of electron is less than the number of protons. For H⁺, an electron with -1 charge is missiing from H, terefore, there is no electron, zero. For Cl^-, it is 18, because when you add one -1 charge to 17 electron Cl atom, there will be an -1 charge overall, so there are 18 electrons in Cl^-. Similar line of arguments will give you the number of electrons in Mg²⁺ to be 10, and O^2- to be 10.

ionic force

+ charge and - charge attract each other

polyatomic ion

Table salt, sodium chloride (NaCl), is an ionic compound. It consists of Na⁺ cation and Cl^- anion.

Vinegar is a solution of acetic acid and water. Acetic acid in water is partially ionic, consisting H⁺ ion and acetate ion (anion), as shown below.

Phase of the compounds. You'll see often notation of chemical formula with its phase. NaCl(s) is solid phase of sodium chloride.

phase	denotation	description
Gas	(g)	gas phase
Liquid	(l)	liquid
Solid	(s)	sold phase
Solution	(aq)	aqueous solution

Chemical Formulas

Chemical formulas — representing the composition of molecules and ions

Atoms in a molecule are held by chemical bond.
Several different chemical formulas

Molecular formula — shows exact number of elements

Structural formula — shows how atoms are connected to each other

Empirical formula — simplest representation of molecule, derived from experiments

₆

Below is an example of formulas for water.

Chemical formulas and structural formula of water
The representation on the right is structural formula for describing the chemical bonding in the molecule. In the figure above, there are lines connected the oxygen atom to two hydrogen atoms. These lines represent chemical bonds.
There are two types of chemical bonds:

Covalent bond—sharing two electrons between two or more nuclei

Ionic bond—force associated with two or more opposite charged ions bond

Here represented in four ways, vanillin molecule. This molecule is responsible for the very pleasant aroma of vanilla.

Chemical Formula	Structural Formula	Organic Chemists	3-D Ball-stick	3d Space Filling
C₈H₈O₃

Two models on the left will be used in CHM3, but other noteworthy ones are also listed. The representation used by organic chemists (labeled as Organic Chemists) the one you learn in Organic Chemistry next year. The 3-D representation can be done in a number of ways. Here we list two, ball and stick model and space filling model.

Formula of Ionic Compounds

As shown above, table salt, NaCl, has ionic character. It can be separated into Na⁺ and Cl^- ions. Again, between Na⁺ and Cl^- ions, there exists the ionic force that holds these ions. Interaction between ions are:

Positive and Positive — Repulsive (repel one another)

Negative and Negative — Repulsive (repel one another)

Positive and Negative — Attractive (come together) So, when ionic compound is made, the ions of opposite charges are combined, and the resulting compound is electrically neutral.

For example, magnesium bromide, where magnesium ion is Mg²⁺ and bromine ion (called bromide ion) is Br^-, must be neutral, then there must be two Br^- so that +2 charge on magnesium ion is cancelled. Thus, magnesium bromide is written MgBr₂.

Nomenclature of Binary Ionic Compounds

Nomenclature — systematic naming scheme

Binary compound — made up of metal cation and main group anion

²⁺

³⁺

—ide

So sodium chloride is: NaCl. First one is cation Na⁺ and the latter is anion Cl^-.

Example: Name the following compounds. LiCl, MgCl₂, BeO, AlCl₃, Al₂O₃

LiCl = lithium chloride

₂

BeO = Berrium oxide

₃

₂

₃

Example: Obtain the chemical formula for the following. Ferric chloride, Potassium telluride, Strontium bromide, Rubidium sulfide.

₃

₂

When the compound contains some number of atoms, we need to know how many. We use Greek-derived prefix for the purpose. For example, TiO₂
is commonly known as Titanium dioxide. Di- represents two. The following table lists the prefixes.

number	prefix	number	prefix
1	mono	11	hendeca
2	di	12	dodeca
3	tri	13	triskadeca
4	tetra	14	tetrakaideca
5	penta	15	pentakaideca
6	hexa	16	hexakaideca
7	hepta	17	heptakaideca
8	octa	18	octakaideca
9	nona	19	nonakaideca
10	deca	20	icosa

Example: Give names of the following compounds. CO, CO₂, N₂O₃, BrF₃, S₂F₂

CO — Carbon monoxide
CO₂ — Carbon dioxide
N₂O₃ — Dinitrogen trioxide
BrF₃ — Bromine trifluoride
S₂F₂ — Disulfur difluoride

Naming with Polyatomic Ions

Somewhat preserved systematic naming in compounds with polyatomic ions. However, many of them use common names. Important ones that you encounter in this course are listed in the following table. Most of them are anions.

We have already talked about acetic acid above. Acetate ion is written as CH₃CO₂^-. If acetate ion is combined with sodium ion, it would be called sodium acetate. Naming scheme is still the same.

Nomenclature of Acid As described above, acid produces H⁺ in aqueous solution. For example, hydrochloric acid is HCl in water, therefore, it is written as HCl(aq). This notation of adding (aq) and having dissociable H⁺ tells that the species is an acid.

Some of the common acids and anion produced from the acids are listed in the following table.

Example: Name the following compounds. HCl(g), HCl(aq), NaOH(s), KMnO₄(s), HNO₃(aq), Fe(ClO₄)₃(s), H₂SO₄(aq)

₄

₃

₄

₃

₂

₄

Introduction to Organic Compounds

The names of hydrocarbon species one carbon to 10 carbons. You can rotate the molecules around.

Name	Formula	Model
Methane	CH₄
Ethane	C₂H₆
Propane	C₃H₈
Butane	C₃H₈
Pentane	C₃H₈
Hexane	C₆H₁₄
Heptane	C₇H₁₆
Octane	C₈H₁₈
Nonane	C₉H₂₀
Decane	C₁₀H₂₂