Lecture 3The Periodic Table, Atomic Structure, Isotopes and Ions, Basic Nomenclature : Ch 2

Dr Harris8/28/12

HW problems: Ch 2: 15, 29, 35, 37, 43, 49, 52, 53

Elements, Molecules and Compounds

• Millions of different materials in the world, all comprised of some combination of only 118 elements

• Similar to how the alphabet combines 26 letters to yield hundreds of thousands of words, elements bond in unique arrangements to give different substances

What exactly is an element?

• Any pure substance that can not be broken down into simpler substances is an element. Elements combine to form molecules. Molecules comprise compounds

• Elements: H, O, Na, Cl (etc.)• Molecules: • H2O (water), • HOOH (hydrogen peroxide)• NaCl (sodium chloride)• HCl (hydrochloric acid)• NaClO4 (sodium perchlorate)

• We refer to a compound as a bulk accumulation of molecules (i.e. A single water molecule vs. a glass full of water composed of billions and billions of molecules)

Metallic vs Nonmetallic Character

• Elements can be metals, nonmetals, or semiconductors (we will discuss semiconductors later)

Metal characteristics

• Malleable • Ductile• Conductive of electricity • Conductive of heat• Have luster and shine

Metallic vs Nonmetallic Character

Nonmetal characteristics• Most nonmetals are gases• Non conductive• Those that are solid are not

conductive, malleable, or ductile

P(s)

S(s)

O2 (g)

Br2 (L)

Metals are all elements LEFT of the black line, except Hydrogen.

Elemental Groups and Diatomic Species

• Elements listed down a column have very similar properties, and tend to behave the same way. These columns are called groups. We will revisit chemical groups later.

• Certain elements are unstable, and hence, do not commonly exist as individual species, but as diatomic molecules

• These include H, O, N, and all of the halogens (group 17)• H H2 (Hydrogen gas)

• O O2 (Oxygen gas)• N N2 (Nitrogen gas)

• F, Cl, Br, I F2, Cl2, Br2, I2 (Fluorine gas, chlorine gas, bromine gas and iodine gas)

Solids, Liquids, and Gases

Solids• Atoms tightly bound• Fixed volume and shape (does

not conform to container)• A chemical is denoted as solid by

labeling it with (s)

Au (s)

Au atoms

Solids, Liquids, and Gases

• Liquids• Atoms less tightly bound than

solids• Has a definite volume, but not

definite shape (assumes the shape of its container)

• Denoted by (L) ex. H2O (L)

Water molecule

Solids, Liquids, and Gases

• Gases• Free atoms• No shape, no definite volume• Can be expanded or compressed

(like engine piston)• Denoted by (g) ; ex. O2 (g)

O2 molecules

Mixtures

AIR • 78% N2

• 21% O2

• water• fructose• CO2(g)• food coloring

Homogeneous Heterogeneous

• Most substances in nature are mixtures, chemicals that exist together without actually bonding. Mixtures can be homogeneous (uniform throughout) or heterogeneous (non-uniform)

Separating Mixtures

Heterogeneous mixture of sugar, sand and iron. How can we separate the components??

• In many instances, it is important to separate mixtures. This can be done by taking advantages of differences in the properties of the components.

Critical Thinking

• You’ve received a sample of dirty water from the local pond to purify in your lab. How could you extract pure water from the contaminated mix?

You would perform a distillation.

Solutions

• All solutions are homogeneous fluids (free flowing), meaning that the composition is completely uniform throughout

• Example: salt water. You add salt (NaCl) to water, it completely dissociates (dissolves; molecules split apart) and spreads evenly throughout the water. There is the same amount of salt everywhere in the glass

• Salt is the solute. Water is the solvent.

• When a substance dissolves in water, it forms an aqueous solution, labeled (aq): Salt water NaCl(aq)

• Air is another example of a solution

Law of Constant Composition

•

• We analyze 1.630 g of CaS and find that it’s 0.906 g Ca. Find the mass of S? Find the mass % of Ca and S?

- First, we know that CaS is composed only of Ca and S. Therefore, all of its mass must come from Ca and S. We can find the mass of S.

• mass Ca + mass S = total mass of CaS 0.906g + X = 1.630 g X = 0.724 g S • Now, we can find the mass % of each element mass % Ca + mass % S = 100%

Example

Group Examples

• A sample of KI has a mass of 2.00 g and is 23.5% K. What is the mass % I? What is the mass of I in the compound.

• A sample of NaCl is 39.3% Na, and is found to contain 1.58 g Na. What is the total mass of NaCl? What is the mass of Cl?

Basic Laws of Chemical Reactions

• Law of conservation of mass: Atoms are not created or destroyed in a chemical reaction, only rearranged

-

• Law of constant composition: Specific molecules have fixed ratios of atoms

• One N atom combines with 3 H atoms to make ammonia gas, NH3 (g). It does not matter how much N or H you have.

• So, if you had 10 N atoms and 1000 H atoms, you would make 10 NH3 molecules, with 970 unused H atoms left over.

- Some elements combine in multiple ratios, like C and O * CO carbon monoxide * CO2 carbon dioxide

Dalton’s Atomic Theory

1. Matter is composed of atoms

2. The atoms of a given element are exactly the same in every way

3. The atoms of different elements differ in mass and chemical behavior

4. Compounds are composed of two or more atoms of different elements bonded together.

5. In a reaction, atoms are rearranged, separated or recombined to form new substances. No atoms are created or destroyed, and the atoms themselves DO NOT CHANGE

Atomic Mass Units• Each element in the periodic table is assigned a relative mass, called the

atomic mass (amu).

• Remember the CaS example. The total mass of CaS is 55.5% Ca and 44.6% S. If all the mass is from Ca and S, then the ratio of the mass % must also be the ratio of the mass

• This suggests that a Ca atoms weighs 1.25 times as much as an S atom

• To develop the periodic table, atom masses are determined by using carbon as a reference

Atomic Mass Units

Calculating Atomic Mass

• What is atomic mass of H2O to 3 sig figs?• H = 1.0079 amu • O = 15.9994 amu

• What is the atomic mass of NaHCO3 to 3 sig figs?• Na = 22.990 amu• C = 12.01 amu

2 (1.0079 )+15.9994=18.0 𝑎𝑚𝑢

22.990𝑎𝑚𝑢+1.0079𝑎𝑚𝑢+12.01𝑎𝑚𝑢+3 (15.999𝑎𝑚𝑢 )=68.0𝑎𝑚𝑢

% Mass Calculations Using Atomic Mass

• What is the % mass of Na in Na2S?

• How many grams of sulfur atoms are there in a 4.50 g sample of Na2S?

𝑇𝑜𝑡𝑎𝑙 𝑎𝑡𝑜𝑚𝑖𝑐𝑚𝑎𝑠𝑠𝑜𝑓 𝑁 𝑎2𝑆=(22.99𝑎𝑚𝑢 ) (2 )+ (32.066𝑎𝑚𝑢 )=78.05𝑎𝑚𝑢

% 𝑁𝑎= 𝑚𝑎𝑠𝑠𝑜𝑓 𝑁𝑎𝑡𝑜𝑡𝑎𝑙𝑚𝑎𝑠𝑠𝑜𝑓 𝑁 𝑎2𝑆

=(22.990 𝑎𝑚𝑢 ) (2 )78.05𝑎𝑚𝑢 =58.90%

% 𝑆=41.1% .411 (4.50𝑔)=1.85𝑔𝑆

Atomic Structure

• Atoms were initially thought to be indivisible, and the smallest particles in existence

• Toward the end of the 19th century, it was discovered that atoms consist of even smaller sub-atomic particles

• G. Johnstone Stoney hypothesized that electricity exists in discrete units, or small packets of equally distributed charge. He called these electric units electrons

• J.J. Thompson was one of the first scientists to proved the existence of such particles. He is most famous for his cathode-ray experiment

Cathode Rays

• Electrical discharge (current) in partially evacuated tubes had been previously observed when high voltages were applied between two electrodes.

• These rays of current are called cathode rays, because they flowed from the cathode (negative end) to the anode (positive end)

• Scientist initially thought that these rays were caused by atoms of the electrodes.

The Electron

• J.J. Thomson modified the typical cathode ray tube experiment in several ways:

1. He used various materials to compose the cathode. It was found that the nature of the beams did not depend at all on what the cathode was composed of, and the mass of the cathode never changed. Hence, these rays were NOT caused by atoms or heavy particles

2. He applied either electrical fields (a) or magnetic fields (b) to the cathode ray. He found that the cathode rays were actually beams of negatively charge particles that are MUCH lighter than atoms, and are the same in every element. This marked the discovery of the electron.

Determination of Electron Mass and Charge

• Through later experiments, it was determined that the mass of an electron is 9.109 x 10-31 kg

• The charge of an electron is -1.602 x 10-19 C, where C is the symbol for coulombs, the SI unit of charge

• For simplicity, we report the relative charge of an electron as -1

Radioactivity• Shortly after the discovery of electrons, Antoine Henri Becquerel

discovered radioactivity, the process by which atoms spontaneously break apart.

• By monitoring emissions from a radioactive material under an electric field, Ernest Rutherford found that radiation consists of three types of radiation• alpha (α) particles• beta (β) particles masses below are amu• gamma (ϒ) particles charges are relative

• Positive α particles drawn toward negative terminal

• Negative β particles drawn toward positive terminal

• Neutral ϒ particles are unaffected by electric field

3 types of Radiation

The “Plum Pudding” Model

• Since atoms are electrically neutral, and electrons are negatively charged, Thomson knew that a positively charged sub-atomic particle must also exist

• Thomson envisioned the atom as electrons evenly distributed in a “sea” of positive charge, the so-called “plum pudding” model (shown to the right)

• Ernest Rutherford then formed an experiment to see how a high energy beam of positively charged α particles would interact with atoms

positive charge

electrons

Discovery of the Nucleus

• A very thin sheet of gold film was placed in front of a beam of α-particles

• Most of the α-particles passed through the gold film

• However, some α-particles were scattered at large angles. How could this happen?

• Rutherford realized that most of the mass of an atom is concentrated in a small positively-charged region. He called this region the nucleus.

Rutherford’s Model of the Atom

• Most of the volume of an atom is empty space, which electrons spread sparsely throughout. The α-particles easily pass through this space.

• However, the nucleus is very dense, and very positively charged. The α-particles that approach this region are strongly repelled.

Nucleus

Electron cloud

α particle

Protons and Neutrons Comprise the Nucleus• Later experiments showed that protons and neutrons reside in the

nucleus and comprise the bulk of the mass of an atom

• Protons and electron have equal but opposite charge

Particle Relative Charge Mass (amu)

Proton +1 1.007

Neutron 0 1.008

Electron -1 .000548

Charges shown in table are relative to the charge of a proton. A proton has an actual charge of 1.602 x 10-19 C, an electron has a charge of -1.609 x 10-19 C. Opposite charges attract!Like charges repel!!

Atomic Number

• The number of protons in an atom is called the atomic number.

• For a given element, the number of protons DOES NOT CHANGE. As shown to the right, only Carbon has 6 protons.

• In a neutral atom, the number of protons is equal to the number of electrons.

Mass Number

• The mass number of an element is the sum of the protons and neutrons.

• The mass numbers listed on the periodic table are average values.

• The reason for these averages is that elements exist in nature as multiple isotopes.

Isotopes

• Isotopes are variations of elements with the same number of protons but different numbers of neutrons.

• For example, the most common isotope of hydrogen contains one proton, one electron, and no neutrons (99.985% of all hydrogen atoms).

• An isotope of hydrogen is deuterium, which has one neutron (.0115 %)

• A third isotope, tritium, has two neutrons (~ 0%).

𝐻11 hydrogen

𝐻12

𝐻13

deuterium

tritium

mass number

atomic number

Average atomic mass is obtained using the % abundance and the isotope mass.

𝐴𝑣𝑒𝑟𝑎𝑔𝑒𝑎𝑡𝑜𝑚𝑖𝑐𝑚𝑎𝑠𝑠=∑ (𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒 )𝑥 (𝑖𝑠𝑜𝑡𝑜𝑝𝑖𝑐𝑚𝑎𝑠𝑠)

Example• Revisiting the previous example of the hydrogen

isotopes, we found the following isotopic masses and abundances in the table.

• Using these values, calculate the average atomic mass of Hydrogen. Does it match the value given in the periodic table?

ISOTOPE % Abundance Isotopic mass (amu)

99.9850 1.00782503

0.0115 2.01410178

~0 3.01604927

𝑎𝑣𝑔 .𝑚𝑎𝑠𝑠=( .999850 ) (1.00782503 )+( .000115 ) (2.01410178 )+(0)(3.01604927)

¿1.0079𝑎𝑚𝑢

Group Problem

ISOTOPE Protons Neutrons Electrons % Abundance Isotopic mass (amu)

98.93 12

1.07 13.003 354 8378

~0 14.003 2420

• The 3 main isotopes of Carbon are C-12, C-13, and C-14. Fill in the table below with the correct number of protons, neutrons, and electrons.

• Then, calculate the average atomic mass. Does your value match that of the periodic table?

How To Determine % abundance

• The average atomic mass of Boron, as given on the periodic table, is 10.811 amu

• It is known that two isotopes of Boron exist, • (isotopic mass: 10.013 amu)

• (isotopic mass: 11.009 amu)

• What are the % abundances of each isotope?

Continued.

• We know that:

• We also know that the total % abundance must be 100%• We allow • Therefore:

10.811=(𝑥 ) (10.013𝑎𝑚𝑢 )+(1−𝑥)(11.009𝑎𝑚𝑢)

• Solving for x, we find that

% ¿% ¿

𝐴𝑣𝑒𝑟𝑎𝑔𝑒𝑎𝑡𝑜𝑚𝑖𝑐𝑚𝑎𝑠𝑠=∑ (𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒 )𝑥 (𝑖𝑠𝑜𝑡𝑜𝑝𝑖𝑐𝑚𝑎𝑠𝑠)

Ions

• Thus far, we’ve learned than an element is essentially defined by it’s atomic number

• Each element has an exact number of protons. • For example, Hydrogen has only one proton. If you force a

second proton onto the atom, you no longer have hydrogen… you now have Helium.

• We have also learned that atoms of a particular element can have variations in the number of neutrons. Atoms of the same element with varying numbers of neutrons are called isotopes.

• Next, we will discuss ions.

Ions

• Ions are electrically charged atoms, resulting from the gain or loss of electrons.

• Positively charged ions are called cations. You form a cation when electrons are lost

• Negatively charged ions are called anions. You form anions when electrons are gained

• An cation is named by adding the word “ion” to the end of the element name

• Anions are named by adding the suffix –ide to the end of an element

Ions

𝐿𝑖+¿ ¿

𝑁𝑎+¿ ¿

𝑀𝑔2+¿¿

𝐴𝑙3+¿¿

Lithium ion

Sodium ion

Magnesium ion

Aluminum ion

𝐶𝑙−

𝑆2−

𝑂2−

𝑃3−

Chloride

Sulfide

Oxide

Phosphide

Ions

𝑆1632Sulfur-32

𝑆2−1632Gains 2 electrons

Sulfide-32

ISOTOPE Protons Neutrons Electrons

16 16 16 2- 16 16 18

𝐴𝑙1327 𝐴𝑙3+¿

1327 ¿Loses 3 electrons

Aluminum-27 Aluminum ion-27

ISOTOPE Protons Neutrons Electrons

13 14 13 3+ 13 14 10

Nomenclature

• There are special rules to naming molecules. In this chapter, you will see two types of molecules: IONIC and COVALENT

• Ionic bonds are formed between metals and nonmetals• To name an ionic compound, you do the following• write the name of the metal• Follow it with the name of the nonmetal, but change the

ending of the name of the nonmetal to –ide

• Example: KF • K is a metal. F is a non metal.

• We write the name as: Potassium Fluoride

Na2S = Sodium Sulfide; MgO = Magnesium Oxide

Nomenclature

• Covalent bonds are formed between nonmetals and nonmetals

• To name a covalent compound, you do the following• Include a prefix corresponding to

the first nonmetal. Do not use mono on the first nonmental.

• Use a prefix to name the second nonmetal. We only use –mono for oxygen. Drop the ending of the second nonmetal and replace it with –ide

SF4 = sulfur tetrafluorideN2O = dinitrogen monoxideP2O3 = diphosphorus trioxideSiC = silicon carbide

Rules of Hydrogen• Hydrogen is strange. It’s a nonmetal, but can sometimes act as a metal and is

listed on the metal side of the periodic table.

1. If hydrogen is listed first, and the nonmetal is of group 6, treat it as a metal and name the compound accordingly (except water).

Ex. H2S = hydrogen sulfide

Compounds of group 17 are strong acids and are named as such. We drop –gen and end the second non metal with –ic acid

HCl = hydrochloric acid ; HF = hydrofluoric acid

2. If hydrogen is listed last, treat the molecule as covalent. Change the ending to –ide and use the appropriate prefix.

NaH = sodium hydride; AlH3 = aluminum trihydride AsH3 = Arsenic trihydride;