Chem 101

PptsWeek ending 2/3/12

Chapter 2 continued

John Dalton

• An English scientist and teacher• He knew of these observations and offered an

explanation for them• His explanation is known as:

DALTON’S ATOMIC THEORYThe main ideas of his theory include:

1. Elements are made of tiny particles called atoms.2. All atoms of a given element are identical. Atoms of a given

element are different from those of any other element.3. Atoms of one element can combine with atoms of other

elements to form compounds. A given compound always has the same relative number and types of atoms

4. Atoms are indivisible in chemical processes. ** Atoms are not created or destroyed in chemical reactions.

A chemical reaction simply changes the way atoms are grouped together.

* Dalton’s Theory offered simple explainations for some basic laws of chemistry such as:

The Law of Conservation of Mass Mass is neither created or destroyed. If atoms are conserved in a reaction then mass must also be

conservedThe Law of Constant Composition Tells us that a cmpd regardless of its origin or method of

preparation always contains the same elements in the same proportions by weight

Law of Multiple Proportions When 2 elements combine to form more than 1 cmpd the

masses of one element which combines with a fixed mass of the other elelment are in a ratio of small whole numbers such as 2:1

Law of Multiple Proportions

Example:Element A + B combine to form 2 cmpds.

Cmpd 1 Cmpd 2 A2B ABThe weight of A combined with a fixed amt of

B in cmpd 1 would be 2x that of the second.

Dalton’s Atomic Teory

Like all new ideas Dalton’s theory was not accepted immediately

- this did not dissuade Dalton - Dalton predicted that N and O could combine

to form several compounds: NO , N2O and NO2

- when the existence of these compounds was verified….. Dalton’s Theory became widely accepted

A Look at Compounds

A compound is a distinct substance composed of atoms of two or more elements that always contains exactly the same relative masses of those elements

• H2O always has two H atoms and one O atom

Chemical Formulas Chemical formulas- used to show the types and

number of each type of atom• Atoms are indicated by the elements symbol• The number of each type of atom is represented by a

subscript (appearing to the right and below the element symbol)

ie. SO3 contains 1 atom of sulfur and 3 atoms of oxygenWrite the formula for the compound containing 2atoms of nitrogen and 5 atoms of oxygen.Ans. N2O5

Chemical Formulas continued

• Glucose has 6 carbon atoms, 12 hydrogen atoms and 6 atoms of oxygen.

Write its formula.Ans. C6H12O6

• The unique formula or arrangement of atoms in a molecule makes one compound

different from another

Compounds with the same chemical formula:

CH3 – CH2 –OH CH3-O-CH3

C2H6O C2H6O ethyl alcohol dimethyl ether• The difference lies in the order of the bonded atoms• Formulas that show the order and arrangement of

the specific atoms are called structural formulas

• Dalton’s Theory provided a convincing explanation for the composition of compounds

• Scientists believed that elements consisted atoms but…….

What did the atom look like?

The Structure of the Atom

• Much pondering about the structure occurred during the 1800’s

• It was not until 1900 that convincing evidence regarding the structure became available

First Convincing Evidence for Subatomic Particles

• Came from experiments involving the conduction of electricity through gases at low pressures using a device called a Cathode Ray Tube

• In 1897 J. J. Thomson (English physicist) took this apparatus partially evacuated it and connected it to a high voltage source (spark coil)

• An electric current flows thru the tube• Associated with this flow are colored rays of light which

originate at the (-) end of the cathode* Thomson found that these rays were bent by both electric

and magnetic fields* Careful study of the nature of this deflection demonstrated

that the the rays consisted of a stream of negatively charged particles which he called electrons.

2.2

• In the absence of any field, the cathode ray (which is -) strikes at B.

• Cathode ray strikes at A in the presence of a magnetic field• Electric plates create an electric field perpendicular to the

direction of the cathode rays (cathode ray is -) causing them to strike the screen at C when the field is on.

Cathode Ray Tube

2.2

Passing an electric current makes a beam appear to move from the negative to the positive end.

•Went on to measure the mass to charge ratio of the electron finding it to be:

m/e = 5.69 x 10-9g/C

e/m = -1.76 x 108 C/g

•Since he found the ratio to be the same regardless of what gas was in the tube, this implied that the electron was a fundamental particle common to all atoms

•Provided the first evidence that atoms are made of even smaller particles

J.J. Thomson and the Cathode Ray Tube

1897

(1906 Nobel Prize in Physics)

J. J. Thomson

• Thomson showed that atoms of any element can be made to emit tiny negative particles

• He called these negative particles electrons• Thomson new that although the atoms contained

negative particles the atoms overall charge had to be zero

• He postulated that the atom must also contain positive particles to balance exactly the negative charge of the electrons

Thompson’s Model• Found the electron

– 1 unit of negative charge– Mass 1/2000 of hydrogen atom– Later refined to 1/1840

• Concluded that there must be a positive charge since atom was neutral

• Atom was like plum pudding– A bunch of positive stuff, with

electrons able to be removed.

Millikan1909

• Millikan determined the charge on an electron with his famous Oil Drop Experiment

• In this experiment small droplets of oil which had picked up extra electrons were allowed to fall between 2 electrically charged plates

• The drops were observed: When the voltage between the plates was increased the

negatively charged drop fell more slowly (it was attracted to the + plate)

At some point the drop will be balanced and stationary.

Millikan’s Experiment continued:

• Knowing this voltage and the mass of the drop, it was possible to calculate the charge on the drop

• Millikan found the charge to always be an integral multiple of a smallest charge

• Assuming the smallest charge to be that on an electron he arrived at a value of 1.60 x 10-19C

• Combining this value with the charge/mass ratio he calculated the mass of an electron

e- charge = -1.60 x 10-19 C

Thomson’s charge/mass of e- = -1.76 x 108 C/g

e- mass = 9.10 x 10-28 g

Measured mass of e-

(1923 Nobel Prize in Physics)

2.2

Henri Becquerel1896

• Studying a uranium mineral called pitchblende he discovered that it spontaneously emitted high energy radiation.

• Further studies by the Curies and Ernest Rutherford revealed 3 types of Radiation

1. Alpha (α) - positively charged(+2) and heavy2. Beta (β)- negatively charged (-1)3. Gamma ()- neutral

(Uranium compound)2.2

• With the growing evidence that the atom was composed of even smaller particles, attention was being given to how they fit together

• Recall, early in the 1900’s JJ Thomson proposed the “plum pudding” model of the atom

Ernest Rutherford

• A student of J. J. Thomson

• Believed in the “Plum Pudding” Model

• In 1911 he performed his now famous

“Gold Foil Experiment”

“Plum Pudding” Model of the Atom

• Developed by J.J. Thomson

• The atom was a uniform pudding of positive charge with enough negative electrons scattered within to counter balance the positive charge

                                   

                                        

Plum Pudding Model orRaisin Bun Model

               

Rutherford’s Experiment

• Rutherford and his students bombarded a piece of gold foil using alpha particles

• Here’s what it looked like…….

Rutherford’s “Gold Foil” Experiment

• Rutherford directed α- particles 7500 times the

mass of an electron at a thin sheet of gold foil only a few atoms thick

• The detector produced tiny flashes if it was hit by an α- particle

• What he expected to see

– Alpha particles should pass through without change in direction

– Positive charges were spread out evenly. Alone they were not enough to stop an alpha particle

• What he got

• How he explained it– Atom is mostly empty– Small dense, positive piece at the center– Alpha particles are deflected if they get close

enough to positive center

A picture of the Atom Evolves

• Based on his alpha-particle scattering experiment on gold, Rutherford concluded that the atom consisted of a hard central core where most of the mass of the atom rested.

Other Subatomic Particles

• Protons were discovered by Rutherford in 1919.

• Neutrons were discovered by James Chadwick in 1932.

Subatomic Particles• Protons and electrons are the only particles that have

a charge.• Protons and neutrons have essentially the same

mass.• The mass of an electron is so small we ignore it.

Nuclear Structure• Facts about the nucleus:

Protons and neutrons have roughly the same mass (1amu=1.67x10-4g), and each is about 2000 times as massive as the electron.The number of protons is the same as the number of electrons (not shown) which orbit the nucleus at a distance of about 10-

8cm.

• **If the nucleus were a grape the electrons would be 1 mile away!!!

No charge.Help hold protons in the nucleus.Without neutronsthe + charges would repel one another.

+ charge

If all atoms contain the same components (protons, neutrons and electrons) why do different atoms have different chemical properties?

The answer lies in the number and arrangement of the electrons.

***It is the electrons that are responsible for the chemical properties of an atom of an element.

Representing Elements• Each and every atom of an element is described on the basis

of the number and types of nuclear particles A Q

X – atomic symbol ZA = Mass number The number of protons and the number of neutrons

Z = Atomic number The number of protons In a neutral atom the number of protons = the number of electrons

Q = Charge if not neutral

Atomic Symbols 23ie. Na 11

Sodium-23 contains 11 electrons, 11 protonsand 12 neutrons.

24Sodium 24: Na contains 11 electrons, 11 protons 11 and 13 neutrons. It is an example of an isotope

Isotopes

• Atoms of the same element that contain an identical number of protons but differ in the number of neutrons

12 13 14• C(carbon -12) C (carbon-13) C (carbon-14) 6 6 6

6e, 6p, 6n 6e, 6p, 7n 6e, 6p, 8n

- three different forms of carbon - differ only in the number of neutrons in the nucleus - illustrates the concept of isotopes

More Isotopes 1 2 3 H H H 1 1 1 Hydrogen-1 Hydrogen-2 Hydrogen-3 deuterium tritium

• In nature elements are usually found as a mixture of isotopes• An isotopes mass is determined by comparison to a standard,

carbon-12 (has a mass of 12 atomic mass units or amu)• An elements atomic mass is obtained by taking a weighted

average of the atomic masses of all isotopes of that element present in nature

Putting it all together

• How many protons , neutrons and electrons are present in 96Mo ?

42

Plan: The number of p, n and e are determined from the atomic # and mass #

Atomic # = 42 and Mass # = 96

Atomic # = # protons → 42p # protons = # electrons in a neutral atom → 42e Mass # = # protons + # neutrons → 96 – 42 = 54n

In any room where chemistry is taught, or practiced you are certain to find a chart called the……….

Periodic Table:

• A systematic catalog of elements.

• Elements are arranged in order of atomic number.

The Periodic Table• Shows all the known

elements• Each box contains the

element’s atomic number (# of protons = # of electrons) written over the one or two letter symbol

• Each box contains the element’s atomic mass (# of protons + # of neutrons) written below the symbol

Symbols and much, much more

• Not only does the Periodic table provide symbols, atomic number and atomic mass of each element,……

• The periodic table tell us a good deal about each element

As you peruse the table you cannot help but note that:

• The elements are arranged in order of increasing atomic number in nice horizontal and vertical columns

Periodicity

When one looks at the chemical properties of elements, one notices a repeating pattern of reactivities.

Dimitri Mendeleev• In 1869 he arranged the

elements based on similarities in chemical properties into

“families”• Mendeleev listed the

families vertically and called them groups

- groups are referred to by the number over the column

- many groups also have names

• Mendeleev called the horizontal rows periods they are designated with numbers

• Elements in a period each have common characteristics

• Each period ends with a member of the family of elements called the Noble Gases

Noble Gases - chemically un-reactive elements that exist in nature as individual atoms

Groups

These five groups are known by their names.

Periodic Table

Nonmetals are on the right side of the periodic table (with the exception of H).

Periodic Table

Metalloids border the stair-step line (with the exception of Al and Po).

Periodic Table

Metals are on the left side of the chart.

Chemical FormulasThe subscript to the right of the symbol of an element tells the number of atoms of that element in one molecule of the compound.

Molecular CompoundsMolecular compounds are composed of molecules and almost always contain only nonmetals.

Diatomic Molecules

These seven elements occur naturally as molecules containing two atoms.

Types of Formulas

• Empirical formulas give the lowest whole-number ratio of atoms of each element in a compound.

• Molecular formulas give the exact number of atoms of each element in a compound.

Types of Formulas

• Structural formulas show the order in which atoms are bonded.

• Perspective drawings also show the three-dimensional array of atoms in a compound.

Ions

• When atoms lose or gain electrons, they become ions.– Cations are positive and are formed by elements on

the left side of the periodic chart.– Anions are negative and are formed by elements on

the right side of the periodic chart.

Ionic Bonds

Ionic compounds (such as NaCl) are generally formed between metals and nonmetals.

Writing Formulas

• Because compounds are electrically neutral, one can determine the formula of a compound this way:– The charge on the cation becomes the subscript on the

anion.– The charge on the anion becomes the subscript on the

cation.– If these subscripts are not in the lowest whole-number

ratio, divide them by the greatest common factor.

Common Cations

Important groups in the Periodic Table

• Group IA – VIIA – known as The Main Group or Representative Elements

Group IA- Alkali Metals most highly reactive metals

Group IIA – Alkaline Earth Metals highly reactive metals Groups IIIA, IVA and VA - are not generally referred to by a family nameGroup VIA – ChalcogensGroup VIIA – Halogens highly reactive nonmetals

Group VIIIA

• Group VIIIA – known as the Noble or Inert Gases

- Noble Gases form few chemical compounds - He, Ne, and Ar do not form any compounds…

all exist in nature as individual atoms

Group B Elements• Called the Transition Metals• They include many familiar metals ie. Fe, Cr, Ni, Sn…• They also include the noble metals Cu, Ag, and Au (Grp IB) -Noble metals are the rare metals of coins and jewelry. -Noble metals are comparatively chemically inert to rust and corrosion

Inner Transition Metals• Lanthanides – group between 57La and 72Ha

• Actinides – group between 89La and 104Rf

The Periodic Table at a Glance• Allows us to classify an element very broadly into two

classes:1. Metals2. Non-metals - the point of separation is the heavy stair step line - B, Si, Ge, As, Sb and Te which border the line are called

metalloids or semimetals

Physical State and the Periodic Table

• the periodic table tells us about an elements physical state at T= 25oC or standard reference temperature

• Except for hydrogen all gaseous elements are found at the extreme right of the table

He N O F Ne Cl Ar Kr Xe Rn• There are only two liquids….metal Hg and non-metal Br2

• ****all other elements are solids

• Of the non-metals many exist as diatomic molecules rather than individual atoms

• This includes all gaseous elements except the Noble gases

H2, N2 , O2, F2, Cl2, Br2, I2

• Natural form of P is P4

• Most common form of S is S8

• Carbon exists in three different forms called allotropes

- all three allotropes of carbon have different properties

- the 3 allotropes are diamond, graphite and buckminsterfullerene

A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical forces

H2 H2O NH3 CH4

A diatomic molecule contains only two atoms

H2, N2, O2, Br2, HCl, CO

A polyatomic molecule contains more than two atoms

O3, H2O, NH3, CH4

2.5

Formation of Cations from a Neutral Atom

• Cations or positively charged ions are formed when one or more e- are lost from a neutral atom to create a species with a + charge

-1e-

Na → Na+ + 1e-

11p+, 11e- 11p+, 10e-

sodium ion -2e-

Mg → Mg+2 + 2e-

12p+, 12e- 12p+,10e-

magnesium ion

Cations are named by retaining the elements original name

Formation of Anions from Neutral Atoms

• Anions are negative ions formed when a neutral atom gains extra electrons to create a species with a - charge

Cl + 1e- → Cl –

17p+, 17e- 17p+, 18e- chloride ion

• Anions are named by taking the root name of the atom and changing the ending to –ide

Br - bromide ion O2- oxide ion S2- sulfide ion

A monatomic ion contains only one atom

A polyatomic ion contains more than one atom

2.5

Na+, Cl-, Ca2+, O2-, Al3+, N3-

OH-, CN-, NH4+, NO3

-

13 protons, 10 (13 – 3) electrons

34 protons, 36 (34 + 2) electrons

Do You Understand Ions?

2.5

How many protons and electrons are in ?Al2713

3+

How many protons and electrons are in ?Se7834

2-

Ionic Compounds

• Compounds formed from the strong electrostatic attraction between oppositely charged ions (a cation and an anion)

• The strong electrostatic attraction between oppositely charged ions is called an ionic bond

ie. Na+ + Cl- → NaCl Sodium Chloride Formula represents the simplest ratio of cation to anion present in sodium

chloride. It is called a formula unit.

Formula Units

• For Ca+2 and Cl-

For every Ca +2 ion two Cl- ions are neededCaCl2 is the formula unit for calcium chloride

For ionic compounds, the formula is always the same as the empirical formulaAn empirical formula shows the simplest whole-number ratio of the atoms in a substance

• the sum of the charges on the cation(s) and anion(s) in each formula unit must equal zero

The ionic compound NaCl

2.6

Polyatomic Ions

• Groups of atoms with a chargeNH4

+

SO42-

NO3-

Metals vs. Nonmetals

• Metals have a tendancy to lose electrons and form cations

• Most Non-metals (except the Noble Gases) gain electrons and form anions

** The Periodic Table can tell us: The charge of the cation formed by a metalandThe charge of the anion formed by a non-metal

Group Numbers and Ion Charge

• Group I Metals- all form cations with a +1 charge• Group II Metals- all form cations with a +2 charge• Group III Metals- all form cations with a +3 charge

For Groups I-III charges of cation formed are identical to the Group Number

Transition Metals

• All the transition metals form cations with various positive charges

• There is no easy way to predict the charge of the cation that will be formed

• Common charges of the transition metals are found in Table 2.3 on pg 61 ….they must be memorized

Non-Metals

• Non-metals form negative ions by gaining electrons

• Group VII- all gain one electron to form 1- ions• Group VI- atoms of elements in this group gain two electrons to form 2- ions• Group V- atoms gain 3 electrons forming anions with a 3- charge

It is important to Remember:

• Isolated atoms do not form ions on their own

• Ions form when metallic elements combine with non-metallic elements to form compounds called ionic compounds

• Chemical compounds must have a net charge of zero

Therefore if a compound contains ions:

1. There must be both cations and anions present

2. the numbers of cations and anions must be such that the net charge is zero

**These simple rules are integral to your ability to write Formulas for Ionic Compounds

Formulas of Metal / Non-metal Binary Ionic Compounds

• When writing the formula for a binary compound it is important to remember:

Total charge + total charge → cmpd with zero on cation on anion net chargeie. Magnesium + chlorine → ?

Plan: use periodic table to find charges on ions of elements

• Metal and nonmetal combine to neutralize charge• Always write the metal first and the non-metal second• Use subscripts to indicate the relative number of ions or atoms• Consider - Mg+2, Cl1-

– cross multiply charges– Mg2+ + 2Cl1- = MgCl2 magnesium chloride

More Examples of Metal /Nonmetal Compounds

• Barium + Oxygen → ?– barium an alkaline earth metal - +2– Ba - +2– O - -2

• Ba+2 + O-2 → BaO (barium oxide)• Lithium + Nitrogen → ? - lithium an alkali metal - +1 - Li- +1 - N- +3• Li +1 + N-3 → Li3N (lithium nitride)