the atom and the periodic table. james chadwick 1932 james chadwick british found an electrically...

The Atom and The Periodic Table

James Chadwick1932 James Chadwick

• British

• Found an electrically neutral particle which resides in the nucleus and has almost the same mass as the proton. He named this “neutral proton”

the neutron.

Discovery of the Neutron

James Chadwick bombarded beryllium-9 with alpha particles, carbon-12 atoms were formed, and neutrons were emitted.

n10

+He42

+Be94 C12

6

Atom: •smallest particle of an element that retains the chemical properties of the element.-can be broken down, however they would lose their chemical identity

The Atom

One of Dalton’s postulates said that, “All atoms of the same element are identical.”

(i.e. same mass and properties)

IS THIS TRUE?

NO!!! NO!!! Example: Boron is mined in Death Valley, CA.

There are two “types” of Boron.

Both have 5 p+ but one has 5 no while the other has 6no.

They are exactly alike chemically, but different in mass.

ISOTOPESIsotopes: same number of p+, but different

number of no

-most elements have 2 stable isotopes-EXCEPTIONS who only have 1 = Al, F, P

-Sn has 10!!!!

-Refer to an isotope by its mass number (p+ + no)

for example: uranium-238 or 238U

The number of p+ = ATOMIC NUMBER•represented by “Z”•if Z = 5, then the element is Boron

In the atom, what is the same as the number of p+? WHY?p+ = e- Atoms are electrically neutral.

Hence, the number of p+ determines the identity of the element and the number of no determines the isotope of the element.

Atomic Number (Z)

Nucleus - Proton (+1)Neutrons (0)

If two elements have the same atomic number they are the same element. If two elements have different atomic numbers they are different.

Dalton’s postulate now reads:“All atoms of an element contain the same number of p+ but

can contain different numbers of no.”

A particular type of atom is called a NUCLIDE (another name for isotope).

•Protium, Deuterium, and Tritium are all nuclides of hydrogen.

Particles that make up the nucleus are called NUCLEONS.

•The proton and neutron are nucleons.

# protons

# protons + # neutrons mass number

The total number of nucleons = p+ + no = mass number (“M”)

Isotopic notation:

XM

Z

Isotopic Notation

X = chemical symbol

M = mass #

Z = atomic #

Isotopic Notation

Symbols

Find the – number of protons– number of neutrons– number of electrons– Atomic number– Mass number

Br8035

= 35

= 45

= 35

= 35

= 80

SymbolsSymbols

If an element has 60 protons and 84 neutrons what is the

– Atomic number

– Mass number

– number of electrons

– Complete symbol

Nd14460

= 60

= 144

= 60

Symbols

If a neutral atom of an element has 78 electrons and 117 neutrons what is the

– Atomic number

– Mass number

– number of protons

– Complete symbol

Pt19578

= 78

= 195

= 78

Symbols

Find the

• number of protons

• number of neutrons

• number of electrons

• Atomic number

• Mass number

Na2311

1+

Sodium ion

= 11

= 12

= 10

= 11

= 23

Symbols

If an element has an atomic number of 23 and a mass number of 51 what is the

– number of protons

– number of neutrons

– number of electrons V5123

= 23

= 28

= 23

Metals lose electrons to form positive ions (cations):

Li ---> Li+ + e-

3 p+ 3 p+ 3 e- 2 e-

4 n0 4 n0


If # of protons > # of electrons it has a positive charge and we call it a cation.

Nonmetals gain electrons to form negative ions (anions):

F + e- ---> F- 9 p+ 9 p+

9 e- 10 e-

10 n0 10 n0


If # of protons < # of electrons it has a negative charge and we call it an anion.

ProtonsIsotopic Notation

92

Neutrons Electrons

34

11

146

45

12

92

36

10

5927

3+Co

3717

1–Cl

55 7+Mn

25

23892

U

2311

1+Na

7934

2–Se

20 1817

30 1825

32 2427

The p+ and no are essentially equal in mass.

As you can see, the electron has very little mass when compared to both the proton and the neutron.

99.95% of an atom’s mass is found in the nucleus.

Protons are over 1800 times larger than electrons. So, chemists say electrons have no mass. This is not exactly true, it’s more like they have negligible mass.

Atomic Mass

Subatomic particles

Electron

Proton

Neutron

Name Symbol ChargeRelative mass

Actual mass (g)

e-

p+

no

-1

+1

0

1/1840 = 0

1

1

9.109389 x 10-28

1.6762623 x 10-24

1.6749286 x 10-24

The mass of atoms is measured in amu or (u).

Amu = 1/12 the mass of the carbon-12 nuclide

1 u = 1.66054 x 101 u = 1.66054 x 10-24-24 g g

Carbon-12 is the standard. One C-12 atom has a mass of 12 u. The mass of the other elements is relative to this mass.

carbon atom

(12 amu)

(1 amu)

(1 amu)(1 amu)

(1 amu)(1 amu) (1 amu)

(1 amu) (1 amu)

(1 amu) (1 amu)(1 amu) (1 amu)

For example….Methane

For carbon 1 in approximately 90 atoms are carbon-13

The rest are carbon-12 the isotope that is 98.9% abundant.

So, for approximately 90 methane molecules…1 carbon is carbon-13

Where’s Waldo?

C-13

Subatomic Particles• Quarks• component of

protons & neutrons

• 6 types

– 3 quarks = 1 proton or 1 neutron

He

A mass spectrometer

Mass Spectrometry

• A mass spectrometer is a device that separates positive gaseous ions according to their mass-to-charge ratios.

• If a stream of positive ions having equal velocities is brought into a magnetic field:• All the ions are deflected from their straight line

paths into circular paths• The lightest ions are deflected the most making a

tighter circle• Conversely, the heaviest ions are deflected the least• A record of the separation of ions is called a mass

spectrum.

Diagram of a simple mass spectrometer, showing the separation of neon isotopes

The mass spectrum of neon

Mass Spectrometry

--

Photographic plate196 199 201 204

198 200 202

Mass spectrum of mercury vaporMass spectrum of mercury vapor

Stream of positive ionsStream of positive ions

+

Mass Spectrum for Mercury

196 197 198 199 200 201 202 203 204

Mass numberMass number

Rel

ativ

e nu

mbe

r of

ato

ms

Rel

ativ

e nu

mbe

r of

ato

ms

30

25

20

15

10

5

196 199 201 204

198 200 202


The percent natural abundances The percent natural abundances for mercury isotopes are:for mercury isotopes are:

Hg-196 0.146%Hg-196 0.146% Hg-198 10.02%Hg-198 10.02% Hg-199 16.84%Hg-199 16.84% Hg-200 23.13%Hg-200 23.13% Hg-201 13.22%Hg-201 13.22% Hg-202 29.80%Hg-202 29.80% Hg-204 6.85%Hg-204 6.85%

(The photographic record has been converted to a scale of relative number of atoms)

Mass spectrums reflect the abundance of naturally occurring isotopes.

Hydrogen

Carbon

Nitrogen

Oxygen

Sulfur

Chlorine

Bromine

1H = 99.985% 2H = 0.015%

12C = 98.90% 13C = 1.10%

14N = 99.63% 15N = 0.37%

16O = 99.762% 17O = 0.038% 18O = 0.200%

32S = 95.02% 33S = 0.75%

34S = 4.21% 36S = 0.02%

35Cl = 75.77% 37Cl = 24.23%

79Br = 50.69% 81Br = 49.31%

Natural Abundance of Common Elements

Atomic Mass• How heavy is an atom of oxygen?

• There are different kinds of oxygen atoms.

• More concerned with average atomic mass.

• Based on abundance of each element in nature.

• Don’t use grams because the numbers would be too small

Abundance = Percent (% ) = (part/whole) = massindividual/masswhole

Average = Abundance1 (mass1) + Abundance2(mass2) + etc

AVERAGE ATOMIC MASS

Steps:•Multiply the mass of each isotope by its Abundance.•Add up all of the products from step 1.

(.2000) (10.0129) + (.8000) (11.00931) =

2.00250 + 8.807448 = 10.810028

= 10.81003 u

Isotope % Abundance Atomic mass

B-10 20.00 % 10.0129 u

B-11 80.00 % 11.00931 u

Example:

Mass Spectrum for Mercury

196 197 198 199 200 201 202 203 204

Mass numberMass number

Rel

ativ

e nu

mbe

r of

ato

ms

Rel

ativ

e nu

mbe

r of

ato

ms

30

25

20

15

10

5

196 199 201 204

198 200 202




(The photographic record has been converted to a scale of relative number of atoms)



(0.00146)(196) + (0.1002)(198) + (0.1684)(199) + (0.2313)(200) +

(0.1322)(201) + (0.2980)(202) + (0.0685)(204) = x 0.28616 + 19.8396 + 33.5116 + 46.2600 + 26.5722 + 60.1960 + 13.974 = x

x = 200.63956 amu

Hg200.6

80

(% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAM

ABCDEFG

ExampleChlorine has two naturally occurring isotopes, 35Cl (34.9689 amu) and 37Cl (36.9659 amu). If chlorine has an atomic mass of 35.4527 amu, what is the percent abundance of each chlorine isotope?


Cl-35 x 34.9689 u

Cl -37 1 – x 36.9659 u

Average Atomic mass 35.4527


Cl-35 x 34.9689 u

Cl -37 1 – x 36.9659 u

Average Atomic mass 35.4527

x (34.9689) + (1 - x) (36.9659) = 35.4527

34.9689 x + 36.9659 - 36.9659 x = 35.4527

36.9659 - 1.9970 x = 35.4527

1.5132 = 1.9970 x

.7577366049 = x

Abundance1 (mass1) + Abundance2(mass2) + etc = Average Atomic mass

the atom and the periodic table. james chadwick 1932 james chadwick british found an electrically...

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