IB Chemistry 1: Topic 2

Atomic Structure: atomic particles, atomic number, mass # , isotopes and

atomic mass

Defining Atoms

&Electrons in

Atoms

Democritus (460-370 BC)

Originated idea of the atom

For the next 2000 years…

Alchemy

• Good or bad?

• Contributions?

Isaac Newton – 1704 AD

John Dalton (1766 - 1844)

1803 Dalton’s Atomic Theory

Proust - 1799

At the time, current thinking…

Several changes have been made to Dalton’s theory.

Modern Atomic Theory

• Dalton said:Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties

• Modern theory states:Atoms of an element have a characteristic average mass which is unique to that element.

Modern Atomic Theory

Dalton said:Atoms cannot be subdivided, created, or destroyed

Modern theory states:Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!

J.J. Thomson (1856-1940)

1897 Discovered the electron(“plum pudding” model)

Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Discovery of the Electron

In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.

Cathode Ray TubeCathode

Tube

Anode

Stand

Alligator Clip

Cathode Ray Tube in Action

• http://www.chem.uiuc.edu/clcwebsite/video/Cath.mov

Mass of the Electron

1909 – Robert Millikan determines the mass of the electron.

The oil drop apparatus

Mass of the electron is 9.109 x 10-31 kg

Conclusions from the Study of Electrons

• Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.

• Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons.

• • Electrons have so little mass that atoms

must contain other particles that account for most of the mass

Thompson’s Plum Pudding Model

Rutherford (1871-1937)

1911 Discovered the nucleus(gold foil experiment)

Rutherford’s Gold Foil Experiment

• Alpha particles are helium nuclei • Particles were fired at a thin sheet of gold foil • Particle hits on the detecting screen (film) are

recorded

Try it YourselfIn the following pictures, there is a target hidden

by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

The Answers

Target #1 Target #2

Rutherford’s Findings

• Most of the particles passed right through • A few particles were deflected • VERY FEW were greatly deflected

The nucleus is small The nucleus is dense The nucleus is positively charged

“Like howitzer shells bouncing off of tissue paper!”

Conclusions:

Since opposite charges attract each other, why don’t the

electrons fall into the nucleus?

Niels Bohr (1885-1962)

1913 proposed Planetary model

Neils Bohr

I pictured electrons orbiting the nucleus much like planets orbiting the sun.

But I was wrong! They’re more like bees around a hive.

WRONG!!!

The Bohr Model of the Atom

Based upon the work of several men, a new mathematical model was

developed to describe the structure of the atom.

1926 The Quantum-Mechanical

Model

Louis de Broglie (1892-1987)Werner Heisenberg (1901-1976)Erwin Schrodinger (1887-1961)

Previous models were physical models based upon

the motion of objects.

This model is primarily a mathematical model and had never been proven....

…until recently!

The New York Times, 9/7/99

Now we know for sure!

Atomic Number• Atomic number of an element is the number

of protons in the nucleus of each atom of that element.

Element Atomic # # of protons

# of electrons

Carbon 6

Phosphorus 15

Gold 79

• In a neutral atom:# electrons = # protons

6

15

79

6

15

79

The Atomic Scale• Most of the mass of the atom is in the

nucleus (protons and neutrons)

• Electrons are found outside of the nucleus (the electron cloud)

• Most of the volume of the atom is the electron cloud.

Particle Charge Mass (g) Location

Electron e- -1

9.109 x 10-28

(1/1840 amu)

Electron cloud

Proton p+ +1

1.673 x 10-24

(1 amu)Nucleus

Neutronn 0

1.675 x 10-24

(1 amu)Nucleus

Atomic Particles

The Atomic Scale• Most of the mass of the

atom is in the nucleus (protons and neutrons)

• Electrons are found outside of the nucleus (the electron cloud)

• Most of the volume of the atom is empty space

“q” is a particle called a “quark”

About Quarks…•Protons and neutrons are NOT fundamental particles.

•Protons are made of two “up” quarks and one “down” quark.

•Neutrons are made of one “up” quark and two “down” quarks.

•Quarks are held together by “gluons”

Reading the Periodic Table

Atomic Number 3

Li6.941Lithium

Element Symbol

Element Name

Atomic Mass

# p+ # e- (in a neutral atom)

# p+ + # n0

# n0 = Atomic Mass - #p+

Isotopes• Elements occur in

nature as mixtures of isotopes.

• Isotopes are atoms of the same element that differ in the number of neutrons

Mass Number• Mass number is the number of protons

and neutrons in the nucleus of an isotope.

Mass # = p+ + n0

Symbols of Isotopes

Carbon-12

Mass # C12Atomic Number

146

Carbon-14

Mass # CAtomic Number

3517

Atomic Number Chlorine-35Mass # Cl

6

Mass NumberMass # = p+ + n0

Isotope p+ n0 e- Mass #

Oxygen - 10

- 33 42

-31 15

8 8 1818

Arsenic 75 33 75

Phosphorus 15 3116

O188

As7533

P3115

Isotope Protons

Electrons

Neutrons

Nucleus

Hydrogen–1

(protium)

1 1 0

Hydrogen-2

(deuterium)

1 1 1

Hydrogen-3

(tritium)

1 1 2

Isotopes…Again (must be on the test)

Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.

Isotope SymbolComposition of

the nucleus % in nature

Carbon-12

12C6 protons 6 neutrons 98.89%

Carbon-13

13C6 protons 7 neutrons 1.11%

Carbon-14

14C6 protons 8 neutrons <0.01%

•Atomic mass is the weighted average of all the naturally occuring isotopes of that element.•Multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.

Carbon = 12.011

Atomic Masses

Isotope name Isotope mass

(amu) percentage

Silver-107 106.90509 51.86

Silver-109 108.90470 remainder

Silver has two naturally occurring isotopes:

Find the missing percentage.

Find the average atomic mass of an atom of silver.

Silicon has three naturally occurring isotopes:

Look over the data before you begin the problem. Estimate the value of the answer before you begin the calculation. Will the weighted average be closer to 28, 29, or 30?

Find the average atomic mass of silicon.

Isotope nameIsotope mass

(amu)Relative

AbundanceSilicon-28 27.98 92.21Silicon-29 28.98 4.70Silicon-30 29.97 3.09

Iron has four naturally occurring isotopes:

Estimate the average mass.

Find the average atomic mass of iron.

Isotope nameIsotope

abundanceIsotope mass

(amu)Iron-54 5.90% 53.94Iron-56 91.72% 55.93Iron-57 2.10% 56.94Iron-58 0.280% 57.93