Lecture_At1._Struc_new

Unit 3

• Atomic Structure• Chapter 4

Objective 1

Describe ancient Greek models of matter.

Democritus-

• Democritus- coined the term atom. He believed that all matter consisted of extremely small particles that could not be divided.

• He called these particles atoms.

Democritus

• But he used the term incorrectly. • He said that there were atoms of wood,

atoms of glass, atoms of water, etc. • He thought that the shapes and sizes of the

atoms in a material determined the properties of the material.

• For example, the atoms of a liquid had to be round and smooth. The atoms of solids had to be rough and prickly.

Democritus

Aristotle

Artistotle-

• Said that substances were made of only four elements.

• They were: earth, air, water and fire.

Alchemy• During ancient

times alchemy was popular. This was the concept that elements could be converted into gold.

Objective 2

List the main points of Dalton’s atomic theory and describe his evidence for the existence of atoms.

John Dalton

John Dalton proposed the:

• Law of Partial Pressures- A gas in a mixture of gases contributes the same pressure it would produce if it were the only gas in a container.

Law Of Partial Pressures

John Dalton proposed the:

• Law of Definite Proportions- elements combine to form compounds in certain fixed ratios by mass.

Law of Definite Proportions

• Example: Magnesium reacts with oxygen to produce magnesium oxide.

Dalton found that 100 g of Magnesium always reacted with 65.8 g of oxygen.

So 10 g of Magnesium would react with ___________g of oxygen. The

reacting masses always were the same ratio.

Magnesium Reacting with Oxygen

Law of Definite Proportions

• How many grams of hydrogen react with oxygen to make water? Let’s use the periodic table to figure this out!

Law of Definite Proportions

• 2H2 + O2 2H2O

• 4 g + 32 g 36 g

• John Dalton’s greatest contribution was Dalton’s Atomic Theory.

The main points are as follows:

a. All elements are composed of atoms.b. All atoms of the same element have the

same mass, and atoms of different elements have different masses.

c. Compounds contain atoms of more than one element.

d. In a particular compound, atoms of different elements always combine in the same way.

• Dalton’s Atomic Theory had to be modified to present day thinking. Look at each of the main points again and decide which ones are wrong.

 b.is wrong

Because there are isotopes of elements. There are three isotopes of oxygen. Oxygen-16, Oxygen-17 and Oxygen-18. Each element of oxygen has 8 protons, but a different number of neutrons.

Objective 3

Explain how Thomson and Rutherford used data from experiments to produce their atomic models.

J. J. Thomson

Thomson- discovered electrons.

• He used a device called a cathode ray tube.

• It is a glass tube filled with low pressure gas.

• An electric current runs through the gas in the tube.

Cathode Ray Tube

• He found that a beam traveled from the – to the + terminal.

• He found that the beam could be bent or deflected when exposed to a positive plate.

Cathode Ray Tube

With A Magnet

Cathode Ray Tube

• He called the beam cathode rays. • It was attracted to the positive plate. • A magnet could bend the cathode ray.

Thomson-discovered electrons

• His conclusion was that the beam was negatively charged, a stream of electrons.

• His results were the same no matter which metal he used to form the metal disks. The particles produced were the same.

• He concluded that an electron has a mass about 1/2000 that of a hydrogen atom.

Thomson Plum Pudding Model

• Atoms were known to be “neutral”, so Thomson devised his model to account for this.

• He envisioned the pudding to represent a positive field with negatively charged electrons spaced throughout, like the plums in the plum pudding.

• The positive charges would equal the number of negative charges.

Thomson Plum Pudding Model

+

+

+

+

+

+

• In America we would have the chocolate chip cookie model.

• The dough representing the positive field and the chocolate chips the electrons throughout the dough.

• Rutherford developed the Nuclear Atom Theory.

• His thoughts were based on the Gold Foil experiment performed by his research assistant Ernest Marsden.

Lord Rutherford

Earnest Marsden

Gold Foil Experiment, text p. 104

• Alpha particles are emitted by a radioactive sample from inside a box.

• These alpha particles were then used like the bullets of a gun to aim and shoot at a piece of gold foil.

• Insert picture

Gold Foil Experiment

• At the time Thompson’s model of the atom was accepted.

• It was believed that the alpha particles would pass straight through the foil and not be deflected.

Gold Foil Experiment

• What happened? • Some of the

particles passed straight through, some were deflected or curved and most astonishing some were deflected straight back!

Rutherford’s Nuclear Atom

• Rutherford concluded that there must be a nucleus that is a dense core that contains positive charges.

• The electrons then traveled around the nucleus.

Houston Astrodome

How big is an atom?

• Your text explains that if the atom’s volume were that of the Houston Astrodome (height of 202 feet), the nucleus would be approximately the size of a marble.

• So most of the atom is empty space and that is why so many of the alpha particles passed right through the foil.

• By 1920, Rutherford predicted the existence of a third particle.

• His research assistant James Chadwick later proved its existence, the neutron.

James Chadwick

Objective 4

• Describe Bohr’s model of the atom.

• Niels Bohr worked for Rutherford. He devised the Planetary Model of the atom.

Niels Bohr

• The electrons move in spherical orbits at fixed distances from the nucleus.

• The electrons of an atom have fixed energy levels.

• n equals the energy level.

Planetary Model• The orbit nearest the nucleus is of the lowest energy

and is called the ground state. • If an electron moves to a higher energy level, energy

of a particular amount is needed (a photon or quanta or energy).

• So an electron of hydrogen could absorb energy and become “excited.”

• The electron would now be traveling in a higher energy level.

• If the electron falls back to ground state, energy would be released in the form of light or energy.

Objective 5

• Explain how the electron cloud model represents the behavior and locations of electrons in atoms.

• Bohr’s Planetary model had to be revised.

Erwin Schrodinger

Erwin Schrodinger developed quantum mechanics.

• His idea was that an electron is a particle traveling with a wave-like motion.

• Schrodinger said that the electrons can only exist in a probability area known as an electron cloud.

• So the electrons do not travel in a fixed path.

• They will be found in a specified region or area known as an electron cloud.

Orbitals

• An orbital is a region of space around the nucleus where an electron is likely to be found. 

• Only two electrons may go in a given orbital.

Models of the Atoms

• Plum pudding-Thomson• Nuclear Atom-Rutherford• Planetary Atom-Bohr• Electron Cloud Model-Schrodinger

Objective 6

Identify three subatomic particles and compare their properties.

Subatomic Particles

• In the nucleus:• Protons are positively charged

• Neutrons have no charge• Protons and neutrons have just about

an equal mass.

Subatomic Particles

• Surrounding the nucleus:• Electrons are negatively charged• The electron’s mass is very, very small,

1/1836 of a proton.

• The properties of the subatomic particles are summarized in your text in Figure 10, p. 109.

Objective 7

Distinguish the atomic number of an element from the mass number and use these numbers to describe the structure of atoms.

• Atomic number- is equal to the number of protons in an atom of that element.

• The atomic number of each element is unique. Look at a periodic table. What is the atomic number of the following elements?

SulfurIronSilver

Element Atomic Number

S 16

Fe 26

Ag 47

Objective 8

• Define and identify isotopes.

• Mass number- is the sum of the protons and neutrons in an atom’s nucleus. 

• An atom of aluminum has 13 protons and 14 neutrons. It’s mass number is 27.

• You would write it Al-27.

• You can find the number of neutrons by subtracting.

Number of neutrons =

mass number - atomic number

Determine the number of neutrons in the following isotopes.

Isotope Mass Number Atomic number No. of Neutrons

O-16O-17O-18Cl-37Cl-35Cu-63Cu-65

Isotope Mass Number Atomic number No. of Neutrons

O-16 16 8 8O-17 17 8 9O-18 18 8 10Cl-37 37 17 20Cl-35 35 17 18Cu-63 63 29 34Cu-65 65 29 36

What is the symbol for the isotope composed of the following?

Isotope Protons Neutrons Electrons

24 27 24

20 20 18

34 44 34

53 74 54

Isotope Protons Neutrons Electrons

Cr-51 24 27 24

Ca-40 +2 20 20 18

Se-78 34 44 34

I-127 -1 53 74 54

Objective 9

• Determine the electron configuration for elements 1-20.

Electron Configuration for Elements

• Hydrogen• One electron is in the first energy level.• It is in an s sublevel of energy.• An s orbital is an electron cloud shaped like

a sphere.

1s1

Helium

• Two electrons.• Both placed in the first energy level.• These electrons are paired.• They are in an s orbital.• The s orbital is full.

1s2

Lithium

• 3 electrons.• The first two go in the first energy level.• The third one is in a bigger sphere the 2s.

1s22s1

Beryllium

• Write it’s electron configuration.• 1s22s2

• There are other electron cloud shapes besides a sphere.

• See next slide.

Orbitals

Boron

• 5 electrons.• Two in the first energy level.• Two in the second energy level.• The first four electrons go in s orbitals.• The fifth electron enters a p orbital.• It has the shape of a dumb-bell.• 1s22s22p1

Objective 11

• Understand Bohr’s model of energy levels.

• Niels Bohr performed extensive research with the hydrogen spectrum.

• As electrons fall to n=2 they produce a line spectrum of various colors.

• The energy gained when an electron moves up to higher levels can be measured and the energy released produces light that can be seen with an instrument called a spectroscope.

• Each element produces a unique line spectra that can be detected with a spectroscope.

• This is evidence that energy levels exist.

• Spectroscopes are mounted on space probes to “read the atomic spectra” emitting from a planet’s atmosphere.

• That way, scientists know what elements are present on that planet.

Objective 12

Distinguish the ground state from excited states of an atom based on electron configurations.

• Ground state- the most stable electron configuration is the one in which the electrons are in orbitals with the lowest possible energies.

• Electrons are placed into an atom from lowest energy to highest in a specific order.  

Carbon

• 1s22s22p2

• This is the ground state for carbon.• 1s22s23s2 would represent carbon with

an electron excited to a “higher energy level.”

What is the electron configuration for Neon?

• 10 protons and 10 neutrons in the nucleus. We have 10 electrons to distribute.

• Where do they go?

1s 22s 22p 6Total 10

1s22s22p6

1s22s22p6

• Notice the second energy level has a total of eight electrons in it. That is very special. It gives an element great stability.

• It is called an octet. So all of the Noble gases: neon, argon, krypton, xenon and radon follow the octet rule.

• Because of this the Noble gases do not react much with other atoms. They were termed inert.

• Now with extreme temperatures and pressure the Noble gases have been made into compounds.

Name the element that has the following electron configuration.

Element Electron Configuration How many

electrons? Atomic Number?

1s22s22p4

1s2

1s22s22p63s23p3

Element How many electrons

Atomic Number

Oxygen 8 8

Helium 2 2

Phosphorus 15 15

Identify the following as elements in the ground state or excited.

• 1s12s1

• excited• 1s22s2

• Ground state• 1s12s23p2

• excited• 1s22s23s1

• excited