The Atom Atomic Models Think with me…about sugar crystals, you can see that they are small crystals and every crystal is identical.Think with me…about

The AtomThe AtomAtomic ModelsAtomic Models

• Think with me…about sugar crystals, you can see that they are small crystals and every crystal is identical.–You may grind these particles into a

very fine powder, but each tiny piece is still sugar.• If you dissolve the sugar in water,

the sugar particles become virtually invisible.

From Philosophy to ScienceFrom Philosophy to Science

–You could even look at the sugar solution under a microscope and you’d still not be able to see the sugar.• However, you know it is still there

because you can taste it.• These kind of observations and logic

patterns led ancient philosophers to ponder the design of the universe


• There were two schools of thought of the composition of the cosmos… – is everything in the universe

continuous and infinitely divisible–Or, is there a limit to how small you can

get?• Particle theory was not the most

popular early opinion, but was supported as early as Democritus in ancient Greece.


• Democritus proposed that all the matter is composed of tiny particles called “Atomos”–These “particles” were thought to be

indivisible• Aristotle did not accept Democritus’

atom, he was of the “matter is continuous” philosophy–Because of Aristotle’s popularity his

theory was adopted as the standard


•By the 1700’s nearly all chemists had accepted the modern definition of an element as a particle that is indivisible

• It was also understood at that time that elements combine to form compounds that are different in their properties than the elements that composed them–However, these understandings

were based on observations not empirical evidence


• There was controversy as to whether elements always combine in the same proportion when forming a particular compound.– In the 1790’s, chemistry was

revolutionized by a new emphasis on quantitative analysis because of new and improved balances

• This new technology led to the discovery of some new scientific understandings


• The Law of Conservation of Mass:–Proposed by Antoine Lavoisier–States that mass is neither created

nor destroyed during ordinary chemical rxns or physical changes.

–Which means the total mass of the reactants must equal the total mass of the products.


++

Carbon, C Oxygen, O Carbon Monoxide, COMass x Mass y Mass x + Mass y

++

Carbon, C Oxygen, OMass x Mass y

Carbon Monoxide, COMass x + Mass y

• The Law of Definite Proportions:–The fact that a chemical compound

contains the same elements in exactly the same proportions by mass regardless of the size of the sample or the source of the compound• NaCl is NaCl no matter if it is table

salt (small crystals) or rock salt (large crystals)


Copper Carbonate

• The Law of Multiple Proportions:– If 2 or more different compounds

are composed of the same 2 elements, then the ratio of the masses of the 2nd element combined with a certain mass of the 1st element is always a ratio of small whole numbers


++ ==Carbon Oxygen Carbon Monoxide,

1:1

==Carbon Dioxide,

1:2

++

Carbon Oxygen

1 1

1 2

• In 1808, John Dalton proposed an explanation for each of the proposed laws–He reasoned that elements were

composed of atoms & that only whole #’s of atoms can combine to form

compounds–His ideas are now called the Atomic

Theory of Matter

Atomic TheoryAtomic Theory

1.All matter is composed of extremely small particles called atoms

2. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, & other properties

ELEMENT2

ELEMENT2

ELEMENT 3

ELEMENT 3

ELEMENT 1

ELEMENT 1

ELEMENT4

ELEMENT4


3. Atoms cannot be subdivided, created, or destroyed

4. atoms of different elements combine in simple whole # ratios to form chem compds

5. in chemical rxns, atoms are combined, separated, or rearranged

++ ++


• Through these statements, evidence could be gathered to confirm or discount its claims–Not all of Dalton’s claims held up

to the scrutiny of experimentation–Atoms CAN be divided into even

smaller particles–Not every atom of an element

has an identical mass


• Dalton’s Atomic Theory of Matter has been modified.

• What remains is…1.All matter is composed of atoms2.Atoms of any one element differ in

properties from atoms of another element

• One of the disputed statements of Dalton was that atoms are indivisible


– In the 1800’s it was determined that atoms are actually composed of several basic types of smaller particles

– it’s the number and arrangement of these particles that determine the atom’s chemical properties.

• The def. of an atom that emerged was, the smallest particle of an element that retains the chemical properties of that original element.


• All atoms consist of 2 regions that contain the subatomic particles–The nucleus–The electron cloud around the nucleus

• The nucleus is a very small region located near the center of the atom– In every atom the nucleus contains

at least 1 proton, which is positively charged particle and usually contains 1 or more neutral particles called neutrons

Atomic StructureAtomic Structure

• The electron cloud is the region that surrounds the nucleus –This region contains 1 or more elec-

trons, which are negatively charged subatomic particles

–The volume of the electron cloud is much larger than the nucleus


• The discovery of the first subatomic particle took place in the late 1800’s.–A power source was attached to two

metal ends of an evacuated glass tube, called a cathode ray tube.

–A beam of “light” appears between the two electrodes called a cathode ray.

Discovery of the ElectronDiscovery of the Electron

Cathode Ray

Electric CurrentElectric Current

Steering CoilsSteering Coils

The electron beam is painting all 525 lines 30 times per sec, it paints a total of 15,750 lines per sec.

The electron beam is painting all 525 lines 30 times per sec, it paints a total of 15,750 lines per sec.

• Investigators began to study the ray and they observed that…

1.An object placed in the path of the ray cast a shadow on the glass

2.A paddle wheel placed the path of the cathode ray began to spin

3.Cathode rays were deflected by a magnetic field

4.The rays were deflected away from a negatively charged object



• The first 2 observations support the idea that the ray is composed of tiny individual particles traveling through the vacuum tube

• The second set of observations support the evidence that the ray is composed of a substance that is negatively charged.


• J.J. Thomson studied the rays and proved that they were tiny negative particles being emitted from the metal atoms. –Dubbed these tiny particles “electrons”

• Robert Millikan then used an ingenious investigation to calculate the mass to charge ratio of an atom–He determined that the electrons

were not part of the mass of the atom.

First Atomic ModelFirst Atomic Model

• What can their work help us conclude about the atom?–atoms are composed of smaller

particles, and one of these compo-nents is negatively charged

–atoms are neutral, so there must be an opposing (+) charge

–because E’s are essentially mass-less, an opposing substance that makes up the mass of the atom

Negative particles embedded in a

sphere of positive plasma-like matter.

THINK… Chocolate Chip Cookie

Negative particles embedded in a

sphere of positive plasma-like matter.

THINK… Chocolate Chip Cookie

Discovery of the ProtonDiscovery of the Proton

Discovery of the NeutronDiscovery of the Neutron

• In 1932, the English physicist James Chadwick discovered yet another subatomic particle.– the neutron is electrically neutral– It’s mass is nearly equal to the proton

• Therefore the subatomic particles are the electron, proton, and neutron.

electron e- -1 0 9.11x10-28

proton p+ +1 1 1.67x10-24

neutron n0 0 1 1.67x10-24

Structure of the AtomStructure of the Atom


• Scientists still didn’t really understand how the particles were put together in an atom.–This was a difficult question to resolve,

given how tiny atoms are.• Most thought it likely that the atom

resembled Thomson’s model

Rutherford ModelRutherford Model

• In 1911, Ernest Rutherford et al. provided a more detailed picture of the internal structure of the atom

• In his experiment, Rutherford directed a narrow beam of alpha particles at a very thin sheet of gold foil. –Alpha particles (a) are He atoms that

have been stripped of their electrons


• According to Thomson’s model, the heavy, positive alpha particles should pass easily through the gold, with only a slight deflection –And mostly that’s how it happened.–However, they found 1 in every

8000 particles had actually been deflected back toward the source.

http://micro.magnet.fsu.edu/electromag/java/rutherford/


• Rutherford suggested a new structural model of the atom.–He stated that all the positive charge

and the mass is concentrated in a small core in the center of the atom, AKA nucleus

–And that the atom is mostly empty space with electrons surrounding the positively charged nucleus like planets around the sun.



• Rutherford’s planet system model was an improvement over earlier models, but it was still not complete.–Physics says that electrons can’t orbit

the nucleus without losing energy,• Losing energy would cause the

electron to spiral into the nucleus.• The attraction of the electron to the

nucleus would cause it to spiral into the nucleus as well

Bohr ModelBohr Model

• Niels Bohr proposed a new model that would allow the electrons to be outside the nucleus and in orbit around the nucleus.–His model coupled Rutherford’s model

with a new concept of energy in Physics called quantum mechanics, • Bohr proposed that the electrons

aren’t on any random orbit around the nucleus, they are on “special” orbits

http://en.wikipedia.org/wiki/Image:Niels_Bohr.jpg


• Bohr’s Model restricts the orbits on which an electron can be–The bases for what orbit an electron is

allowed is entirely based on how much energy the electron has• If it has any more energy or any less

energy it would be forced to be on a different path of different energy

–The energy of the electron is quantized, which means it is of a very specific quantity


–Each path or level of energy that the electron is on is given a label of “n”• Such that n=1 is the closest energy

level to the nucleus• n=2 is higher in energy and outside of,

but adjacent to n=1, and so on…–Each energy level can only hold

a certain number of electrons (2n2)• n=1 can hold 2 electrons• n=2 can only hold 8 electrons• n=3 can hold 18 electrons• Etc.


Bohr Model of the AtomModel describes the paths

of electrons as energy levels. The electrons are only allowed

to have a certain amount of energy which restricts their path around

the nucleus.


• With the exception of Hydrogen, every nucleus contains 2 kinds of particles protons and neutrons– they make up the mass of the atom

(Mass Number = Protons + Neutrons)• Proton has a charge equal to but

opposite of the charge of an elec.–Atoms are neutral because they

contain equal #’s of protons & electrons


• The atoms of different elements differ in the # of protons in their nuclei and therefore in their positive charge–The # of protons the atom contains

determines the atom’s identity• Only Oxygen contains 8 protons• Only Fluorine contains 9 protons• Only Neon contains 10 protons


• The nucleus is composed of a densely packed cluster of protons, which are all electrically positive–Don’t like charges repel?–Why don’t they fly apart?

• When 2 protons are in very close proximity, there is a strong force of attraction between them.–similar attraction exists

when neutrons are close


• These short-range p+-n0, p+-p+, & n0-n0 forces hold the nuclear particles together, A.K.A strong nuclear forces.–When these nuclear forces are strong

enough the atom is stable– If the forces are not strong enough

the atom (heavier atoms) the atom is unstable and becomes radioactive.

http://www.pbs.org/wgbh/aso/tryit/atom/

Counting AtomsCounting Atoms

• Basic Truth: All atoms contain the same basic parts, but atoms of different elements have different numbers of protons.–The PT lists atoms in consecutive

order by their Atomic Number (Z) –The atomic number is directly

related to the number of protons in the nucleus of each atom of that element


• The atomic number is found above the elemental symbol on the PT and it defines the type of element–Atomic #47 can only be Ag and it also

can only have 47 protons in each nucleus

–Because atoms are neutral, we know from the atomic number the atom must also contain 47 electrons.


• The total number of protons & neutrons determines the mass of the atom–Called the Mass Number–A Carbon atom, has 6 protons and 6

neutrons, so its mass number is 12• If you know the atomic number &

mass number of an atom of any element, you can determine the atom’s composition

ATOMS OF THE 1ST TEN ATOMSNAME SYMBOL ATOMIC # p+ n0 MASS # e-

Hydrogen H 1 1 0 1 1Helium He 2 2 2 4 2Lithium Li 3 3 4 7 3

Beryllium Be 4 4 5 9 4Boron B 5 5 6 11 5

Carbon C 6 6 6 12 6Nitrogen N 7 7 7 14 7Oxygen O 8 8 8 16 8Fluorine F 9 9 10 19 9

Neon Ne 10 10 10 20 10


• Every Cl atom has 17 protons, w/o exception, but not every Cl atom has 18 neutrons.–Atoms with the same # of protons but

contain different #s of neutrons are called isotopes.• Since isotopes of an element have

different #s of neutrons they have different masses


• Isotopes are chemically alike because they have identical numbers of protons and electrons– It’s the electrons and protons that are

responsible for chemical behavior• Isotopes can be noted using hyphen

notation (Cl-35 vs Cl-37)–elemental symbol hyphen mass

number

Isotope: one of two or more atoms having the same number of protons but different numbers of neutrons

or Na-23 or Na-24

PracticePractice

Name SymbAtomic

#Mass

## p+ # e- # n0 Isotopic

Symbol

Strontium

W

82

70

As7433

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The Atom Atomic Models Think with me…about sugar crystals, you can see that they are small crystals and every crystal is identical.Think with me…about