Atomic Structure - Denton Independent School District...change their overall atomic mass. An atom...

Atomic Structure

Pre-AP Chemistry

Target Words

Atom

Atomic number

Atomic theory

Electromagnetic spectrum

Electron

Electron cloud

Emission spectrum

Energy level

Hypothesis

Isotope

Law of definite proportions

Lewis dot diagram

Mass number

Neutron

Nucleus

Proton

Scientific law

Theory

Valence electron

What do you know about atoms?

Early Atomic Theory

Philosophically based from ethical, religious, or other

schools of thought, not modern scientific methodology

Greek

Democritus – proposed atomic theory to account for

philosophical disagreements on matter versus void

Indian

Multi-faceted between scientific elemental forms and religious

theories concerning souls and knowledge

Islamic

Atomism reconciles the intent of the divine with the existence

of the physical

Early Scientific Work

France, 1789

Antoine Lavoisier develops the Law of Conservation of Mass

through highly accurate measurements

France, 1799

Joseph Proust develops the Law of Definite Proportions, which

states that when broken down, compounds always have the

same ratio of elemental ingredients

England, 1802

John Dalton builds on previous work to develop the Law of

Multiple Proportions; fixed amounts of element A will always

combine with element B in proportions that reduce to whole

numbers

Dalton’s Atomic Theory (1803)

All matter is made up of atoms.

Atoms are indestructible and cannot be divided into

smaller particles.

All atoms of an element are exactly alike, but are different

from atoms of other elements.

Dalton’s Work Published

Though

brilliantly

and

empirically

derived, his

model was

flawed and

incomplete.

Why?

Look closely at

his illustrated

plates: what do

you see here?

Questions Raised -

Why are there so many different elements?

What makes one atom different from another?

How do atoms combine to form compounds?

Review Slide

Early work on Atomic Theory was based in philosophy,

not science.

Real science-based work started in the late 1700’s in

France and England; Lavoisier and Proust.

Laws of; Conservation of Mass, Definite and Multiple

Proportions led to Dalton formulating his Atomic Theory. All matter is made up of atoms.

Atoms are indestructible and cannot be divided into smaller particles.

All atoms of an element are exactly alike, but are different from atoms of other elements.

Dalton’s Model; a solid ball of the ?element? that is tiny.

Questions were left unanswered!

Cracking into the Mystery of the Atom!

Additional refinement was done throughout the 1800’s to

further find atomic weights and relative masses of

elements (Avagadro)

The inner secrets of the atom were not divulged until

very recently.

J. J. Thompson, England (1897)

British physicist experimenting with electricity and

vacuum tubes.

Constructed a vacuum tube with positive and negative

terminals at either end, and a phosphorescent (glows

when energized) strip.

Science in ACTION!

Thompson’s Thoughts

Something is passing between the two metal nodes.

Is it light?

Is it something else?

How could he know?

Science, but with magnets!

Thompson’s Conclusions

The stream is NOT light; magnets do not bend light.

Therefore the stream is made up of charged particles.

The stream can NOT be something already in the tube, it

is a vacuum.

Therefore the stream MUST be coming from the metal

nodes at either end.

Therefore atoms must have particles that can be separated

and shot as a ray, particles with an electric charge!

More Information and Conclusions

The magnet bends the stream certain ways.

Therefore the charged particles had a NEGATIVE charge.

The particles were EXTREMELY small, 1/1387 the mass of

the smallest atom.

Therefore the atom must have other stuff inside it!

Atoms normally have a NEUTRAL charge.

Therefore the other stuff is POSITIVELY charged.

Thompson’s Model

Thompson described the atom as a ball of positive

charges with electrons drifting around somehow.

Though including new subatomic particles, his model

maintained the ‘solid ball/clump’ structure of Dalton’s

earlier model.

Complications Plague Thompson

It was discovered that the positive terminal also emits a

ray.

Thompson proposed a positive particle so this was expected.

The masses of some elements were found to be slightly

variable – sometimes they weighed more, sometimes a

little less

The rational explanation is a third particle, a neutral one called

the neutron.

Atoms that are chemically identical but of different weights are

called isotopes.

Review Slide

More work on the Atomic Theory provided very important additional details.

Thompson’s experiments with vacuum tubes, cathodes, and magnets demonstrated;

That atoms were made of smaller particles.

The particles were negative, positive, or neutrally charged.

Named the Electron, Proton, and Neutron.

Elements sometimes came in different weights (isotopes) – this was proposed to be because of having more or less neutrons inside.

Important question; How is everything arranged?

Rutherford, 1908

Another British physicist interested in working with

positively charged particles, called α particles (alpha).

He was working on scattering, or finding out how these

particles bounced for complex physics reasons.

Rutherford’s Experimentation

After constructing an

alpha particle gun, it was

aimed at a sheet of gold

foil.

Gold can be easily

flattened until it is a few

atoms thick.

Source of fast moving,

positively charged

particles (α particles)

ZnS fluorescent screen

Gold foil

Deflected

particles

Undeflected

particles

What SHOULD have happened.

What Really happened!

Exercise

1. What kinds of particles are emitted by the radioactive source? What is their charge?

2. Toward what are the alpha particles being directed?

3. What happens when the charged particles strike the surface of the gold foil?

4. What was the purpose of the fluorescent screen?

5. What did Rutherford conclude from the gold foil experiment regarding the amount of empty space in the atom? Why was he able to conclude this?

6. How does the path of a charged particle that strikes the center of a gold atom differ from the path of a particle that passes near the center?

7. Based on this experiment, where is most of the positive charge and mass of an atom found?

Rutherford’s Model

Rutherford or Nuclear model

In the early twentieth century, Rutherford

showed that most of an atom's mass is

concentrated in a small, positively charged

region called the nucleus.

Questions post Rutherford

How are the electrons arranged?

What size are these atoms?

How does energy figure into this?

Niels Bohr

Another physicist, but Danish!

His idea was that electrons orbited the nucleus – but

with quantum energy states keeping them in place instead

of gravity! (This is complicated, more on it later.)

Electrons ‘orbit’ the nucleus in layers, or electron shells,

which depend on the number of electrons and size of the

nucleus.

Each shell can only hold a certain number of electrons

Electron Shells

Each element has a different number of electrons, and so

they have different electron shell arrangements;

Hydrogen for example has one electron:

Oxygen has 8 electrons:

There can be many different layers!

Bohr

Bohr model

After Rutherford's discovery, Bohr proposed

that electrons travel in definite orbits, arranged in shells or levels

Around the nucleus.

Review Slide

Rutherford discovered that the mass of an atom is

concentrated in the center, or nucleus of an atom.

He was unable to determine the exact arrangement of

other particles.

Bohr calculated the locations of electrons using high-

energy equations and complex math.

Bohr demonstrated that electrons orbit in energy layers,

or shells

Atomic Model Review

Rutherford or Nuclear model

In the early twentieth century, Rutherford showed that

most of an atom's mass is concentrated in a small,

positively charged

region called the nucleus.

Bohr model

After Rutherford's discovery, Bohr proposed

that electrons travel in definite orbits, arranged

in shells or levels around the nucleus.

Thompson model

After working with cathode ray tubes, Thompson

created the ‘plum pudding’ model with electrons

Embedded in an unknown positively charged

substrate.

Dalton model

Not much of a model, but Dalton’s understanding was

of a discrete little ball of whatever made atoms different

from one another.

?

Atomic Makeup

Nucleus

Contains protons

Contains neutrons

Electron Shells

Holds all the electrons in orbits

How many subatomic particles?

Every element has different numbers of subatomic

particles making up its nucleus and electron shells.

The number of protons is unique for each element and

does not change.

The number of electrons can vary.

The number of neutrons can vary.

Reading the Periodic Table

The periodic table gives us all the information we need;

An examples is below.

Breaking it down

Every element has a unique atomic number.

The # of protons = atomic number.

The mass of an atom is concentrated in the nucleus so the atomic mass = # protons + # nuetrons.

The # of neutrons = atomic mass – atomic number.

Atoms have no charge (ground state) so the positive charges of the protons must be canceled by the negative charges of the electrons.

The # of electrons = # of protons.

For Example;

Aluminum

Protons = 13 (Atomic Number)

Neutrons = 13 (26[atomic mass]-13[atomic number])

Electrons = 13 (To cancel the positively charged protons.)

Phosphorus




Zinc




Electron numbers can change?

Yes!

Atoms can lose or gain electrons, which means they

gain/lose a charge that is either positive or negative.

As electrons are lost or gained the +/- balance shifts.

More electrons makes the atom become negatively charged.

Fewer electrons makes the atom positively charged.

The newly charged atoms are called ions.

Positive ions are cations.

Negative ions are anions.

Notation and Ions

Al+2



Electrons = 11 (Two protons are not being cancelled.)

P-1



Electrons = 16 (There is an extra negative charge.)

Zn+4



Electrons = 26 (Four protons are not being cancelled.)

Neutron numbers shift as well? Yes.

Atoms naturally exist with variable numbers of neutrons, which can change their overall atomic mass.

An atom with the same atomic number but a different atomic mass is called an isotope.

One of the most common ways to identify an isotope is to write the symbol then the mass; Carbon -14

Neon – 21

Oxygen -16

The atomic mass on the periodic table is a weighted average of all isotopes of that element.

Isotope Examples

Aluminum - 27




Phosphorus - 31




Zinc - 64




Atomic Mass

The atomic mass of an element represents the

average mass of all the isotopes found in nature.

No element exists with only one possible isotope.

Hydrogen has the smallest number of isotopes:

H-1 protium, H-2 deuterium, H-3 tritium.

Calculating Atomic Mass

If you look at your periodic table for hydrogen, the

atomic mass is 1.0079 amu (atomic mass units).

The atomic mass is calculated by adding the % of H-

1 mass found in nature to the % of H-2 mass found

in nature plus the % of H-3 mass.

% H-1 + % H-2 + % H-3 = average mass (atomic

mass)

Generally the formula used is:

% X + % Y + % Z… = atomic mass.

Sample Problem

Silver is found to have two stable isotopes, one has an atomic mass of 106.904 amu and the other weighs 108.905 amu.

The first isotope represents 51.82 % of the mass of the element and the second represents 48.18 %.

What is the atomic mass of the element silver?

Solution

The equation to use is: %X + % Y = average atomic mass And remember to convert your percentage amounts

into fractions (by dividing by 100) before you begin

anything! (0.5182) 106.904 amu + (0.4818) 108.905 amu =

mass 55.398 amu + 52.470 amu = 107.868 amu

Now look at the periodic table to verify the answer.

Try This:

A sample of neon contains three isotopes, neon-

20 (with an isotopic mass of 19.9924 amu), neon-

21 (20.9939 amu) and neon-22 (21.9914 amu).

The natural abundances of these isotopes are

90.92%, 0.257 %, and 8.82 %.

Calculate the atomic weight of neon.

20.17 amu

Atomic Structure - Denton Independent School District...change their overall atomic mass. An atom...

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