Atomic Structure Model Timeline

Dalton Thomson Rutherford Bohr Cloud

1803 1897 1911 1913 1926

Dalton’s Model (p.161)Dalton proposed the first model of atoms; he thought atoms were a

solid sphere

Dalton’s Atomic Theory

1) All elements consist of atoms that cannot be divided

2) All atoms of the same element are exactly alike. Atoms

of different elements are different.

3) An atom of one element cannot be changed into an

atom of a different element.

4) Compounds form when atoms of more than one

element combine in a specific ratio.

Only points

(2) and (4)

are still true

today!

Thomson’s Model (p. 162)Thomson suggested that atoms had negatively charged electrons set in a

soft positive sphere. Each electron is represented by the symbol e.−.

Think of Thompson’s model like a ball of chocolate chip cookie dough. It

is also called the “plum pudding” model.

Can you think of any other analogies for Thomson’s model?

Rutherford’s Gold Foil Experiment (p.162)If atoms were a soft positive sphere, alpha particles fired at it should have

punched straight through. Rutherford was surprised that a few particles were

deflected strongly. Which are the paths of the particles that were not predicted

by Thomson’s atomic model?

Rutherford’s Gold Foil Experiment (p.162)Use the diagrams to compare the expected and observed results of

Rutherford’s gold foil experiment.

Rutherford’s Model (p. 163)

The gold foil experiment resulted in the discovery of the nucleus.

According to Rutherford’s model, an atom was mostly open space.

The “6+” in the model means that there are six protons in the nucleus.

Bohr’s Model (p. 163)Niels Bohr tested Rutherford’s model by observing the light emitted by

the element hydrogen.

Bohr’s Model (p. 163)

His results suggested that electrons move in specific orbitals around

the nucleus of an atom like planets orbiting the sun.

Modern Model of the Atom

Now we understand that orbitals are not “set in stone” as Bohr thought. Rather,

electrons move in cloudlike regions referred to as energy levels. We also know

that atoms are made up of protons, neutrons, and electrons.

.

Electron Cloud

This is the closest

we have been able to come

to visualizing the parts of

the atom. This image is

a mathematical projection

of the electron cloud(s)

around an atom, based

off experimental mass and

charge measurements. The

nucleus is too small and

dense to visualize.

Particles in an AtomAn atom is made up of protons, neutrons, and electrons. What is the

correct charge for each atomic particle?

So…what about QUARKS?

Well, the charges of protons, neutrons, and electrons have to

come from SOMEWHERE…

https://www.youtube.com/watch?v=nlv06lSAC7c

Believe it or not, the parts of the atom CAN be split!

QUARKS are the fundamental particle from which protons,

neutrons, and electrons are made.

Interpreting Atomic Structure Each block on the periodic table contains a chemical symbol, element

name, atomic number, and mass number.

For example, Potassium has the symbol “K,” (2) an atomic number of 19 (1) and

an atomic mass of 39.098 (3) amu. Bananas are rich in potassium!.

Interpreting Atomic Structure

Atomic Number

- number of protons in an atom

- tells you which element you have

Mass Number

- total number of protons and neutrons

- number of neutrons equals the mass number minus the atomic number

Electrons

- number of electrons in an atom is the same as the atomic number (number of protons)

Isotopes An isotope is an atom with fewer or more neutrons than usual.

The mass number on the periodic table is actually a weighted average of

the masses of the isotopes of that element.

What is the missing information for each isotope, below?

6

7

14

IonsAn ion is an atom with fewer or more electrons than usual.

Ions with a positive charge have fewer electrons than protons. Ions with a

negative charge have more electrons than protons. How would you find the

number of electrons if you knew the charge and atomic number of an ion?

Let’s Practice!

Example Video

https://www.youtube.com/watch?v=G3ImfVYSxoc

Remember!

Atomic Number = #Protons OR #Electrons

Atomic Mass = #Protons + #Neutrons

Now complete the worksheet “atomic structure basics”