U2L1 - Lecture Notes - Defining the Atom

Lesson 1 -
Defining the Atom

​Unit 2
The Atom and the Periodic Table

Essential Questions

• What is an atom?

• How can the subatomic particles be distinguished in terms
of relative charge and mass?

• Where are the locations of the subatomic particles within
the structure of the atom?

Defining the Atom
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The Atom

• The smallest particle of an element that retains the properties of the element is called an atom.

• An instrument called the scanning tunneling microscope (STM)
allows individual atoms to be seen.

Defining the Atom
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Multiple Choice

smallest particle of matter that retain the properties of element is ____________________

• atom

• proton

• cathode

• energy

The Electron

• When an electric charge is applied, a ray of radiation
travels from the cathode to the anode, called a cathode ray.

• Cathode rays are a stream of particles carrying a negative
charge. The particles carrying a negative charge are known as electrons.

• This figure below shows a typical cathode ray tube.

Defining the Atom
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Multiple Choice

electrons are discovered by _________experiment

• cathode ray tube

• gold foil

• oil drop

Multiple Choice

electrons are ________________charged

• negatively

• positively

• not

• both positively and negatively

Multiple Choice

The figure shows ________________________

• cathode ray tube experiment

• gold foil experiment

• oil drop experiment

The Electron

• J.J. Thomson measured the effects of both magnetic and electric fields on the cathode ray to determine the charge-to-mass ratio of a charged particle, then compared it to known values.

• The mass of the charged particle was much less than a hydrogen atom, then the lightest known atom.

• Thomson received the Nobel Prize in 1906 for identifying the first subatomic particle—the electron.

Defining the Atom
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The Electron

• In the early 1910s, Robert Millikan used the oil-drop
apparatus shown below to determine the charge of an
electron.

Defining the Atom
Copyright © McGraw-Hill Education

Multiple Choice

The figure shows _____________experiment

• oil drop

• cathode ray tube

• gold foil

Multiple Choice

The experiment ( shown in the figure ) lead to determine __________

• mass and charge of the electron

• the size of the electron

• the mass of the atom

• the charge of the atom

The Electron

• Charges change in discrete amounts—1.602 × 10–19 coulombs,
the charge of one electron (now equated to a single unit, 1–).

• With the electron’s charge and charge-to-mass ratio known,
Millikan calculated the mass of a single electron.

Defining the Atom
Copyright © McGraw-Hill Education

the mass of a
hydrogen atom

The Electron

• Matter is neutral. You know that matter is neutral from
everyday experiences. You do not receive an electric shock
(except under certain conditions) when you touch an object.

• If electrons are negative, then how is matter, which is made
up of electrons, neutral?

• J.J. Thomson proposed a model of the atom to answer this
question.

Defining the Atom
Copyright © McGraw-Hill Education

The Electron

• J.J. Thomson's plum pudding model of the atom states that the atom is a uniform, positively charged sphere containing
electrons.

Defining the Atom
Copyright © McGraw-Hill Education

Multiple Choice

The name of the model of the atom ( shown in the figure) is _______________

• plum pudding model

• nuclear model

• energy levels model

• quantum model

Multiple Choice

A particle that moves around the nucleus is a(n)...

• Proton

• Neutron

• Electron

• Quark

The Nucleus

• In 1911, Ernest Rutherford studied how positively charged
alpha particles interacted with solid matter.

• By aiming the particles at a thin sheet of gold foil, Rutherford
expected the paths of the alpha particles to be only slightly
altered by a collision with an electron.

Defining the Atom
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The Nucleus

• Although most of the
alpha particles went
through the gold foil, a few of them bounced
back, some at large
angles.

Defining the Atom
Copyright © McGraw-Hill Education

The Nucleus

• Rutherford concluded that
atoms are mostly empty space.

• Almost all of the atom's positive
charge and almost all of its mass is contained in a dense region in
the center of the atom called the
nucleus.

• Electrons are held within the atom
by their attraction to the positively charged nucleus.

• The repulsive force between the positively charged nucleus
and positive alpha particles caused the deflections.

Defining the Atom
Copyright © McGraw-Hill Education

Multiple Choice

the central region of an atom where its neutrons and protons are is its 

• nucleus 

• electron cloud

• core 

• center 

The Nucleus

• Rutherford refined the model to include positively charged particles in the nucleus called protons.

• James Chadwick received the Nobel Prize in 1935 for
discovering the existence of neutrons, neutral particles in the
nucleus which accounts for the remainder of an atom’s mass.

Defining the Atom
Copyright © McGraw-Hill Education

- Protons have a mass of 1 amu (atomic mass unit)

- Neutrons have a mass equal to a proton = 1amu

Multiple Choice

The positive particles of an atom are 

• electrons 

• positrons 

• neutrons 

• protons 

Multiple Choice

Particles in an atom that are neutral and have no charge are 

• megatrons 

• electrons 

• neutrons 

• protons 

The Nucleus

• Scientists have determined that protons and neutrons
are composed of subatomic particles called quarks.

• Scientists do not yet understand if or how quarks affect
chemical behavior.

• Chemical behavior can be explained by considering only
an atom's electrons.

Defining the Atom
Copyright © McGraw-Hill Education

The Nucleus

• All atoms are made of
three fundamental
subatomic particles:
the electron, the proton,
and the neutron.

• Atoms are spherically shaped.

• Atoms are mostly empty space,
and electrons travel around the
nucleus held by an attraction to
the positively charged nucleus.

Defining the Atom
Copyright © McGraw-Hill Education

Multiple Select

Which of the following are subatomic particles?

(Choose all that apply)

• Electrons

• Protons

• Neutrons

• Megatrons