Unit 3 Notes Guide – Part 1 Name # Atoms Date: ____________________ Block: Chemistry

I. The Size of an Atoma. We cannot see atoms with even the most powerful microscopes.

b. Since the atom cannot be directly observed, scientists rely on ________________ evidence to

create ______________ to help us understand the atom.

i. Scientific models _______________ always look like the actual object.

ii. A model is an ________________ to use familiar ideas to describe unfamiliar things in a visual

way.

c. A model can be _______________ as new ________________________ is collected.

II. History of the Atomic Modela. Aristotle (~500 BC)

i. According to ____________________, all matter is composed of one of the four fundamental

substances, or _________________: earth, fire, water, and air.

b. Democritus (~400 BC)

i. According to _________________, all matter is made up of ________, ___________________

particles called “___________.”

ii. “____________” is Latin for ____________________________.

c. Robert Boyle (1600s)

i. Author of “The Sceptical Chymist”

ii. Proposed that an _____________ is a substance that could not be further broken down by

_____________________ means.

d. Antoinne Lavoisier (1798)

i. Known as the “_______________________________________________________”

ii. Formed the Law of _________________________ of __________.

No mass is _________, no mass is ________________.

Supported Boyle’s claim that an element could not be broken down by chemical means

e. John Dalton (1800s)

i. Dalton’s Atomic Theory

Elements are made of atoms.

Atoms of the same _____________ are _________________.

Atoms of __________________ elements are different

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Atoms of different elements combine in simple ______________________________ ratios

to form chemical ___________________.

In chemical reactions, atoms cannot be _________________ or ____________________ but

_______________________.

f. JJ Thomson (1897-1904)

i. The Cathode Ray Experiment

Passing an electric current makes a beam appear to move

from the negative to the positive end

By adding an electric field, he found that the moving pieces were

negative

ii. The Plum Pudding Model

Used ________________ ray tubes to discover the _________________.

Since atoms are neutral species, he suggested that _______________ electrons are

floating in a __________________ charged _________________ of matter.

Model resembles “plum pudding”

g. Robert Millikan (1909)

i. Millikan performed his famous __________________ Experiment to determine the exact

____________ of the electron.

ii. Using the mass of the drop and the charge on the plates, he calculated the charge of the electron

to be 1.592x10-19 C (Coulombs = unit of charge)

h. Ernest Rutherford (1909)

i. In 1911, Ernest Rutherford and his associates performed the

________________ experiment to discover the ______________.

ii. Rutherford proposed the existence of the atomic ________________

(Latin for “little nut”) containing all of the ___________________

charge of the atom.

iii. The nucleus occupies __________________ space, but most of the

___________ of the atom.

iv. The Planetary Model

Rutherford later amended his model to include _______________________.

Rutherford’s model is sometimes referred to as the ____________________ model, as

the ________________ discovered by Thomson ______________ the nucleus like

planets orbiting around the sun.

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i. James Chadwick (1932)

i. We know that opposite charges repel, so it did not make any sense to have a condensed nucleus

of only positive charge.

ii. Chadwick discovered the ______________, a ______________ particle in the nucleus, that acts

as a _____________ to prevent the protons from ripping apart the nucleus.

j. Niels Bohr (1913)

i. Proposed that the _____________ of electrons around the nucleus can only exist at certain

_______________________.

ii. These orbits or “____________” can be represented by a ___________________ quantum

number, n. (n=1, 2, 3, etc.)

The Bohr Model (Electron Shell Model) Each energy shell has a __________________

number of electrons it can hold

a. 1st shell = _______________________

b. 2nd shell = _______________________

c. 3rd shell = _______________________

III. Atomic Structure and Subatomic Particlesa. Atoms are composed of three __________________________ particles.

i. Protons

ii. Neutrons

iii. Electrons

b. The mass of subatomic particles is measured in __________________________________ (______)

c. Protons

i. ______________ have a charge of _______ and are found in the ________________ at the

____________ of the atom.

ii. The ___________ of a proton is approximately ____________.

d. Neutrons

i. ______________ have _____ charge and are also found in the _______________.

ii. The _________ of a neutron is approximately ____________.

e. Electrons

i. ______________ have a charge of _______. Electrons reside in the _____________________,

the region of the atom surrounding the nucleus.

ii. Protons are 1836 times larger than electrons, so the _______ of the electron is ______________

iii. According to the Bohr model, electrons circle the atom in specific _________.

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f. Atoms are mostly _____________________.

g. Elements on the Periodic Table

i. All periodic tables may be slightly different

ii. The element ____________ and ____________________ are shown as well as the

__________________________________ and the _____________________________.

iii. Atomic ___________________ refers to the number of _____________ only.

iv. Atomic ________ is the number of ______________ plus the number of _______________.

v. For now, we will assume atoms are neutral, so the number of positive protons and negative

electrons must be equal.

vi. Practice #1: Complete the table. Use a periodic table to identify the element.

Element Atomic Number

Mass Number

# Protons

# Neutrons

# Electrons

K 39 20

15 31 15

2 2

53 78 53

h. Standard Nuclear Notation

i. Another way to represent an element

Example: Write the standard nuclear notation for Nitrogen.

Practice #2: Write the standard nuclear notation for Neon.

IV. Lewis Dot Diagrams & Bohr Modelsa. Bohr Models

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Atomic Number=¿ protons

AtomicMass=¿ protons+¿neutrons

ZAX

i. Recall the Bohr model of the atom describes the electrons surrounding the nucleus in defined

orbits with specific energy.

ii. These orbits are known as ____________ _____________ or ___________.

iii. We can visualize these energy levels like steps on a ladder with energy increasing up the ladder

(farther away from the nucleus)

iv. Bohr models are a visual _____________________________ of an atom that includes

information on the protons, neutrons, electrons, and the energy levels those electrons reside in.

v. Steps to Drawing a Bohr Model

1. Determine the number of protons, neutrons, and electrons for the element

a. # protons = atomic number

b. # neutrons = atomic mass – atomic number

c. # electrons = # protons

2. Draw nucleus, include # protons and # neutrons

3. Determine how many energy levels are needed

4. Draw energy levels needed

5. Fill energy levels with electrons (Remember: 2, 8, 8)

Example:

Practice #3 Practice #4

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# protons = 17

# neutrons = 35 - 17 = 18

# electrons = 17

# protons =

# neutrons =

# electrons =

# protons =

# neutrons =

# electrons =

b. Lewis Dot Diagrams (Electron Dot Diagrams)

i. A Lewis dot diagram is a shortcut to represent the structure of an atom.

ii. Bohr models show _______ electrons, but Lewis dot diagrams only show the ____________

electrons.

Valence electrons are the _____________________ electrons of the atom, the electrons that

reside in the ________________ energy level from the nucleus.

iii. Lewis dot diagrams have 2 parts:

The _______________ (representing the nucleus and the core electrons)

The __________________________ (representing the valence electrons)

iv. By convention, you should not have more than _______ electrons per “side”

v. The maximum number of valence electrons is _______.

Examples:

Practice #5 Practice #6

V. Ionsa. An atom has an overall ______________ charge when the

number of ______________ is equal to the number of

__________________.

i. Positive charge balances negative charge

b. However, atoms may gain or lose electrons for form an

________.

i. A ____________ is a positive ion

ii. An __________ is a negative ion

c. The Charge of an Ion

i. When # protons = # electrons, the charge is ______________ (0).

ii. When there ________ electrons than protons, the charge is ______________.

Anions form by ______________ electrons.

iii. When there are more ______________ than electrons, the charge is ________________.

Cations form by ____________ electrons.

d. The Octet Rule

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Total # electrons:

# valence electrons:

Total # electrons:

# valence electrons:

i. Everything in chemistry is driven by the need to reach ___________________.

ii. A stable atom has a ________ valence shell of electrons.

It is called the octet rule since most shells hold ______ electrons.

iii. Exceptions:

________________ and ____________ only have electrons in the first shell.

Since the first shell can hold only ______ electrons, it follows the ________ rule.

e. Gaining or Losing Electrons?

i. Atoms take the path of least resistance to achieve a complete octet.

ii. Elements that have ________ than 4 electrons will ________ their electrons to drop down to the

previous filled shell.

iii. Elements with ________ than 4 electrons will ________ more electrons to fill their current

shell.

iv. Elements with exactly ______ valence electrons can go ____________ way, but they tend to

lose electrons.

v. __________ __________ (Group 18) already have 8 valence electrons, so they do ______ form

ions.

ExamplesChlorine has 7 valence electrons Magnesium has 2 valence electronsCl gains 1 electron to form Cl- Mg loses 2 electrons to form Mg2+

vi. On the periodic table:

_____ _____

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17 p18 n

12 p12 n

12 p12 n

____ ____ ____ ____ ____ ____

*Transition metals do not follow any rule

Practice #7What charge does the oxygen ion form?

What charge does the potassium ion form?

What charge does the aluminum ion form?

f. Notation for Charge

i. Charge is represented in the __________ __________ hand corner

ii. Lewis Dot diagrams and Bohr models are put in ________________

VI. Isotopes and Atomic Massa. On the periodic table, atomic mass is always represented as a ___________________.

i. How can this be possible if there can only be whole numbers of protons and neutrons?

b. Isotopes are variations of the ______________________ with different numbers of ____________.

i. We differentiate isotopes by their ________________________.

c. The existence of isotopes proves one tenet of _______________________________________

incorrect because atoms of the same element are _______ always identical.

d. The atomic mass on the periodic table is actually the _________________ atomic mass.

i. Average atomic mass if the _______ of the masses of each isotope of an element multiplied by

its ____________________________.

Natural abundance is the ___________________ of that isotope out of all the atoms of that

element.

e. Calculating Average Atomic Mass

i. For each isotope, multiply the atomic mass by the percent natural abundance divided by 100%

atomicmass x percent abundance100

ii. Add together the values for all isotopes

iii. Units for average atomic mass are _________ (atomic mass units)

ExamplesCarbon has two isotopes. 99% of carbon has a mass of 12 amu, and 1 % has a mass of 13 amu. Calculate the average atomic mass.

76% of chlorine atoms are Chlorine-35. The other 24% is Chlorine-37. Calculate the average atomic mass of chlorine.

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-28

16 O -2O -2O

-28 p8 n

Practice #8: Nitrogen-14 has a natural abundance of 99.63% and Nitrogen-15 has a natural abundance of 0.37%. Calculate the average atomic mass of nitrogen.

ExamplesLithium exists as two isotopes, Lithium-6 and Lithium-7. If the molar mass on the periodic table is 6.94 g/mol, which isotope is more abundant?

Lithium-7 because the molar mass is closer to 7 than to 6.

Copper has two naturally occurring isotopes. Cu-63 has an atomic mass of 62.93 amu and an abundance of 69%. What is the atomic mass of the second isotope? Write this isotope in standard nuclear notation.

Practice #9: Magnesium has three naturally occurring isotopes. 78.99% of atoms have a mass of 24 amu, and 11.01% have a mass of 26 amu. What is the mass of the third isotope?

VII. Radioactivity and Half-Lifea. Henri Becquerel (1896)

i. While studying phosphorescence on photographic plates using uranium, Becquerel discovered a

phenomenon he called ________________________.

ii. Radioactivity is the _________________ emission of various, specific particles from an

__________________________.

b. Marie & Pierre Curie (1903)

i. Isolated two previously unknown radioactive elements: polonium and ________________.

c. Nuclear Reactions

i. Radioactivity falls in a different type of chemistry that deals with ________________ reactions.

ii. All nuclear reactions release energy (____________________)

iii. There are two types of nuclear reactions:

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12amu x 99100

=11.88 amu

13amu x 0.01=0.13amu+_________12.01amu

35amu x 0.76=26.60amu37amu x0.24=8.88amu+ ________

35.48amu

Molar Mass: 63.55% 2nd isotope = 31%

63.55amu=62.93 (0.69 )+x (0.31)x=64.93amu Cu65

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Fusion is a nuclear reaction in which two or more atomic nuclei collide at a very high energy

and ________ ________________ into a new nucleus

a. Fusion reactions product a HUGE amount of energy, but they are very difficult to

control.

Fission is a nuclear reaction in which the nucleus of an atom

____________ into smaller parts

a. Fission produces less energy than fusion, but it is still considerable.

b. Fission is also easier to control, making it an ideal source of energy.

iv. Fission reactions…

Involve the _____________ in the ____________________ of the atom

Occur because the nucleus in the atom is ____________________.

v. Remember, Chadwick discovered the neutron as a way to explain the stability of the nucleus.

vi. The stability of the nucleus depends on the _______________ of protons to neutrons.

d. The ____________________________ disintegration of the nucleus creates a slightly smaller atom

with a more stable nucleus

i. There are three types of radioactive decay:

____________ emissions

____________ emissions

____________ emissions

e. Alpha Decay

i. Alpha decay occurs when there are too many ______________ in the nucleus.

ii. An __________________________ is emitted

iii. An alpha particle is the nucleus of a ______________ atom without any electrons (_______)

iv. General equation:

f. Beta Decay

i. Beta decay occurs when there are too many ____________ in the nucleus.

ii. A _________________ converts into a _________________ and a __________________ is

ejected from the nucleus

iii. A beta particle is an _________________ (________ or _____)

iv. General equation:

g. Gamma Decay10

i. ______________________ electromagnetic waves (light)

ii. Also known as gamma radiation (_____________)

iii. Emitted in all decay processes

iv. General equation:

Summary:

Radiation Type Particle Gain or Lose proton ∆ in atomic # ∆ in atomic massα Alpha Emission He2

4      

β Beta Emission e−10      

γ Gamma Emission γ00      

Examples:

Chemical Equation Type of decay

I53131 → He2

4 +¿________

I53131 → e−1

0 +¿________

Practice #10:Chemical Equation Type of decay

Na1122 → e−1

0 +¿________

Po84209 → He2

4 +¿________

Cu2966 → e−1

0 +γ+¿________

K1940 → Ca20

40 +¿________

Rn86222 → Po84

218 +¿________

h. Rate of Nuclear Decay

i. Radioactive decay occurs at different rates for different elements.

ii. The unit of radioactive decay is the _________ ________.

iii. Half-life refers to the time it takes for _____________ of a sample to decay.

iv. Example

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After 1 half-life, 50 of the original 100 grams remain, half the amount of the original sample.

After 1 half-life, 50 grams remain and then undergoes another half-life and 25 grams remain.

After each half-life the sample loses _________ the amount.

v. Calculations with half-life:

¿Half liv es= time passedlengthof half life

Lengthof half life=time passed¿half li v es

ExamplesGraph the first three half-lives.

Is half-life considered a linear or exponential decay?

Determine the half-life of the element from the graph.

If the original sample was 0.20, the time where 0.10 occurs is the length

of one half life.

7 seconds

Practice #11: Graph the first three half-lives.

Example

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Carbon-14 has a half-life of 5,700 years. How much of a 100. g sample will remain after 17,100 years?

Practice #12: Strontium-90 has a half-life of 28.8 years. If you start with a 10-gram sample of strontium-90, how much will be left after 115.2 years?

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¿half lives=171005700

=3half lives

1002

→ 502→ 25

2=12.5grams