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Unit 2 continued-Chemical Foundations

Atoms, Ions, &Elements

The Elements• Most abundant elements in/on

Earth:– Oxygen- 49.2%

– Silicon-25.7%

• Most abundant in the human body:– Oxygen-65.0%

– Carbon-18.0 %

– Hydrogen-10.0%

Microscopic vs. Macroscopic– Chemists use the word element for

macroscopic (things we can see with the eye)- meaning an amount that we would mass on a balance

– Chemists also use the word element to describe microscopic form of an element when we are talking about a single atom of that element.

Element Defined• Element- a substance that

cannot be decomposed into a simpler substances by chemical or physical means.

• It consists of atoms all having the same atomic number.

Element Symbols• Symbols are used to represent the

element.

• They usually consist of the first letter of the first two letters of the element names.

• The first letter is always capitalized, and the second is not.

Most Common Elements

You will need to memorize the elements name and symbol of the elements (on your sheet)—Polyatomic ions included.

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The Language of ChemistryCHEMICAL ELEMENTS

SodiumBromine

Aluminum

The Language of Chemistry

• The elements, their names, and symbols are given on the

PERIODIC TABLE• How many elements

are there?

The Periodic Table

Dmitri Mendeleev (1834 - 1907)- Known as the Father of the Periodic Table.

An atom consists of a

• nucleus

– (of protons and neutrons)

• electrons in space about the nucleus.

The Atom

Nucleus

Electron cloud

Copper atoms on silica surface.

• An atom is the smallest particle of an

element that has the chemical properties of the element.

Distance across = 1.8 nanometer (1.8 x 10-9 m)

ATOM COMPOSITION

•protons and neutrons in the nucleus.

•the number of electrons is equal to the number of protons.

•electrons in space around the nucleus.

•extremely small. One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water.

The atom is mostlyempty space

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ATOMIC COMPOSITION• Protons (p+)

– + electrical charge– Located in the nucleus– mass = 1.672623 x 10-24 g– relative mass = 1.007 atomic mass units (amu)

we round to 1

• Electrons (e-)– negative electrical charge– Located outside of nucleus– relative mass = 0.0005 amu

we round to 0

• Neutrons (no)– no electrical charge– Located in the nucleus– mass = 1.009 amu

We round to 1

Subatomic Particles

• Quarks

– component of protons & neutrons

– 6 types

– 3 quarks = 1 proton or 1 neutron

He

You do not have to know the types, just that quarks exist! Scientists are learning more everyday!

The red compound is composed of• nickel (Ni) (silver)• carbon (C) (black)• hydrogen (H) (white)• oxygen (O) (red)• nitrogen (N) (blue)

CHEMICAL COMPOUNDS are two or more elements chemically combined.

They can be broken back down into the elements by a chemical process. (Usually requires much energy!)

Compounds

– composed of 2 or more elements in a fixed ratio

–properties differ from those of individual elements

– EX: table salt (NaCl)

A MOLECULE is the smallest unit of a

compound that retains the chemical characteristics of the compound. (Covalent compounds are called molecules)

Composition of molecules is given

by a MOLECULAR FORMULA

H2O C8H10N4O2 - caffeine

ELEMENTS THAT EXIST AS DIATOMIC MOLECULES

Remember:

BrINClHOF

These elements

only exist as

PAIRS. Note that

when they combine

to make

compounds, they

are no longer

elements so they

are no longer in

pairs!

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How do we know?

How do we know all of these things

about the atom?

As part of the chemistry curriculum, you will need to know about the development of the model of the atom. In the next few days, we will explore and see what each scientist learned as the current model of the atom developed.

Atomic Models• Models are used to help us visualize something that

cannot be seen

Democritus• This Greek philosopher

is credited with coining the term “atomos”

• This was the term for the indivisible units.

The idea ONLY- He had no Proof!

Another Greek -Aristotle• All substances are made of 4 elements

• Fire - Hot

• Air - light

• Earth - cool, heavy

• Water - wet

• Blend these in different proportions to get all substances

• He was better at debating, this was believed until 1700’s--Alchemy

Late 1700’s - John Dalton

An English Scientist who studied the nature of materials.

He developed what is known as the

Atomic Theory.

The brown box on page 52 of text.. You will also need to know the main ideas of his theory.

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Dalton’s Atomic Theory

John Dalton (1766-1844) proposed an atomic theory

While this theory was not completely correct, it revolutionized how chemists looked at matter and brought about chemistry as we know it today instead of alchemy

Thus, it’s an important landmark in the history of science.

Dalton’s Atomic Theory

1. matter is composed, indivisible particles (atoms)

2. all atoms of a particular element are identical3. different elements have different atoms4. atoms combine in certain whole-number

ratios 5. In a chemical reaction, atoms are merely

rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements.

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Problems with Dalton’s Atomic Theory?

1. matter is composed, indivisible particlesTrue, but Atoms Can Be Divided, but only in a nuclear reaction

2. all atoms of a particular element are identical**WRONG**Does Not Account for Isotopes (atoms of the same element but a different mass due to a different number of neutrons)!

3. different elements have different atomsTrue!

4. atoms combine in certain whole-number ratiosTrue! Called the Law of Definite Proportions

5. In a chemical reaction, atoms are merely rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements.True, except for nuclear reactions that can change atoms of one element to a different element

J.J. Thomson

• Deflected a beam of particles with magnets

• Determined that particles must have a negative charge

Thomson’s Experiment

Voltage source

+-

Metal Disks

Passing an electric current makes a beam

appear to move from the negative to the

positive end

Thomson’s Experiment

Voltage source

+-

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Voltage source

Thomson’s Experiment

By adding an electric field

+

-

Voltage source

Thomson’s Experiment

By adding an electric field he found that the

moving pieces were negative

+

-

By adding an electric field

Thomson’s Model

• Found the electron.

• Couldn’t find positive (for a while).

• Said the atom was like plum pudding.

• A bunch of positive stuff, with the electrons able to be removed.

About Millikan

• Millikan determined the charge of the electron.

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Millikan’s Experiment

Atomizer

Microscope

-

+

Oil

Metal Plates

36

Millikan’s Experiment

Oil

Atomizer

Microscope

-

+

Oil droplets

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Millikan’s Experiment

X-rays

X-rays give some drops a charge by knocking off

electrons38

-

Millikan’s Experiment

+

39

Millikan’s Experiment

They put an electric charge on the plates

++

--

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Millikan’s Experiment

Some drops would hover

++

--

41

Millikan’s Experiment

+

+ + + + + + +

- - - - - - -

Some drops would hover

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Millikan’s Experiment

From the mass of the drop and the charge on

the plates, he calculated the charge on an electron

++

--

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The modern view of the atom was developed by Ernest Rutherford (1871-1937).

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Rutherford’s Experiment

• Ernest Rutherford English physicist. (1910)

• Believed the plum pudding model of the atom was correct.

• Wanted to see how big electrons were.

• Used radioactivity.

• Alpha particles - positively charged pieces given off by uranium.

• Shot them at gold foil which can be made a few atoms thick.

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Rutherford

• A stream of positive alpha particles were directed at foil

• Thomson’s model predicted that particles should pass through neutral material

Rutherford’s experiment.

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Rutherford’s experiment

• When the alpha particles hit a florescent screen, it glows.

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Lead

blockUranium

Gold Foil

Flourescent

Screen

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He Expected

• The alpha particles to pass through without changing direction very much.

• Because…

• The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles.

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What he expected

51

Because

52

Because, he thought the mass was

evenly distributed in the atom

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Because, he thought

the mass was evenly

distributed in the atomResults of foil experiment if Plum Pudding

model had been correct.

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What he got

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How he explained it

+

• Atom is mostly empty.

• Small dense, positive piece

at center.

• Alpha particles are deflected by

it if they get close

enough.

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+

What Actually Happened

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Rutherford

• Rare deflections indicated two things:

A positive mass existed in the atom

The positive center is very small

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Density and the Atom

• Since most of the particles went through, it was mostly empty.

• Because the pieces turned so much, the positive pieces were heavy.

• Small volume, big mass, big density.

• This small dense positive area is the nucleus.

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Modern View

• The atom is mostly empty space.

• Two regions.

• Nucleus- protons and neutrons.

• Electron cloud- region where you might find an electron.

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Which Model is this?Is it correct?

Atomic Number, Mass Number, and Element Symbol

• There are 3 different parts to identify an element.

– Atomic Number• The number of protons in the nucleus of an atom.

• Symbolized by Z

– Mass Number• The sum of the protons and neutrons.

• Symbolized by A

– Element Symbol• The one or two letter symbol given to every element.

XAZ

Na2311

{Plus}

• If you see an element written this way:

Carbon-14

mass number is 14

Atomic Number, Z

All atoms of the same element have the same number of protons in the

nucleus, Z

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Al

26.981

Atomic number

Atom symbol

AVERAGE Atomic Mass

Mass Number, A• C atom with 6 protons and 6 neutrons is the

mass standard • = 12 atomic mass units

• Mass Number (A)= # protons + # neutrons

• NOT on the periodic table…(it is the AVERAGE atomic mass on the table)

• A boron atom can have A = 5 p + 5 n = 10 amu

A

Z

10

5B

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Isotopes

• Atoms of the same element (same Z) but different mass number (A).

• Boron-10 (10B) has 5 p and 5 n

• Boron-11 (11B) has 5 p and 6 n

10B

11B

Figure 3.10: Two isotopes of sodium.

Isotopes & Their Uses

Bone scans with radioactive technetium-99.

Isotopes & Their Uses

The tritium content of ground water is used to discover the source of the water, for example, in municipal water or the source of the steam from a volcano.

Atomic Symbols

Show the name of the element, a hyphen, and the

mass number in hyphen notation

sodium-23

Show the mass number and atomic number in

nuclear symbol form

mass number23 Na

atomic number 11

Isotopes?

Which of the following represent isotopes of the same element? Which element?

234 X

234 X

235 X

238 X

92 93 92 92

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Answer:

234 U

234 Np

235 U

238 U

92 93 92 92

234 Np is not an isotope of Uranium.

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Counting Protons, Neutrons, and Electrons

• Protons: Atomic Number (from periodic table)• Neutrons: Mass Number minus the number of

protons (mass number is protons and neutrons because the mass of electrons is negligible)

• Electrons:– If it’s an atom, the protons and electrons must be the SAME

so that it is has a net charge of zero (equal numbers of + and -)

– If it does NOT have an equal number of electrons, it is not an atom, it is an ION. For each negative charge, add an extra electron. For each positive charge, subtract an electron (Don’t add a proton!!! That changes the element!)

Proton Electron Neutron

+ charge - charge 0 charge

In nucleus Outside nucleus in

specific levelsIn nucleus

1 1/1836 1

Learning Check – Counting

Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.

12C 13C 14C6 6 6

#p+ _______ _______ _______

#no _______ _______ _______

#e- _______ _______ _______

Answers

12C 13C 14C6 6 6

#p+ 6 6 6

#no 6 7 8

#e- 6 6 6

Learning Check

An atom has 14 protons and 20 neutrons.A. Its atomic number is

1) 14 2) 16 3) 34

B. Its mass number is1) 14 2) 16 3) 34

C. The element is1) Si 2) Ca 3) Se

D. Another isotope of this element is1) 34X 2) 34X 3) 36X

16 14 14

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Solution

An atom has 14 protons and 20 neutrons.A. It has atomic number

1) 14

B. It has a mass number of3) 34

C. The element is1) Si

D. Another isotope of this element would be

3) 36X14

IONS • IONS are atoms or groups of atoms with a

positive or negative charge.

• Taking away an electron from an atom gives a

CATION with a positive charge

• Adding an electron to an atom gives an ANIONwith a negative charge.

• To tell the difference between an atom and an ion,

look to see if there is a charge in the superscript!

Examples: Na+ Ca+2 I- O-2

Na Ca I O

Forming Cations & Anions

A CATION forms when an atom loses one or more electrons.

An ANION forms when an atom gains one or more electrons

Mg --> Mg2+ + 2 e- F + e- --> F-

PREDICTING ION CHARGES

In general

• metals (Mg) lose electrons ---> cations

• nonmetals (F) gain electrons ---> anions

Learning Check – Counting

State the number of protons, neutrons, and electrons in each of these ions.

39 K+ 16O -2 41Ca +2

19 8 20

#p+ ______ ______ _______

#no ______ ______ _______

#e- ______ ______ _______

Learning Check – Counting

State the number of protons, neutrons, and electrons in each of these ions.

39 K+ 16O -2 41Ca +2

19 8 20

#P 19 8 20

#N 20 8 21

#E 18 10 18

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One Last Learning Check

Write the nuclear symbol form for the following atoms or ions:

A. 8 p+, 8 n, 8 e- ___________

B. 17p+, 20n, 17e- ___________

C. 47p+, 60 n, 46 e- ___________

Answers

Write the nuclear symbol form for the following atoms or ions:

A. 8 p+, 8 n, 8 e- 16O8

B. 17p+, 20n, 17e- 37Cl17

C. 47p+, 60 n, 46 e- 107Ag+

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Charges on Common Ions-1-2-3

+1

+2

By losing or gaining e-, atom has same number of e-’s as nearest Group 8A atom.

AVERAGE

ATOMIC MASS

• Because of the existence of isotopes, the mass of a collection of atoms has an average value.

• Boron is 20% 10B and 80% 11B. That is, 11B is 80 percent abundant on earth.

• For boron atomic weight

= 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu

10B

11B

Isotopes & Average Atomic Mass

• Because of the existence of isotopes, the mass of a collection of atoms has an average value.

• 6Li = 7.5% abundant and 7Li = 92.5%

– Avg. Atomic mass of Li = ______________

• 28Si = 92.23%, 29Si = 4.67%, 30Si = 3.10%

– Avg. Atomic mass of Si = ______________

Honors only

The Periodic Table

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Periods in the Periodic Table Groups in the Periodic Table

Elements in groups react in similar ways!

Regions of the Periodic Table Group 1A: Alkali Metals

Cutting sodium metal

Reaction of potassium + H2O

Magnesium

Magnesium oxide

Group 2A: Alkaline Earth MetalsGroup 7A: The Halogens (salt

makers) F, Cl, Br, I, At

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Group 8A: The Noble (Inert) Gases

He, Ne, Ar, Kr, Xe, Rn

• Lighter than air balloons

• “Neon” signs

• Very Unreactive because they have full electron levels

XeOF4

Transition Elements

Lanthanides and actinides

Iron in air gives iron(III) oxide