The Atom and Periodic Table General Chemistry: Unit 3 Fall 2010

The Atom and Periodic Table

General Chemistry: Unit 3Fall 2010

Atomic Model Scientists

John Dalton 1803

John Dalton (Wikipedia.org)

Dalton’s Atomic Theory

All matter is made up of tiny, unbreakable particles called atoms (Democritus proposed this in 460-370 BC)

Atoms of the same element are identical, but they are different from atoms of other elements.

Chemical reactions occur when atoms are separated, joined, or rearranged

Successes and Problems

Verified theories and experiments by Democritus and Lavoisier (law of conservation

of matter) and Proust (law of definite proportions).

We know today that atoms are made of smaller particles and can be split, and there can be isotopes of the same element.

Atomic Model Scientists (cont.)

Dmitri Mendeleev 1869 Developed a Periodic

Table based on atomic mass

Dmitri Mendeleev (Wikipedia.org)


Joseph John Thomson 1897 Found rays bent toward a

positively charged plate and away from a negatively charged plate.

Determined cathode rays are made up of negatively charged particles referred to as electrons.

JJ’s Successes

Scientists were able to use JJ Thompson’s cathode ray tube to discover protons.

The amount of charge on an electron and proton is equal but opposite, but the mass of a proton is much greater that that of an electron.

Thompson Again!

He also discovered that Neon consisted of atoms of two different masses.

Later, these were called isotopes: atoms of an element that are chemically alike but differ in mass (# of neutrons).

Scientists were then able to discover the neutron (1930s): a neutrally charged particle of equal mass to a proton.


Ernest Rutherford 1911 Discovered the positively

charged nucleus through gold foil experiment.

Ernest Rutherford (Nobelprize.org)

Rutherford’s Gold Foil Experiment

http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter02/Text_Images/FG02_05.JPG


Niels Bohr 1913

Niels Bohr (The University of New

York)

Bohr Continued

Rutherford’s atomic model did not explain the chemical properties of elements

A description of the behavior of electrons was needed

Bohr described fixed energy levels an electron can possess. In order to move up an

energy level, energy must be absorbed and vice versa but energy is released

The more energy an e- has, the further from the nucleus it is

Bohr concluded

A quantum of energy is the amount of energy needed to move an e- from one level to the next

The higher the energy level, the less energy it takes to move from that level to the next

This idea failed to explain more complex elements… this is where Shrodinger comes in

Erwin Schroedinger(quantum mechanical model)

Electrons are not stuck in a “planetary orbit,” or exact path, around the nucleus.

Instead, they’re spherical regions, of space around the nucleus in which electrons are most likely to be found. You can’t determine the exact location of an e-

Called “electron clouds” or “atomic orbitals” A region of space in which there is a high

probability of finding an electron

Atomic Structure

Atoms are made up of three subatomic particles:

Subatomic = smaller than

an atom

Live where?

Have mass?

What charge?

Protons

Neutrons

Electrons

In nucleus

In nucleus

Outside nucleus

Yes, substantial

Yes, substantial

No, negligible

+

-

none

Elements

Substances made of only one type of atom

Identified by atomic number (protons!!!)

Can not be broken down into simpler substances

Element Symbols

Shorthand name of the element Most are based on the Latin name

Ex: Gold = Au The symbol is either:

1. One capital letter-ex: Carbon = C

2. Two letters…one capitol, one lower case-ex: Krypton = Kr

Electron Clouds

The different layers of clouds hold different numbers of electrons 1st cloud = 2 2nd cloud = 8 3rd cloud = 8

And then it gets complicated...we’ll save that discussion for future chemistry courses

These cloud layers conveniently match the rows

on the periodic table:1st row = 2 elements2nd row = 8 elements

3rd row =8 elements…And then it gets complicated

Electron Clouds (cont.)

To draw the electron clouds: Figure out how many total electrons Fill in the electrons starting with the

first cloud Sulfur has:

16 electrons

Electron Clouds (cont.)

Practice Problem #3 Draw the electron clouds for an Al

atomThis is WAY

too much work…there must be a

simpler way!

Lewis Dot Structures

A smart man named Gilbert Newton Lewis figured out an easier way! For Lewis Dot Structures draw only the

important electrons… The outer, or valence, electrons

Just draw the outer electronsaround the atomic symbol

S

Lewis Dot Structures (cont.)

Practice Problem #4 Draw Lewis Dot Structures for:

A Calcium atom

A Chlorine atom

An Oxygen atom

Ca

O

Cl

Lewis Dot Structures (cont.)

All elements want to be full of electrons: So they gain or lose electrons until they

are full This gives the atom a charge

Negative charge if they gain electrons Positive charge if they lose electrons

Charged atoms are called ions: cations if they are positive and anions if

they are negative

Called the “Octet Rule”

-gaining/losing enough e- to have a full

valence

Atomic Number

Top Number indicates Atomic Number

Atomic Number = Number of Protons Hydrogen:

Atomic Number 1 = 1 proton Magnesium:

Atomic Number 12 = 12 protons Lead:

Atomic Number 82 = 82 Protons IF YOU CHANGE THE

NUMBER OF PROTONS OF AN ATOM, YOU CHANGE ITS IDENTITY!!!!!!!!

1

H12

Mg82

Pb

Atomic Mass Bottom Number indicates Atomic

Mass Atomic Mass = Total Mass (Number of

Protons + Number of Neutrons) Hydrogen:

1 (Protons + Neutrons) Magnesium:

24 (Protons + Neutrons) Lead:

207 (Protons + Neutrons) If we take the Atomic Mass and

subtract the Atomic Number, we can figure out the number of neutrons.

Pb – (Atomic Mass) 207 – Atomic Number) 82 = 125

H1

Mg24

Pb207

Atomic Structure (cont.)

Atomic mass Average mass of an element,

based on amount of each isotope found in nature

Not a whole number because it is an average

Atomic number Number of protons in an

element Also, number of electrons

when it is neutral (has no charge)

Mass number Mass of a particular isotope

7

N14.011

Determining Composition of atoms

Number of neutrons = mass number – atomic number

# of protons = atomic number If neutral atom… # protons = # electrons

Charge is positive? That # fewer electrons than protons

Charge is negative? That # more electrons than protons


For a lithium atom: What is the

atomic number? How many

protons? How many electrons since it is neutral?

What is the atomic weight?

How many neutrons?

3

3

3

6.939

Usually 4

Who was the scientist that came up with the planetary model of the

atom?


Practice Problem #1 For a sodium atom:

What is the atomic number?

How many protons? How many electrons

since it is neutral? What is the atomic

weight? How many neutrons?

11

11

11

22.99Usually 12


Practice Problem #2 For a boron atom:

What is the atomic number?

How many protons? How many electrons

since it is neutral? What is the atomic

weight? How many neutrons?

55

5

10.811

Often 6

Is a tool to organize the elements

By 1860, scientists had discovered 60 elements

They noticed some elements had similar properties.

They also noticed differences between the elements.

1829 Classified elements

into groups of 3 Called them triads.

The elements in a triad had similar chemical properties

Physical properties varied in an orderly way according to their atomic mass

1869 Developed a

Periodic Table based on atomic mass

He left blank spaces

Dmitri Mendeleev (Wikipedia.org)

Realized chemical + physical properties of elements repeated in an orderly way.

Periodicity- the tendency to recur at regular intervals

Luster Conductive Malleable (can be

bent and formed into shapes)

Ductile (can be pulled into wires)

Dull Nonconductive Brittle

(Shaded regions)

Share properties of metals and non-metals

Some are semiconductors

I study metalloids

Columns and Rows

Called “groups” or “families” Called “periods”

An atom can have up to 7 energy levels of electrons.

An element’s period (row) tells us the number of …? For example, a sodium (Na) atom has ____ electron

orbitals? Fluorine (F) has ____ electron orbitals?

An element’s family (aka group) tells us ...? The outer 2 shells of the Group B elements are

considered valence electron orbits. We will be able to ignore Group B for now.

For example Sodium (Na) has ____ valence electrons Fluorine (F) has ____ valence electrons

Group 1A Only one

valence electron

VERY reactive!!!!

Hydrogen is NOT included

Group 2A Two valence electrons Not as reactive as the alkali metals Named because of where they are

found on Earth

Found in the middle of the table

In the “B Groups” Can change their

number of valence electrons

Bottom Rows are also known as the Rare Earth Metals!!!

Group 7A Seven valence

electrons VERY reactive!!!!

Group 8A 8 valence

electrons—outer energy level is full

Very UNREACTIVE—what do they need to be

For carbon (C):(a) How many electron shells does it have?(b) How many valence electrons does it have?(c) Is it a metal, nonmetal, or metalloid?

Answer for carbon (C):(a) 2 electron shells(b) 4 valence electrons(c) nonmetal

For potassium (K):(a) How many electron shells does it have?(b) How many valence electrons does it have?(c) Is it a metal, nonmetal, or metalloid?

Answer for potassium (K):(a) 4 electron shells(b) 1 valence electron(c) metal

For copper (Cu):(a) How many electron shells does it have?(b) Is it a metal, nonmetal, or metalloid?(c) How many protons does it have?

Answer for copper (Cu):(a) 4 electron shells(b) metal(c) 29 protons

For uranium (U):(a) How many electron shells does it have?(b) Is it a metal, nonmetal, or metalloid?(c) How many protons does it have?

Answer for uranium (U):(a) 7 electron shells(b) metal(c) 92 protons

Radioactivity is the process of nuclear decay, in which an unstable nucleus gives off matter and energy.

Nuclei with too many or too few neutrons compared to the number of protons are radioactive.

The three types of nuclear radiation are alpha, beta, and gamma radiation

When alpha radiation occurs, an alpha particlemade of two protons and two neutrons is emitted from the decaying nucleus

• A second type of radioactive decay is called beta decay.

Sometimes in an unstable nucleus a neutron decays into a proton and emits an electron.

Because the atom now has one more proton, it becomes the element with an atomic number one greater than that of the original element.

However, because the total number of protons and neutrons does not change during beta decay, the mass number of the new element is the same as that of the original element.

They have no mass and no charge and travel at the speed of light.

Isotopes Same element, different number of

neutrons Ex. Uranium-235 & Uranium-237 Also written as

UU 23792

23592 &

Some isotopes are radioactive, while

others are not.

_________ and _____________ are always conserved due to the Law of Conservation of Mass.

Mass # atomic #

RaTh 228232

RnHe 22086

42

147

24195

He4290 88

Ra22488

Am N Ra22788

Alpha decay of U-238

Beta decay of Th-235

Gamma decay of Th-235

ThHeU 23490

42

23892

PaeTh 23591

01

23590

00

23590

23590 ThThm

Alpha decay emits a Helium atom

Beta decay decomposes a

neutron to a proton and emits an electron

Gamma decay causes rearrangement of the

nucleus and emits gamma radiation

Alpha, beta, and gamma rays: particles or waves emitted during

radioactive decay

Form of radiation

Symbol Charge Penetration

Alpha

Beta

Gamma

+

-No charge

Least

Middle

Most

α

β

γ

Most damaging radiation is… Alpha, if it gets in the body

If alpha radiation gets in the

body it does NOT

leave!

People are exposed to radiation Naturally

Cosmic rays, radioisotopes from rocks, soil, groundwater

People are exposed to radiation Human sources

Nuclear weapons testing, air travel, nuclear power, X-rays, medical treatments with radiation

Some smoke detectors give off alpha particles that ionize the surrounding air.

Radon is…

A noble gas that decays into radioactive materials

Radon in homes is a concern because… It seeps in through cracks in foundations It stays trapped in air-tight homes It decays into heavy metals that emit alpha

particles

10-14% of U.S. deaths from lung cancer attributed

to radon

Radon levels in the U.S. Zone 1

> 4 pCi/L Zone 2

2-4 pCi/L Zone 3

2 pCi/L

12% of lung cancer deaths are caused by

Radon

2 ways of using radioisotopes in medicine To locate

problems To kill cancer

cells

Tracers are… Radioisotopes injected in

people for diagnostic purposes.

They are injected and then traced as they travel through the body.

Irradiation Kills cancer cells It is applied directly to a cancerous

location on the body

Irradiation is… Exposure to

radiation of any kind

Usually refers to food irradiation

The FDA requires that all irradiated foods be

labeled with the above symbol, the radura

Irradiate strawberries, poultry, potatoes, etc. Reduces spoiling Kills bacteria and

parasites that lead to foodborne illnesses

Half-life is… The time

it takes for half the atoms to decay

Hydrogen-3 has a half-life of 12 years…start with 1,000 atoms After 12 years

There will be 500 atoms After 24 years

There will be 250 atoms After 48 years

There will be 62.5 atoms

7,000 atoms of Radon-222, half life of 3.28 days, in 6.56 days there are how many atoms? 1,750 atoms

100,000 atoms of Uranium-238, half life of 4.5 billion years, in 13.5 billion years there are how many atoms? 12,500 atoms U238

92

Rn22286

60,000 atoms of Potassium-40, half life of 1,280,000,000 years, how long until 7,500 atoms left? 3,840,000,000 years

Radon-222, half life of 3.28 days, how long until 12.5% of initial sample? 9.84 days

K4019

Rn22286

500 atoms of Radon-222, half life of 3.28 days, it has been 19.68 days since the experiment began, how much did you begin with? 32,000 atoms

Carbon-14, half life of 5,730 years, what fraction remains after 11,460 years? ¼

Rn22286

C146

Uranium powers our nuclear power plants. After the fuel is spent, we are left with radioactive waste.

High-level radioactive waste with U-238 is a problem because…

Uranium’s half life is 4.5 billion years, it will be

around for a long time.

Nuclear fission: The process of splitting a nucleus into several smaller nuclei.

Nuclear fusion Combining of two nuclei to form a new,

heavier nucleus

Fusion is much more

powerful than Fission

Documents

The Atom and Periodic Table General Chemistry: Unit 3 Fall 2010