Unit 02Atomic Structure

Just How Small is an Atom?You don’t need to write.

• A speck 0.1 mm in diameter (about half the size of a period at the end of the sentence) requires one million atoms.

• It would require a million atoms, edge to edge, to match the thickness of a page of paper.

Can you see an atom?• Technically, you cannot "see" anything smaller than the shortest

wavelength of light that you can see it with. • But there are ways to "visualize" it, like Atomic Force Microscopy.

But these are all just measurements converted to computer images, and are not in any real sense "seeing" the atom.

• You can't see atoms in any normal sense of using an optical microscope.

• You don't get an optical image, but it does allow you to map out an image of the atoms of a molecule. To do this you use a metallic tip which interacts with the atoms you want to image. As you move the tip over the atoms, you pass a current, called a tunneling current, between the tip and the atom. This current is extremely sensitive to the distance between the atom and the tip.

- REMEMBER FROM: Elements, Mixtures, and Compounds -

Element - a pure substance made up of one

type of atom.

- organized on periodic table- each element has a unique number of protonsprotons…its atomic number

Atomic Structure:Atoms contain three subatomic particles…

1. Protons…2. Neutrons…3. Electrons…

These are located in NUCLEUS!

Electrons surround the nucleus in

orbitals

positive chargeneutral chargenegative charge

Atomic Structure

An atom is considered electrically neutral.Electrically neutral means the number of

protons (+) = the number of electrons (-)

4 red protons4 red protons = = 4 blue electrons4 blue electrons

Properties of Subatomic Particles

Particle Symbol

Relative

electrical

charge

Relative mass Actual mass (g)

Electron e- 1- 1/1840 9.11 × 10-28

Proton p+ 1+ 1 1.67 × 10-24

Neutron n0 0 1 1.67 × 10-24

Protons and Neutrons have the same mass.

A. Discovery of the Atom Ernest Rutherford discovered the nucleus by shooting

alpha particles (have a positive charge) at a very thin piece of gold foil.

He predicted that the particles would go right through the foil at some small angle.

Rutherford’s Gold Foil Experiment

• some particles (1/8000) bounced back from the foil

• this meant there must be a “powerful force” in the foil to hit particle back

Predicted Results Actual Results

Rutherford’s Gold Foil Experiment

Discovery of the AtomPurpose:

The students will find the shape of different items and relate this to the early scientist that made discoveries about the shape and size of the atom.

Procedure: 1. Title the left side of your spiral Discovery of the Atom. 2. For each item you will write the letters then draw your predicted shape of the item.3. Then you will write 1 sentence describing why they think your prediction is the shape of the item.

A: Item in brown bag – Use your hands to feel the shape of the item. B: Item in clay – Using the toothpicks provided find the shape of the object

enclosed in the modeling clay. C: Black box – Maneuver the black box with a marble inside to discover the

shape of the object enclosed.

B. Models of the Atom

“Plum pudding” atom negatively charged e- stuck into a lump of positively charged material

– similar to chocolate chip cookies

J.J. Thomson

“The Blow Pop”

Ernest Rutherford • In Rutherford’s gold foil experiment he

discovered electrons surround a dense positive nucleus

Bohr Model • electrons are arranged in fixed orbits around

the nucleus.

ex. Orbits gum

Quantum Mechanical Model

• Quantum mechanics was developed by Erwin Schrodinger

• Estimates the probability of finding an e- in a certain position

• Electrons are found in an “electron cloud”

I. Nuclear Symbols

B11

5

A. Mass Number• mass # = protons + neutrons

always a whole number

NOT on the Periodic Table!

© Addison-Wesley Publishing Company, Inc.

B. Isotopes• Atoms of the same element with

different mass numbers. (different number of neutrons)

C126Mass #

Atomic #

Nuclear symbol:

Hyphen notation: carbon-12

B. Isotopes

You must know how to find:A. # of protons = atomic numberB. mass # = # of n0 + # of p+ (atomic #)

– What’s in the nucleus of the atom

C. # of electrons = # of protons (in a neutral atom)

Boron

5

B10.811

atomic number

(average) atomic mass(Not the same as the mass #)

C. Nuclear Symbols

How to write a Nuclear Symbol

B11

5

Mass Number

= p+ + n0

Atomic Number

= p+

Element Symbol

-3

Charge if ion

C. Nuclear Symbols

• Chlorine-37

–atomic #:

–mass #:

–# of protons:

–# of electrons:

–# of neutrons:

17

37

17

17

20

Cl3717

Mg12

27

Nuclear Symbol Examples

Atomic Number Mass NumberNumber

of ProtonsNumber

of Neutrons

Number of

Electrons

12 1227 1512

Cl17

35

Atomic Number Mass NumberNumber

of ProtonsNumber

of Neutrons

Number of

Electrons

17 1735 1817

D. Relative Atomic Mass

• 12C atom = 1.992 × 10-23 g

1 p = 1.007276 amu

1 n = 1.008665 amu

1 e- = 0.0005486 amu

© Addison-Wesley Publishing Company, Inc.

atomic mass unit (amu)

1 amu = 1/12 the mass of a 12C atom

E. Average Atomic Mass

• weighted average of all naturally occuring isotopes

• on the Periodic Table• round to 2 decimal places

100

(%)(mass(mass)(%) )

Avg.AtomicMass

Avg.AtomicMass

E. Average Atomic Mass

• EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O.

100

(18)(0.20)(17)(0.04))(16)(99.76 16.00amu

Avg.AtomicMass

E. Average Atomic Mass

• EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37.

10

(37)(2)(35)(8)35.40 amu

Example: A sample of cesium is 75% 133Cs, 20% 132Cs and 5% 134Cs. What is the average atomic mass?

Answer:.75 x 133 = 99.75.20 x 132 = 26.4.05 x 134 = 6.7

132.85 = average atomic mass

II. The Periodic Table

Periodic Law Periodic Law – properties of elements can be predicted by their position on

the periodic table

A. History of the Periodic Table

• Dmitri Mendeleev (1871)

– Developed the first periodic table– It was arranged by atomic mass because atomic

number had not been discovered– He was able to predict properties of elements

A. History of the Periodic Table

• Henry Moseley (1913)

- developed the modern periodic table

- arranged in order of increasing atomic number

B. Organization of the Periodic Table• Period

– horizontal rows numbered 1 - 7– Energies of outermost electrons are similar

• Groups/ Families– vertical columns– have similar chemical & physical properties

B. Organization of the Periodic Table

Group 1 (IA) – Alkali Metals

Group 2 (IIA) – Alkaline Earth Metals

Group 18 (VIIIA) – Noble Gases

Group 17 (VIIA) - Halogens

Left of stair step

On the stair step

Right of stair step

METALS

NONMETALS

Metals- Nonmetals

-Metalloids-

B. Organization of the Periodic Table

METALLOIDS

TRANSITION “Group B”

INNER TRANSITION

REPRESENTATIVE“Group A”

Group A- RepresentativeGroup B - Transition

B. Organization of the Periodic Table

The Extended Periodic Table