# Unit 02 Atomic Structure.

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Unit 02 Atomic 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 -
- a pure substance made up of one type of atom. - organized on periodic table - each element has a unique number of protons…its atomic number

Atomic Structure: Atoms contain three subatomic particles… 1. Protons…
2. Neutrons… 3. Electrons… positive charge These are located in NUCLEUS! neutral charge negative charge Electrons surround the nucleus in orbitals

Atomic Structure An atom is considered electrically neutral. Electrically neutral means the number of protons (+) = the number of electrons (-) 4 red protons = 4 blue electrons

Properties of Subatomic Particles
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 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

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

Discovery of the Atom Purpose: 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

J.J. Thomson “Plum pudding” atom negatively charged e- stuck into a lump of positively charged material – similar to chocolate chip cookies

Ernest Rutherford In Rutherford’s gold foil experiment he discovered electrons surround a dense positive nucleus “The Blow Pop”

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 B 11 5

mass # = protons + neutrons
A. Mass Number mass # = protons + neutrons always a whole number NOT on the Periodic Table! © Addison-Wesley Publishing Company, Inc.

B. Isotopes Mass # Atomic #
Atoms of the same element with different mass numbers. (different number of neutrons) Nuclear symbol: Mass # Atomic # Hyphen notation: carbon-12

B. Isotopes

You must know how to find:
C. Nuclear Symbols You must know how to find: # of protons = atomic number mass # = # of n0 + # of p+ (atomic #) What’s in the nucleus of the atom # of electrons = # of protons (in a neutral atom) Boron 5 B 10.811 atomic number (Not the same as the mass #) (average) atomic mass

How to write a Nuclear Symbol
Charge if ion Element Symbol B Mass Number = p+ + n0 -3 11 5 Atomic Number = p+

C. Nuclear Symbols Chlorine-37 atomic #: mass #: # of protons:
# of electrons: # of neutrons: 17 37 20

Cl Mg Nuclear Symbol Examples 35 17 27 12 17 35 17 17 18 12 27 12 12
Number of Protons Number of Electrons Number of Neutrons Atomic Number Mass Number 17 35 17 17 18 Mg 27 12 Number of Protons Number of Electrons Number of Neutrons Atomic Number Mass Number 12 27 12 12 15

D. Relative Atomic Mass 12C atom = 1.992 × 10-23 g
atomic mass unit (amu) 1 amu = 1/12 the mass of a 12C atom © Addison-Wesley Publishing Company, Inc. 1 p = amu 1 n = amu 1 e- = amu

E. Average Atomic Mass weighted average of all naturally occuring isotopes on the Periodic Table round to 2 decimal places Avg. Atomic Mass

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. Avg. Atomic Mass 16.00 amu

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. Avg. Atomic Mass 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 = average atomic mass

II. The Periodic Table 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 31

A. History of the Periodic Table
Henry Moseley (1913) - developed the modern periodic table - arranged in order of increasing atomic number 32

B. Organization of the Periodic Table
horizontal rows numbered 1 - 7 Energies of outermost electrons are similar 33

B. Organization of the Periodic Table
Groups/ Families vertical columns have similar chemical & physical properties 34

Group 1 (IA) – Alkali Metals
35

Group 2 (IIA) – Alkaline Earth Metals
36

Group 18 (VIIIA) – Noble Gases
37

Group 17 (VIIA) - Halogens
38

B. Organization of the Periodic Table
Metals- Nonmetals- Metalloids- Left of stair step Right of stair step On the stair step NONMETALS METALLOIDS METALS

REPRESENTATIVE “Group A”
B. Organization of the Periodic Table Group A- Representative Group B - Transition REPRESENTATIVE “Group A” TRANSITION “Group B” INNER TRANSITION

The Extended Periodic Table
41