History of the Atom

History of the Atom

Democritus 

All material bodies are made up of indivisibly small particles called “atomos.”

Empedocles

-Greek Philosopher
-All substances are made of earth, fire, water, and air
-Supported by Aristotle

Law of Conservation of Matter/Mass

• Matter (Mass) is neither created or destroyed during ordinary chemical reactions or physical changes

• Antoine Lavoisier (1782)

• In a sealed container: 2.0 g of hydrogen always reacted with 16.0 g of oxygen to give 18.0 g of water

Fill in the Blank

Sodium chloride can be formed by the reaction of sodium metal and chlorine gas. Using the law of conservation of matter/mass, if 45.98 g of sodium combines with chlorine gas to form 110.89 g of

sodium chloride, what mass of chlorine gas was used in the reaction?

Law of Definite Proportions

• Elements that compose a compound are always the same proportion by mass

• Joseph Proust (1799) - Studied composition of many compounds

• Regardless of the source, water is always 11% H and 89% O by mass

John Dalton

• Started teaching at the Quaker school (he also went to) in his village in Cumberland, when he was just 12 years old.

• Meteorologist & Studied Color Blindness

• Atomic Theory in early 1800's

Dalton's Atomic Theory

1. Each element is composed of extremely small particles called atoms 

2. All atoms of a given element are identical, but atoms from different elements are different 

3. Atoms cannot be subdivided; atoms are neither created nor destroyed in chemical reactions 

4. Atoms combine in whole-number ratios to form compounds 

5. Atoms are separated, combined, or rearranged in chemical reactions

Open Ended

Scientists have since found 2 parts of Dalton's Atomic Theory incorrect. What is at least one of them?

Ernest Rutherford

• 4th child out of 12 children 

• worked under J.J. Thomson at Cambridge

• His associates for the GOLD FOIL Experiment were Hans Geiger and Ernest Marsden

Multiple Choice

Which scientist developed an instrument that can detect radioactivity?

• Ernest Rutherford

• Ernest Marsden

• Hans Geiger

• JJ Thomson

Gold Foil Experiment

• A piece of thin gold foil was bombarded by fast moving alpha particles (positively charged radioactive particles)

• Most of the particles went straight through....

• Conclusion: Atoms are mostly composed of empty space.

• A small number of the particles were deflected and redirected...

• Conclusion: The particles are influenced by something that is small, dense, positively charged .....nucleus 

Planetary Model Of an Atom

The nucleus is at the center of the atom and the electrons surround the nucleus.  Similar to how the planets surround the sun.  

Open Ended

State the 3 basic subatomic particles that were discovered.

Atomic Math

• Atomic Number = number of protons    

• This number identifies the element 

  How many protons does Carbon have?

• Atoms are electrically neutral 

• Number of protons = Number of electrons

Fill in the Blank

Identify the element whose atoms contain only 7 electrons.

Isotopes

• Atoms of the same element that have different masses (more neutrons)

• Mass Number = # protons + # neutrons

• Different for each type of isotope for an element

• NOT the same as the mass of the element on the periodic table

• # neutrons = mass number – atomic number

Multiple Choice

1. An aluminum isotope consists of 13 protons, 13 electrons, and 14 neutrons. Its mass number is

• 13

• 14

• 27

• 40

Representing Isotopes:

Hyphen Notation

• Carbon-12, Carbon-13, Carbon-14

• Write the name of the element – (hyphen) mass number

Multiple Choice

Determine the number of protons, neutrons, and electrons in tin-120.

• 50 protons, 50 electrons, and 70 neutrons

• 70 electrons, 50 protons, 50 neutrons

• 120 neutrons, 50 protons, and 70 electrons

• 70 neutrons, 70 protons, and 50 electrons

Representing Isotopes: Nuclear Symbol Notation

• Use the symbol of the element, superscript on the left is the mass number, and subscript is the atomic number

Multiple Choice

Which of the following atoms contains the greatest number of neutrons

• $$_{15}^{31}P$$  

• $$_{14}^{28}Si$$  

• $$_{16}^{36}S$$  

• $$_{17}^{35}Cl$$  

Relative Atomic Mass

• Relative Atomic Mass Oxygen-16: actual mass of 2.657 x 10^ -23  kg

• RELATIVE ATOMIC MASS: Mass of an isotope based on a standard isotope with a given mass in amu

• Standard Isotope: Carbon-12: Given the relative mass of 12 amu

• 1 Atomic Mass Unit (AMU) is the approximate mass of a proton or neutron

Fill in the Blank

What would be the sum if all of the percent abundances are added together?

Open Ended

1. An element consists of two isotopes. Isotope 1 has a mass of 190.9606 amu and has a percent abundance of 37.30%. Isotope 2 has a mass of 192.9629 amu and has a percent abundance of 62.70%.

a. What is the average atomic mass of the element?

b. Identify the element

Open Ended

Chlorine has two naturally occurring isotopes. If 24.22% of chlorine is found as chlorine - 37 with an atomic mass of 36.9659 amu, what is the mass of the other isotope?