atoms and isotopes

Atoms & Isotopes: 

Atoms & Isotopes Refining Our Understanding of Atomic Theory

What We Know About Atoms: 

What We Know About Atoms Atomic Theory Through Bohr Atoms are the smallest operational unit of matter Atoms are nuclear Protons and neutrons in a small, dense nucleus Electrons outside in orbitals or shells Elements differ from each other by the number of protons in their nucleus Same number of electrons in orbitals

Some Examples of Atoms: 

Some Examples of Atoms Elements With One and Two Protons One proton: hydrogen atom Hydrogen has one electron also No problem with nucleus holding together The Hindenburg disaster

Some Examples of Atoms: 

Some Examples of Atoms Elements With One and Two Protons Two protons: Helium atom Helium has two electrons also Chemical properties different from H No known chemical reactions But with two protons, charges repel inside nucleus More on this in a bit Photo: Lockheed-Martin

More Elements: 

More Elements Properties Determined by Number of Electrons Lithium - 3 protons; 3 electrons Beryllium - 4 protons; 4 electrons Boron - 5 protons; 5 electrons Carbon - 6 protons; 6 electrons Nitrogen - 7 protons; 7 electrons Oxygen - 8 protons; 8 electrons Fluorine - 9 protons; 9 electrons Neon - 10 protons; 10 electrons

Sample Test Questions: 

Sample Test Questions What element is characterized by 7 electrons? Answer: nitrogen What element has 3 protons? Answer: Lithium Principle: The number of protons in the nucleus is called the atomic number

The Rest of the Story: 

The Rest of the Story Where’s the rest of the atomic mass? Clearly, a complex interaction of forces keeps the nucleus together Electrostatic force drives protons apart Neutrons function to keep the positive electricity together Idea for extra credit reading & report: How?

The Case of Helium: 

The Case of Helium Keeping 2 protons from blowing up Helium atoms always contain at least one neutron The most common kind of helium contains two neutrons Thus the atomic mass of He is 4 amu (Oh, and alpha particle is a He nucleus)

See Table 5.2: 

See Table 5.2 Common number of neutrons in each element Name Symbol Atomic # Protons Neutrons Mass # Electrons Hydrogen H 1 1 0 1 1 Helium He 2 2 2 4 2 Lithium Li 3 3 4 7 3 Beryllium Be 4 4 5 9 4 Boron B 5 5 6 11 5 Carbon C 6 6 6 12 6 Nitrogen N 7 7 7 14 7 Oxygen O 8 8 8 16 8 Fluorine F 9 9 10 19 9 Neon Ne 10 10 10 20 10

See Table 5.2: 

See Table 5.2 Common number of neutrons in each element # protons + # neutrons = Mass Number Mass Number - Atomic Number = # neutrons Not all nuclei are the same They differ by # neutrons Called isotopes of the element Same chemistry; different mass

Isotopes of Carbon: 

Isotopes of Carbon Three natural carbon isotopes Mass number 12: most common Mass number 13: rare but stable Mass number 14: rare & radioactive Written: 12 C 13 C 14 C

“Hyphen” notation: 

“Hyphen” notation Three natural carbon isotopes Mass number 12 is C-12 Mass number 13 is C-13 Mass number 14 is C-14 This notation can be used for any isotope of any element

Isotopes do not have the same abundance: 

Isotopes do not have the same abundance Suppose we had two kinds of carbon, C-12 and C-13. If their abundance was the same, the measured atomic mass of C would be 12.5 If the abundance of C-12 was 75% and of C-13 25%, then we would calculate the atomic mass of carbon in nature as follows:

Isotopes do not have the same abundance: 

Isotopes do not have the same abundance If the abundance of C-12 was 75% and of C-13 25%, then we would calculate the atomic mass of carbon in nature as follows M = (12*.75) + (13 *.25) = 12.25 So the general rule for calculating atomic mass in nature is: M = (AM1 * Ab1) + (AM2 * Ab2) + ... Thus, there are two isotopes of chlorine, Cl-35, which is 76% abundant, and Cl-37, which is 24% abundant. Solve.

Chlorine problem: 

Chlorine problem Thus, there are two isotopes of chlorine, Cl-35, which is 76% abundant, and Cl-37, which is 24% abundant. M = (35*.76) + (37*.24) = 35.5 Which is about what we see on the periodic table