AP Chemistry Chapter 3 The Structure of the Atom
Democritus was the early (around 400BC) Greek philosopher who is credited with the concept of the atom (atomos) –which means invisible
Dalton(around 1800AD) is an English school teacher who proposed the law of conservation of mass, the law of definite proportions, and the law of multiple proportions. His many experiments with gases proved these laws are true, if atoms exist. Dalton is also known as the Father of the (Modern) Atomic Theory
Dalton’s atomic theory: • All matter is composed of very small particles called atoms • Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in these properties.
Atoms cannot be subdivided, created, or destroyed • 4.Atoms of different elements combine in simple whole-number ratios to form chemical compounds. • 5.In chemical reactions, atoms are combined, separated, or rearranged.
Two aspects of Dalton’s atomic theory proven to be incorrect: • We now know atoms are divisible. • b. Atoms of the same element can have different masses.
J. J. Thomson is the man credited with the discovery of the electrons in the late 1800’s, using cathode ray tubes
Knowledge of electrons led to two inferences about atomic structure: 1.Because atoms are electrically neutral, they must contain positive charge to balance the negative electrons. 2. Because electrons have so little mass, atoms must contain other particles to account for most of their mass
Nucleus of the atom—discovered by Lord Ernest Rutherford Gold foil experiment—actually done by Hans Geiger and Ernest Marsden
Observations: • Majority of the alpha (α)particles penetrated foil undeflected. • b.About 1 in 20,000 were slightly deflected • c.About 1 in 20,000 were deflected back to emitter
Conclusions: 1. Mass of the atom and the positive charge are concentrated in small regions called nucleus • Most of the atom is empty • 3.Magnitude of charge on the nucleus is different for different atoms
4. Number of electrons outside the nucleus = number of units of nuclear charge (to account for the fact that the atom is electrically neutral) Atoms are electrically neutral because they contain equal numbers of protons and electrons
A couple years later Rutherford presented evidence for a neutral particle which was also in the nucleus and contained a similar mass to that of a proton – called a neutron
Mass of one proton = mass of neutron = mass of 1837 electrons Thus the total mass of an atom is basically the sum of the protons and neutrons, called the atomic mass or mass number, abbreviated A
Atomic number—the number of protons in the nucleus of the atom. --number of protonsidentifies the element and is equal to the number of electrons (of a neutral atom) --symbol is Z
Isotopes are atoms of the same element that have different masses because they have different numbers of neutrons but they still have similar chemical properties
Nuclide is the general term for any isotope of any element Atomic Mass Unit (amu) is exactly 1/12 the mass of a carbon-12 atom
Average atomic mass is the weight average of the atomic masses of the naturally occurring isotopes of an element. Ave. Atomic mass = %abundace(mass of isotope 1) + %abundance(mass of isotope 2) +…..
Example 1: Element Sciencium has two isotopes. Sciencium-301 has an abundance of 59.5%, and Sciencium-304 is the other. What is the average atomic mass? 301amu x .595 = 179 amu 304amu x .405 = 123 amu 302 amu
Example 2: Element Pepsium has an average atomic mass of 335. Two isotopes of Pepsium exist. If Pepsium-327 is 30.5% abundant, then what is the second isotope? 327amu x 0.305 = 99.7 amu ?amu x 0.695 = ? amu 335 amu 1 – 0.305 = 0.695
Example 2: Element Pepsium has an average atomic mass of 335. Two isotopes of Pepsium exist. If Pepsium-327 is 30.5% abundant, then what is the second isotope? 327amu x 0.305 = 99.7 amu ?amu x 0.695 = ? amu 335 amu Step 1: Find the missing weighted mass
Example 2: Element Pepsium has an average atomic mass of 335. Two isotopes of Pepsium exist. If Pepsium-327 is 30.5% abundant, then what is the second isotope? 327amu x 0.305 = 99.7 amu ?amu x 0.695 = 235amu 335 amu 335 – 99.7 = 235.3
Example 2: Element Pepsium has an average atomic mass of 335. Two isotopes of Pepsium exist. If Pepsium-327 is 30.5% abundant, then what is the second isotope? 327amu x 0.305 = 99.7 amu ?amu x 0.695 = 235amu 335 amu Step 2: Find the missing mass
Example 2: Element Pepsium has an average atomic mass of 335. Two isotopes of Pepsium exist. If Pepsium-327 is 30.5% abundant, then what is the second isotope? 327amu x 0.305 = 99.7 amu 338amu x 0.695 = 235amu 335 amu 235 ÷ 0.695 = 338
Mass Spectrometry How we know isotopes.
A mass spectrometer has three parts: 1. ionizer 2. magnetic field 3. detector And might look like this
If a sample of a pure element is placed in the spectrometer, then all the ions formed will have the same charge. For example Cl → Cl- Cl Cl- Ionizer
The ions then pass through a magnetic field that will change their paths. Which will change direction more, something heavy or something light? - Cl Cl- Cl- + Magnetic Field Detector
The locations tell the masses – one group of Cl had a mass of 35 amu’s and the other had a mass of 37 amu’s.
As the bar for 35Cl is 3 times bigger than 37Cl (measuring with a ruler) thus 35Cl is about 75% abundant and 37Cl is about 25% abundant.
1) When referring to nuclear reactions people commonly think of nuclear fission (the splitting of large atoms into smaller pieces)
1) and nuclear fusion (the combining of small atoms into one large one), but on earth these reactions do not occur naturally.
2) Naturally occurring nuclear reactions result from the unusual number of neutrons of an isotope which makes it unstable (unusually high in energy). This often results in the isotope changing from one element into another element in an attempt to become more stable (lower in energy).
These reactions are called nuclear reactions, as they involve changes in the nucleus. • During these nuclear reactions, rays and particles are given off, which is called radiation.
C) Sometimes an unstable nucleus will change into many different elements as it tries to become more stable. This is called radioactive decay.
3) When radioactive decay occurs, there are three different types of radiation that can be given off. Each type has a different mass, and sometimes a charge.
A) The first type of radiation to be discovered was called alpha radiation and came from alpha particles.
Because the mass numbers must be equal, • 226 = 222 + x. So the mass of the alpha particle must be 4.
Because the atomic numbers must be equal, • 88 = 86 + x. So the atomic number of the alpha particle must be 2.
iii) The element Helium has a mass of 4 and an atomic number of 2, so the alpha particle is just like a helium atom without any electrons;
B) The second type of radiation to be discovered was called beta radiation and came from beta particles.