PreAP Chemistry Chapter 4 Chapter 4 Annotation Questions Due to the box NOW! Late is only worth 60%, but is better than 0%!

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: 1.All matter is composed of very small particles called atoms

Dalton’s atomic theory: 1.All matter is composed of very small particles called atoms 2.Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in these properties.

3.Atoms cannot be subdivided, created, or destroyed

4.Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

3.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: a.We now know atoms are divisible.

Two aspects of Dalton’s atomic theory proven to be incorrect: a.We now know atoms are divisible. b. Atoms of the same element can have different masses.

Atom --smallest particle of an element that retains the properties of that element.

J. J. Thomson is the man credited with the discovery of the electrons in the late 1800’s, using cathode ray tubes

Millikan calculated the mass of the electron (very, very small)

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.

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

What would this look like?

Nucleus of the atom—discovered by Lord Ernest Rutherford

Gold foil experiment—actually done by Hans Geiger and Ernest Marsden

Observations: a.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

2. Most of the atom is empty

Conclusions: 1. Mass of the atom and the positive charge are concentrated in small regions called nucleus 2.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 protons identifies 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

Mass Number of Isotope Number of ProtonsNumber of Neutrons Isotopes of Carbon

Mass Number of Carbon Isotopes Name of Isotopes 8 carbon-8 9 carbon-9 10 carbon carbon carbon carbon carbon carbon carbon carbon carbon carbon carbon-20

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 = %abundance(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? 301 amu x.595 = 179 amu 304 amu 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? 327 amu x = 99.7 amu ? amu x = ? amu 335 amu 1 – = 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? 327 amu x = 99.7 amu ? amu x = ? 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? 327 amu x = 99.7 amu ? amu x = 235 amu 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? 327 amu x = 99.7 amu ? amu x = 235 amu 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? 327 amu x = 99.7 amu 338 amu x = 235 amu 335 amu 235 ÷ = 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 - ClCl - 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 - Magnetic Field - + Cl - Detector

The computer attached to the detector gives a readout like this:

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.

The size of the bars indicate the relative amounts of each isotope.

As the bar for 35 Cl is 3 times bigger than 37 Cl (measuring with a ruler) 35 Cl is about 75% abundant and 37 Cl is about 25% abundant.

Unstable Nuclei and Radioactive Decay

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).

A) These reactions are called nuclear reactions, as they involve changes in the nucleus.

B)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.

i)Because the mass numbers must be equal, 226 = x. So the mass of the alpha particle must be

i)Because the mass numbers must be equal, 226 = x. So the mass of the alpha particle must be 4.

ii)Because the atomic numbers must be equal, 88 = 86 + x. So the atomic number of the alpha particle must be.

ii)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; 4 Heor 4  2 2

B) The second type of radiation to be discovered was called beta radiation and came from beta particles.

i) Because the mass numbers must be equal, 14 = 14 + x. So the mass of the beta is

i) Because the mass numbers must be equal, 14 = 14 + x. So the mass of the beta is zero.

ii) Because the atomic numbers must be equal, 6 = 7 + x. So the atomic number must be

ii) Because the atomic numbers must be equal, 6 = 7 + x. So the atomic number must be -1.

iii) The electron has a mass of zero and a charge of -1, so the beta particle is just like an electron; 0 β

C) The last type of radiation to be discovered was called gamma radiation and came from gamma particles.

i)Because the mass numbers must be equal, 238 = x. So the mass of the gamma particle must be

i)Because the mass numbers must be equal, 238 = x. So the mass of the gamma particle must be zero.

ii) Because the atomic numbers must be equal, 92 = x. So the atomic number must also be zero.

iii) The gamma particle was the last to be found because it has no mass and no charge; 0 0 γ

Geiger Counter

Alpha, Beta, and Gamma Radiation