Atoms: The Building Blocks of Matter

The particle theory of matter was supported as early as 400 BC by the Greeks (Democritus) –He called these particles atoms (Greek for indivisible) Aristotle followed Democritus and felt that matter was continuous

Foundations of Atomic Theory Several theories proposed in the late 1700’s and early 1800’s Law of conservation of mass - mass is neither created nor destroyed during ordinary chemical reactions

Law of definite proportions – a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound Law of multiple proportions – If two or more compounds are composed of the same two elements, then the ratio of the masses of the elements is a ratio of small whole numbers

Dalton’s Atomic Theory All matter is composed of extremely small particles called atoms Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties

Atoms cannot be subdivided, created, or destroyed Atoms of different elements combine in simple whole-number ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged

Modern Atomic Theory Today we know that atoms themselves are divisible We also know that individual atoms can have different masses (isotopes) In general however, Dalton’s original atomic theory still holds

The Structure of the Atom The atom is defined as the smallest particle of an element that retains the chemical properties of that element Atoms consist of the nucleus (core – protons and neutrons) and the electrons traveling around the nucleus

The Structure of the Atom The first subatomic particle discovered was the electron (mid 1800’s) The discovery involved the use of a cathode ray tube Gases at atmospheric pressure don’t conduct electricity well

The current passed from the cathode to the anode –The rays created shadows and could turn a paddle wheel –They deflected as though they were negative –Called these cathode rays

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JJ Thomson was able to use these cathode ray tubes to determine the charge to mass ratio of the particles (electrons) in 1897 Robert Millikan performed an oil drop experiment that allowed him to calculate the charge on a single electron

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With this information, he was able to calculate the charge and mass of an electron Since atoms are neutral and the presence of electrons was confirmed, atoms were assumed to have some sort of positive charge

Thomson proposed the plum pudding model Ernest Rutherford performed the gold foil experiment and disproved the plum pudding model

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Atoms are very small ( pm) –Nuclei are pm –Nuclei are very dense (2 x 10 8 metrictons/cm 3 )

Counting Atoms The atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element –The identity of the element is based on this number

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons

The mass number is the number of protons and neutrons in the nucleus

Nuclide is the general term for any isotope of any element Nuclides can be referenced with a hyphen notation or a nuclear symbol

One atomic mass unit is defined as 1/12 th of the mass of the carbon-12 atom –The mass is therefore approximately the mass of a proton or a neutron The mass of an electron is amu, a proton is amu, and a neutron is amu’s. Although isotopes have different masses, they do not differ significantly in their chemical behavior

Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element

Ions Formed when an atom gains or loses an electron a. Charge = # of protons - # of electrons Ex) Mg +2 = lost 2 electrons # of protons: 12 # of electrons: 10 Charge: +2 Ex) N -3 = gained 3 electrons # of protons: 7 # of electrons: 10 Charge: -3

Molar Mass and Avogadro’s number can be used for conversions

Nuclear Forces The number of protons is the atomic number (Z) The number of protons and neutrons is the mass number (A) General Symbol

The protons and neutrons of a nucleus are called the nucleons A nuclide is the general term applied to a specific nucleus with a given number of protons and neutrons –Can be shown with a symbol (previous slide) –Can also be shown with name (radium – 228)

Isotopes are nuclides with the same number of protons, but different numbers of neutrons

Radioactive Decay Radioactivity is the spontaneous change of an unstable nucleus to form a more stable one The release of particles and energy from this process is called radioactivity

If a nucleus has too many neutrons, it can decay by turning a neutron into a proton and emitting a beta particle –Occurs if the N/Z number is too large Electron capture is the reverse of this process

Some nuclei that have too many protons can become more stable by emitting positrons (the antiparticle of an electron) These will collide with an electron and annihilate each other to release energy

Very large atoms will emit alpha radiation An alpha particle consists of two protons and two neutrons Many decay processes leave the nucleus in an unstable state in which it releases gamma radiation

Nuclear equations must be balanced –The superscripts and subscripts must be equal on both side of the arrow

Nuclear Fission Nuclear fission occurs when a very heavy nucleus splits into two smaller nuclei When Uranium-235 is bombarded with a neutron, it undergoes fission

The three released neutrons can strike other nuclei and cause them to decay If there is a critical mass, the reaction will continue This continuation is called a chain reaction

The fission of 1 gram of uranium generates as much energy as the combustion of 2700 kg of coal This process is used in a nuclear reactor The radioactive material decays to release heat, which creates steam, that runs a steam turbine to produce energy The moderator slows the neutrons down, so that they can be captured Control rods absorb neutrons to stop the chain reaction

There is a subcritical mass of U-235 in nuclear reactors –Cannot explode –Can overheat and “meltdown”-many safety mechanisms to prevent this In Chernobyl (1986) technicians briefly removed the control rods during a safety test

Nuclear fusion occurs when small nuclei combine to form a larger atom Stars perform fusion is their cores

Scientists are attempting to create a fusion reaction for energy generation –So far, we are only at a break even point with energy generation/consumption Currently done in a strong magnetic field

Currently 100 nuclear reactors generate 20% of the energy needs in the United States Nuclear waste disposal is an issue

Half Life The rate of radioactive decay is measured in half lives Half life is the time it takes for half a radioactive sample to decay Can be used to date materials –Carbon-12/Carbon-14

The more unstable the nucleus, the shorter it’s half life A body was found in the Alps in 1991 –Was dated to BC using C-14 dating C-14 dating is only good for living things in the last 50,000 years K-40 can be used for geologic dating –Half life of 1.28 billion years