AS Chemistry: Chapter 1-Atomic structure

• Protons and neutrons are called nucelons, as they are found in the nucleus
• The electrons surround the nucleus
• Protons and neutrons have a relative mass of 1, but electrons have a relative mass of 1/1840
• Nucleic forces between protons and neutrons are much stronger than the electrostatic forces  between protons and electrons which prevent repulsion.
• However, nucleic forces only act over short distances.

Isotopes:

• Atomic number=the number of protons within a nucleus (and therefore the number of electrons)
• Mass number= the number of protons + the number of neutrons
• Isotopes contain the same numbers of protons and electrons, but varying numbers of neutrons. for this reason, they share the same atomic number , but the mass number may vary.
• Isotopes react chemically the same way, as they have they same electronic configuration as their counterparts.

• Sometimes isotopes are unstable and the nucleus of the atom itslef breaks down giving off bits of the nucleus or energetic rays, causing radioactivity.
• Each radioactive isotope decays at a rate measured by its half-life, known as the time for half of its radioactivity to decay.
• Carbon-14 has a half-life of 5730 years and can date organic matter.
• The levels of carbon-14 in an organism remain the same, while its living. 
• Once the organism dies, the carbon-14 begins to decay, and the levels of carbon-14 can be measured to determine the age of the specimen.
• For example, a sample with half the numner of carbon-14 found in its living counterpart would have been dead for 5730 years.

Example question:

The radioactivity in a wooden bowl was found to be 1/8 of that found in a sample of living wood: How old is the wood from the bowl?

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Ans: 1/8=1/2*1/2*1/2=3 half-lives; 5730*3=17,190

• Relative atomic mass= average mass of 1 atom/(one-12th) mass of 1 atom of Carbon-12
• relative molecular mass=average mass of molecule/(one-12th)mass of 1 atom of Carbon-12
• the mass spectrometer is the most accurate tool to determine relative atomic masses, and can be used by forensic scientists to help identify substances, such as illegal drugs.

TOF(time of flight mass spectrometer):

Stages of the Mass spectrometer (TOF):

• Vacuum-prevents the ions produced colliding with molecules from the air.
• Ionisation-the sample is dissolved in a volatile solvent and forced through a fine hollow needle that is connected to the positive terminal of a high voltage supply. This produces charged droplets which have lost electrons to the positive charge of the supply. the solvent evaporates from the droplets into the vacuum, and the droplets get smaller until they are a single cation.
• Acceleration-the cations are attracted to a negatively-charged plate and accelerate towards  it. Lighter ions and more highly charged ions achieve a higher speed.
• Ion drift-the ions pass  through a hole in the negatively-charged plate, forming a beam and travel along a tube, called the flight tube, to a detector.
• Detection-when ions with the same charge arrive at the detector, the lighter ones are 1st as tey have higher velocities. the flight times are recorded. the positive ions pick up an electron from the detector, which causes a current to flow.
• Data analysis- the signal from the detector is passed to a computer which generates a mass spectrum.

RAM= (mass no*percentage from peak)+('')/100

RAM of neon= (22*10)+(20*100)/100

=22.2