Topic 2.3: Electronic Arrangement

2.3.1 Describe the electromagnetic spectrum

Electromagnetic radiation comes in different forms of differing energy. Gamma rays are a particularly high energy form and the visible light we need to see the world is a lower energy form. All electromagnetic waves travel at the same speed and can be distinguished by their different wavelength.

This is the full electromagnetic spectrum

Note: They are all called light, it depends whether we can see it or not. Hence a section is called the visible light.

2.3.2 Distinguish between the continuous spectrum and line spectrum

White light is a mixture of light waves of differing wavelengths or colors. We see this when sunlight passes through a prism to produce a continuous spectrum.

Line spectrum shows some colors of the continuous spectrum, this is because it is an emission spectrum.

The colors present in the emission spectrum are the same as those that are missing from the absorption spectra. As different elements have different line spectra they can be used like bar-codes to identify unknown elements.

2.3.3 Explain how the lines in the emission spectrum of hydrogen are related to electron energy levels.

Excite atoms using heat can give off pretty light. It excites the electron to a higher energy shell, but these lights are actually made up of a electrons releasing energy when they jump back to the lower energy shell.

The hydrogen atom gives out energy when an electron falls from a higher to lower energy level. Hydrogen produces visible light when the electron falls to the second energy level (n=2). The transition to the first energy level correspond to a higher energy change and are in the ultraviolet region of the spectrum. Infrared radiation is produced when an electron falls to the third or higher energy.

http://www.youtube.com/watch?v=6rHerkru60E is a good video that explains the electron energy levels.

The lines converge at higher energies because the energy levels inside the atoms are closer together. When an electron is at the highest energy (n = infinity), then the atom has been ionized.

Most notable discovery is that the line spectra is exactly the same as the energy levels.

2.3.4 Deduce the electron arrangement for atoms and ions up to Z = 20

Atoms

Hydrogen - 1
Helium - 2
Lithium - 2, 1
Beryllium - 2, 2
Boron - 2, 3
Carbon - 2, 4
Nitrogen - 2, 5
Oxygen - 2, 6
Fluorine - 2, 7
Neon - 2, 8
Sodium - 2, 8, 1
Magnesium - 2, 8, 2
Aluminium - 2, 8, 3
Silicon -2, 8, 4
Phosphorous - 2, 8, 5
Sulfur - 2, 8, 6
Chlorine - 2, 8, 7
Argon - 2, 8, 8
Potassium - 2, 8, 8, 1
Calcium - 2, 8, 8, 2

The general rule of thumb is that first shell has a maximum of 2, second shell has a maximum of 8 and third shell has a maximum of 8.

Ions

Hydrogen - 0
Helium - 2
Lithium - 2
Beryllium - 2
Boron - 2
Carbon - 2, 8
Nitrogen - 2, 8
Oxygen - 2, 8
Fluorine - 2, 8
Neon - 2, 8
Sodium - 2, 8
Magnesium - 2, 8
Aluminium - 2, 8
Silicon -2, 8, 8
Phosphorous - 2, 8, 8
Sulfur - 2, 8, 8
Chlorine - 2, 8, 8
Argon - 2, 8, 8
Potassium - 2, 8, 8
Calcium - 2, 8, 8

General rule of thumb, when it is an atom becomes an ion. It wants a full outer electron shell.

Topic 2.2: The mass spectrometer

2.2.1 Describe and explain the operation of a mass spectrometer

Mass spectrometer is used to measure different isotopes and their relative abundances.

The mass spectrometer has five basic operations:

Vaporization: A vaporized sample is injected into the instrument. This allows the individual atoms of element to be analysed

Ionization: The atoms are hit with high-energy electrons which knock out electrons, thus producing positively charged ions.

Acceleration: The positive ions are attracted to negatively charged plates. They are accelerated by an electric field and pass through a hole in the plate.

Deflection: The accelerated positive ions are deflected by a magnetic field placed at the right angles of their path. The amount of deflection is proportional to the charge/mass ratio. Ions with smaller mass are deflected more than heavier ions. Ions with higher charger are deflected more as they interact more efficiently with the magnetic field.

Detection: Positive ions of a particular mass/charge ratio are detected and a signal sent to a recorder. The strength of the signal is a measure of the number of ions with that charge/mass ratio that are detected.

2.2.2 Describe how the mass spectrometer may be used to determine relative atomic mass using the 12C scale

Relative atomic mass uses the 12 C scale because using the actual mass of individual atoms are beyond our experiences. For example hydrogen atom is 1.67 x 10^-24 g in mass or 1.001 in RAM.

As carbon is a very common element which is easy to transport and store because it is a solid, its isotope, 12 C, was chosen as the standard in 1961.

2.2.3 Calculate non-integer relative atomic masses and abundance of isotopes from given data.

This is a simple Mass Spectrum. The horizontal axis shows the mass/charge ratio of different ions on the carbon-12 scale and the relative abundance is shown on the vertical axis. Normally the relative abundances come in percentages.

So the percentages should be 75% and 25%.

(75 x 35) + (25 x 37) = 3550 = total mass

3550/100 = 35.5 = average mass

There are normally more m/z than 2 therefore you should really practice using a variety of numbers.

Topic 2.1: The Atom

2.1.1 State the position of protons, neutrons and electrons in the atom.

Proton and neutrons are both present in the nucleus of the atom, they are also known as nucleons. While electrons are present in the energy levels.

2.1.2 State the relative masses of relative charges of protons, neutrons and electrons.

Protons have relative mass of 1 and relative charge of +1. It has a positive charge.

Neutrons have relative mass of 1 and relative charge of 0. It has no charge.

Electron have relative mass of 5 x 10^4 or 0.0005 and relative charge of -1. It has a negative charge.

2.1.3 Define the terms mass number (A), atomic number (Z), and isotopes of an element.

Mass number or relative atomic mass is how heavy an atom is compared to hydrogen. a.k.a (A)

Atomic number is the number of proton in an atom. a.k.a (Z)

Isotopes are the same element with the same number of proton but different number of neutrons.

Protium, Deuterium and Tritium are all isotopes of hydrogen. (Example)


2.1.4 Deduce the symbol for an isotope given its mass number and atomic number.

The symbol of an element is found by matching the "atomic number" with its corresponding one on the periodic table. It is important to use the atomic number because there are individual unique element for each atomic number.

2.1.5 Calculate the number of protons, neutrons and electrons in atoms and ions from mass number, atomic number and charge.

To calculate the number of protons, simply find Z or look for the chemical symbol in the periodic table.

To calculate the number of neutrons, simply find A-Z. Since A is the number of both proton and neutrons, we can get the only neutron by subtracting the protons.

To calculate the number of electrons, simply find Z and its charge (n). Since protons should equal electrons in an atom, then the electron will be Z. However, if it is an ion then extra steps are required. If the charge is +1, then subtract one from Z and vice versa. 

2.1.6 Compare the properties of the isotopes of an element.

Isotopes show the same chemical properties, as a difference in the number of neutrons makes no difference to how they react and so they occupy the same place in the Periodic Table.

The difference in mass lead to different physical properties such as boiling and melting points. Heavier isotopes move more slowly at a given temperature and these differences can be used to separate isotopes

2.1.7 Discuss the uses of radioisotopes.

Carbon 14 dating

Carbon 14 has eight neutrons which is too many to be stable. It can reduce the neutron to proton ratio when a neutron changes to a proton. This is beta radiation.

The relative abundance of carbon-14 present in living organism is constant due to breathing and carbon dioxide. However, when an organisms dies, it no longer absorbs carbon 14 and carbon 14 fall owing to nuclear decay. As this process occurs at a regular rate, we can determine the death of an organism to 5730 years.

Cobalt-60 Radiotherapy

Treatment of cancer and other diseases using ionizing radiation. The treatment damages the genetic material inside a cell and making it impossible to grow, though this damages both cancer cells and normal cells.

Iodine 131 Medical Tracer

Iodine 131 is an emitter of both beta and gamma rays can be used to investigate the activity of the thyroid gland and treat thyroid cancer It has a half-life of 8 days so it is quickly eliminated from the body.

Topic 2: The Atomic Structure

Topic 2 of the IB HL Chemistry syllabus is the Quantitative Chemistry. IBO recommends to spend 4 hours on this topic.

This topic has 3 sub-chapters: "The atom", "The mass spectrometer" and "Electron Arrangement". Each are separated with numerical values in order of mentioned.

These are all basic syllabus statements, it is recommended to bring a Casio Graphical Calculator instead of Texas. Casio Calculators have the periodic table installed already.