Edexcel A-level Chemistry Relative atomic mass
This page covers the following topics:
Atoms consist of protons, neutrons and electrons. Each element has a characteristic number of protons, called the atomic number. This can be found in the periodic table. Neutral atoms have the same number of protons and electrons.
For charged ions, the number of negatively charged electrons changes. Negative ions will have a higher number of electrons since they have a negative charge that comes from the negative electrons. Thus, positive ions will have the usual number of electrons that belongs to the atoms minus their charge which indicates the number of electrons lost.
The number of neutrons in an atom can be determined from the atomic mass, also found in the periodic table; number of neutrons = atomic mass − atomic number. The number of protons is the same as the atomic number. As protons and neutrons are a lot heavier than electrons, the centre of mass of atoms is located at the nucleus.
Isotopes are atoms of the same element, which have the same number of protons but a different number of neutrons. They are present in different levels of abundance. Some isotopes are radioactive.
Many isotopes are denoted by using the chemical symbol of an element and two additional numbers. The top number in such notation is the mass of an isotope in atomic units and the lower number is the charge of the nucleus, which matches the atomic number of an element.
Relative atomic mass is a weighted average mass of all the isotopes of an element. Weighted means that it takes into account the natural abundance of each isotope.
To find the relative atomic mass from given atomic masses of isotopes and their abundances, multiply corresponding atomic masses and their abundances in decimals and add the multiplication products together. If you use abundances as percentages, divide your final result by 100%.
To find a missing abundance or a mass of a particular isotope, use the aforementioned steps with a missing variable x for an unknown number and equate the final result to a given relative atomic mass. This will yield a linear equation, which after solving should give you the value of x.
Mass spectrometry is an analytical technique which separates gaseous ions by their mass-to-charge ratio (m/z). A sample is vaporised and can be ionised by using electron impact (removing electron) or electrospray ionisation (adding H⁺). After ionisation molecules are accelerated towards the detector.
As the ions travel through the electromagnetic field, they are deflected by a specific radius of curvature, which is dependent on their m/z. Only ions with the target m/z will reach the detector.
Another method is time-of-flight (TOF) mass spectrometry. This technique fires ions into a vacuum column and measures how long it takes to reach the detector at the other end.
Lighter ions, with a smaller m/z, travel faster. Both these methods produce spectra, showing the proportion of sample ions with each m/z value.
When analysing m/z diagrams, the highest value in it is usually the molecular peak. The base peak is considered the m/z value at the highest column value.
A key principle for TOF-mass spectrometry is that the accelerated ions all have the same kinetic energy, KE = ½mv². Rearrange this equation to find an equation for velocity, showing how it is related to the mass of the ion.
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