If you choose any element in the periodic table, you can read the atomic mass at the top right of the element card, how heavy the atom is. However, this mass is only the mass that occurs most frequently in nature. An element can have variants of different weights, which are called isotopes.
Isotope – definition and explanation
the isotopes of an element all have the same number of protons but a different number of neutrons in the nucleus.
The atomic structure is the basis for understanding isotopes. An atom is made up of one atomic nucleus and one atomic shell together. are in the atomic nucleus protons and neutrons, electrons in the atomic shell. The number of protons determines which element it is.
If there is only one proton in the nucleus, it is hydrogen with the element symbol H. If there are six protons, carbon with the element symbol C is present. In the periodic table, the element cards are numbered with the element symbol at the top left. This number reflects the number of protons.
The number of neutrons is a non-negligible part of the atomic mass, because neutrons are about as heavy as protons. Protons and neutrons together make up about 99.9% of the atomic mass. The rest comes from the significantly lighter electrons.
Although an element always has the same number of protons, it can have a different number of neutrons in its nucleus. However, since neutrons make up a large part of the atomic mass, isotopes have different weights.
Atomic mass is given in the atomic mass unit «u», where 1 u is approximately 1.66x correspond to 10-27 kg. Proton and neutron each have a mass of about 1 u.
Hydrogen has three isotopes that occur naturally. They are called protium, deuterium and tritium. Protium has no neutron, deuterium has one neutron, and tritium has two neutrons.
Note that protium, deuterium and tritium all have only one proton in the nucleus, otherwise they would not be isotopes of hydrogen. Tritium atoms are about three times as heavy as protium atoms. This is because tritium atoms have a mass of 3 u, composed of one proton and two neutrons, while protium atoms with only one proton in their nucleus have a mass of only 1 u.
The spelling of isotopes
In reaction equations you only ever find the element symbols, which reveal how many protons an atom of this element has. However, they do not specify how many neutrons the atom under consideration has. In fact, the neutron anzahl or the total mass of the atom is of secondary importance for the reaction behavior. Only the reaction speed is reduced by higher atomic masses. However, a special notation for isotopes is of great importance, for example for nuclear physics.
In particular, nuclear physics examines what are known as decay processes, in which one element decays into two new elements. Without an understanding of decay processes, humanity would have to do without the diagnosis and therapy of pathological tissue changes. You can find more about this in the article on nuclear physics in the subject physics.
For the notation you need the element symbol, the number of protons and the number of neutrons. To distinguish between isotopes, add two numbers to the lower left and upper left of the element symbol X. The number Z in the lower left corresponds to the number of protons in the atom.
A different element symbol must be used for any other number of protons because it is then a different element. In contrast, the top left shows the sum of neutrons and protons in the nucleus. So if you subtract the number on the bottom left from the number on the top left, you get the number of neutrons in the nucleus.
The number of protons is also referred to as the atomic number or from the atomic number. The number at the top left is also referred to as the mass numberbecause the sum of neutrons and protons also gives the atomic mass in the atomic mass unit u.
How would the isotopes protium, deuterium and tritium mentioned in the previous example look like in this notation? Since these are the three isotopes of hydrogen, all of them have the number 1 as the atomic number. Since protium has no neutron in its nucleus, its mass number is 1 + 0 = 1. Deuterium has 1 neutron, which is the mass number here is 1 + 1 = 2. Finally, tritium has 2 neutrons and here the mass number is 1 + 2 = 3.H11+0 = H H11+1 = H12 H11+2 = H1311
Measurement of atomic masses
But how do you find out the mass of an atom? There is a specific procedure for that mass spectrometry. Mass spectrometry uses the fact that the atomic nuclei of isotopes are all the samee have charge, but different masses.
The nuclei of an element’s isotopes have the same positive charge because they all have the same number of protons.
Although the same force would act on all isotopes in an electric or magnetic field, each isotope would be deflected differently. Because the heavier an atomic nucleus is, the more inert it is and the weaker it is deflected.
The mass spectrometer records how strongly an atomic nucleus is deflected. From this it can then be calculated what mass this atom must have in order to experience a deflection of the registered amount. Get a better overview of this topic You in the article on mass spectrometry.
unstable isotopes
Not all isotopes are stable. This means that some isotopes break down into another element after a certain period of time, emitting radiation that is harmful to living beings. The so-called nuclide chart shows whether an isotope is stable and how it decays if it is unstable. There are different types of radiation, which are explained in more detail in the article on radioactivity.
The x-axis of this nuclide chart shows the number of neutrons and the y-axis shows the number of protons. If all isotopes of an element are to be considered, a fixed number of protons must be taken. Then, using the colors and the legend, consider at what neutron count an isotope is unstable or stable.
For example, you can draw a horizontal line through the number 50 on the y-axis for the isotopes of tin and you would be able to look at all the isotopes of tin because tin has 50 protons in the nucleus. The neutron count for a tin atom varies depending on where you are on the horizontal line.
It is noticeable that the stable isotopes of all elements form a kind of center line, which is shown in black. Unstable isotopes located to the right of this center line, with the same number of protons, have more neutrons compared to their stable element on the black center line.
Unstable isotopes to the left of this center line, again with the same number of protons, have fewer neutrons compared to their stable element on the black center line.
It is therefore reasonable to assume that the prerequisite for the stability of an isotope must be a certain ratio between the number of neutrons and protons. In other words, there must be neither too many nor too few neutrons.
Isotopes Example: The radiocarbon method
An important isotope that is found in the human body is the so-called Carbon-14 isotope, C614. The carbon-14 isotope is unstable and therefore spontaneously breaks down into another element, namely nitrogen (N). Due to its decay, one can determine the age of deceased organisms.
The method for determining the age is based on the fact that in the body of living beings there is a certain ratio between the carbon-14 isotope and the stable Carbon-12 isotope prevails. Because the carbon-12 isotope is 1012 times more common than the carbon-14 isotope. Both isotopes are ingested through food and this ensures that the ratio in the living organism remains the same.
In a deceased organism, on the other hand, no further carbon atoms are added. Over time, the number of carbon-14 isotopes in the body of the deceased decreases as they decay, while the number of stable carbon-12 isotopes remains the same.
The fewer carbon-14 isotopes there are in relation to the carbon-12 isotopes, the older the deceased organism is.
With the help of the so-called half-life of the carbon-14 isotope, it is even possible to calculate exactly how long an organism has died from the ratio of the two carbon isotopes to one another. But what exactly does this formula look like? And how do the carbon isotopes get into our food in the first place? You can find out more about this in the article on the radiocarbon method.
The half-life is a number that indicates the time after which half of all unstable isotopes in a starting quantity have decayed. In the case of the carbon-14 isotope, it is 5730 years. Each isotope has a specific half-life.
Isotopes – The most important thing
- The isotopes of an element all have the same number of protons but different numbers of neutrons
- Isotopes have different weights
- For the notation of isotopes, write the number of protons and the sum of protons and neutrons (=mass number) at the bottom left and top left of the element symbol
- There are stable and unstable isotopes. Unstable isotopes break down into other elements, emitting radiation harmful to humans. An isotope is stable if it has a certain ratio of protons to neutrons. There must be neither too many protons nor too many neutrons.
proof
- Binnewies, Finzel, Jackel, Schmidt, Willner, Rayner-Canham (2016). General and Inorganic Chemistry. Jumper Spectrum.
- Böhme, Uwe (2013). Inorganic chemistry for dummies. Wiley.