Even ancient philosophers suggested that matter is built from atoms. However, the fact that the very "bricks" of the universe consist of the smallest particles, scientists began to guess only at the junction of the XIX and XX centuries. The experiments proving this made a real revolution in science in due time. It is the quantitative ratio of the components that distinguishes one chemical element from another. Each of them is assigned a place in the periodic table according to the serial number. But there are species of atoms that occupy the same cells in the table, despite the difference in mass and properties. Why this is so and what isotopes in chemistry are described below.
Atom and its particles
Exploring the structure of matter through bombardment with alpha particles, E. Rutherford proved in 1910 that the main space of an atom is filled with void. And only in the center is the core. Negative electrons move around it in orbitals, making up the shell of this system. Thus was created a planetary model of the "bricks" of matter.
What are isotopes? Remember from the chemistry course that the core also has a complex structure. It consists of positive protons and chargeless neutrons. The number of the first determines the qualitative characteristics of the chemical element. It is the number of protons that distinguishes substances from each other, giving their nuclei a certain charge. And on this basis they are assigned a serial number in the periodic table. But the number of neutrons in the same chemical element differentiates them into isotopes. The definition in chemistry of this concept can therefore be given the following. These are types of atoms that differ in the composition of the nucleus, have the same charge and serial numbers, but have different mass numbers, due to differences in the number of neutrons.
Designations
Studying chemistry in 9th grade and isotopes, students learn about accepted conventions. The letter Z marks the charge of the nucleus. This figure coincides with the number of protons and therefore is their indicator. The sum of these elements with neutrons marked by the N symbol is A - mass number. The family of isotopes of one substance, as a rule, is indicated by the icon of the chemical element, which in the periodic table is assigned a serial number that matches the number of protons in it. The upper left index added to the indicated icon corresponds to the mass number. For example, 238 U. The charge of an element (in this case, uranium, marked by serial number 92) is indicated by a similar index below.
Knowing these data, one can easily calculate the number of neutrons in a given isotope. It is equal to the mass number minus the serial number: 238 - 92 = 146. The number of neutrons could be less, this chemical element would not cease to be uranium. It should be noted that most often in other, simpler substances, the number of protons and neutrons approximately coincides. Such information helps to understand what an isotope is in chemistry.
Nucleons
A certain element is endowed with individuality precisely in the number of protons, and the number of neutrons does not affect it in any way. But the atomic mass is made up of these two indicated elements, having the common name "nucleons", representing their sum. However, this indicator does not depend on the number of electrons forming a negatively charged atomic shell. Why? One has only to compare.
The mass fraction of the proton in the atom is large and is approximately 1 a. E. m. or 1,672 621 898 (21) · 10 -27 kg. The neutron is close to the performance of this particle (1.674 927 471 (21) · 10 -27 kg). But the mass of an electron is thousands of times less, it is considered insignificant and is not taken into account. That is why, knowing the superscript of an element in chemistry, it is not difficult to find out the composition of the nucleus of isotopes.
Hydrogen isotopes
Isotopes of some elements are so famous and widespread in nature that they received their own names. A vivid and simplest example of this is hydrogen. In vivo, it is found in its most common variety of protium. This element has a mass number of 1, and its core consists of one proton.
So what are hydrogen isotopes in chemistry? As you know, the atoms of this substance have the first number in the periodic table and, accordingly, are endowed with a charge number of 1 in nature. But the number of neutrons in the atom’s nucleus is different. Deuterium, being heavy hydrogen, in addition to the proton, has one more particle in its core, that is, a neutron. As a result, this substance exhibits its own physical properties, unlike protium, having its own weight, melting point and boiling point.
Tritium
Tritium is the hardest made. This is superheavy hydrogen. According to the definition of isotopes in chemistry, it has a charge number of 1, but a mass number of 3. It is often called a triton, because in addition to one proton, it has two neutrons in the nucleus, that is, it consists of three elements. The name of this element, discovered in 1934 by Rutherford, Olyphant and Hartek, was proposed even before its discovery.
This is an unstable substance with radioactive properties. Its core has the ability to split with the release of beta particles and electron antineutrinos. The decay energy of this substance is not very large and amounts to 18.59 keV. Therefore, such radiation is not too dangerous for a person. Ordinary clothes and surgical gloves can protect against it. And this radioactive element obtained with food is quickly excreted from the body.
Uranium isotopes
The various types of uranium, which are known to science today, are far more dangerous. Therefore, when speaking about what isotopes are in chemistry, it is impossible not to mention this element. Despite the variety of uranium species, only three are found in the nature of its isotopes. These include 234 U, 235 U, 238 U. The first of them, having suitable properties, is actively used as fuel in nuclear reactors. And the latter is for the production of plutonium-239, which itself, in turn, is indispensable as a valuable fuel.
Each of the radioactive elements is characterized by its own half-life. This is the length of time during which a substance splits in a ratio of ½. That is, as a result of this process, the amount of the remaining part of the substance is halved. This time span is huge for uranium. For example, for isotope-234 it is estimated at 270 millennia, and for the other two indicated species it is much more significant. The record half-life of uranium is 238, lasting billions of years.
Nuclides
Not every type of atom, characterized by its own and strictly defined number of protons and electrons, is so stable that there exists at least any long period sufficient to study it. Those that are relatively stable are called nuclides. Stable formations of this kind are not exposed to radioactive decay. Unstable are called radionuclides and also, in turn, are divided into short-lived and long-lived. As is known from chemistry lessons of class 11 on the structure of isotope atoms, osmium and platinum possess the largest number of radionuclides. One stable one has cobalt and gold, and tin has the largest number of stable nuclides.
Isotope sequence number calculation
Now we will try to summarize the information described earlier. Having understood what isotopes are in chemistry, it's time to find out how to use the knowledge gained. Consider this with a specific example. Suppose it is known that a certain chemical element has a mass number of 181. Moreover, the atomic shell of a given substance contains 73 electrons. How can I, using the periodic table, find out the name of this element, as well as the number of protons and neutrons in the composition of its nucleus?
We proceed to solve the problem. The name of a substance can be determined by knowing its serial number, which corresponds to the number of protons. Since the number of positive and negative charges in an atom is equal, then it is 73. So, this is tantalum. Moreover, the total number of nucleons in the sum is 181, which means that the protons of this element are 181 - 73 = 108. It's quite simple.
Gallium isotopes
The gallium element in the periodic system has the serial number 71. In nature, this substance has two isotopes - 69 Ga and 71 Ga. How to determine the percentage of gallium species?
Solving problems for isotopes in chemistry is almost always associated with information that can be obtained from the periodic table. This time should do the same. Determine the average atomic mass from the indicated source. It is equal to 69.72. Denoting by x and y the quantitative ratio of the first and second isotopes, we take their sum to be 1. So, in the form of an equation this will be written: x + y = 1. It follows that 69x + 71y = 69.72. Expressing y through x and substituting the first equation in the second, we get that x = 0.64, and y = 0.36. This means that 69 Ga is 64% in nature, and the percentage of 71 Ga is 34%.
Isotope transformations
Radioactive fission of isotopes with their transformation into other elements is divided into three main types. The first of these is alpha decay. It occurs with the emission of a particle, which is the nucleus of a helium atom. That is, this formation, consisting of a set of pairs of neutrons and protons. Since the number of the latter determines the charge number and atom number of the substance in the periodic system, as a result of this process, one element transforms into another, and in the table it shifts to the left by two cells. In this case, the mass number of the element is reduced by 4 units. We know this from the sections of chemistry about the structure of isotope atoms.
When the nucleus of an atom loses a beta particle, which is essentially an electron, its composition changes. One of the neutrons transforms into a proton. This means that the qualitative characteristics of the substance change again, and the element shifts one cell to the right in the table, practically without losing mass. Typically, such a conversion is associated with electromagnetic gamma radiation.
Radium isotope conversion
The above information and knowledge from chemistry of class 11 about isotopes again help to solve practical problems. For example, the following: 226 Ra during decay turns into a chemical element of group IV, having a mass number of 206. How much alpha and beta particles should it lose?
Given the changes in mass and the group of the daughter element, using the periodic table, it is easy to determine that the lead formed by the isotope splitting is lead with a charge of 82 and mass number 206. And given the charge number of this element and the initial radium, it should be assumed that its core lost five alpha -particles and four beta particles.
The use of radioactive isotopes
Everyone is well aware of the harm that can be caused to living organisms by radioactive radiation. However, the properties of radioactive isotopes are useful to humans. They are successfully used in many industries. With their help, it is possible to detect a leak in engineering and construction structures, underground pipelines and oil pipelines, storage tanks, heat exchangers in power plants.
The indicated properties are also actively used in scientific experiments. For example, the tsetse fly is a carrier of many serious diseases for humans, livestock and domestic animals. In order to prevent such males, these insects are sterilized by weak radiation. Isotopes are also indispensable when studying the mechanisms of certain chemical reactions, because atoms of these elements can label water and other substances.
In biological research, labeled isotopes are often also used. For example, it was in this way that it was established how phosphorus affects the soil, growth and development of cultivated plants. With success, the properties of isotopes are also used in medicine, which made it possible to treat cancerous tumors and other serious diseases, and determine the age of biological organisms.