What makes up the atomic nucleus
At the center of every atom lies a nucleus. It includes two types of particles:
- Protons — possess a positive charge.
- Neutrons — carry no charge.
The only exception to this rule is the atom of the lightest element — hydrogen. Its nucleus contains exactly one proton, and neutrons are absent. All other multi-charged nuclei obligatorily contain both types of particles. At the same time, the stability of the nucleus depends on a specific ratio of the number of protons to neutrons.
Nuclear charge and chemical element
The number of protons determines the nuclear charge ($Z$). This value is the primary feature by which atoms are classified. A chemical element is a collection of atoms with the same nuclear charge.
Since the atom as a whole is an electroneutral particle, exactly as many negatively charged electrons must orbit the nucleus as there are protons inside it.
In the D.I. Mendeleev Periodic Table, the atomic number of an element strictly corresponds to its nuclear charge. The modern formulation of the Periodic Law states that the properties of elements and the substances they form are in periodic dependence precisely on the charges of their atomic nuclei. The mass of atoms increases along with the charge, but is not the main factor determining their properties.
Mass number
The masses of a single proton and neutron are nearly equal and amount to approximately $1$ atomic mass unit (a.m.u.). Electrons are so light that their contribution to the total mass of the atom is disregarded. Therefore, the mass of the entire atom is always close to an integer value.
Mass number ($A$) is the total number of protons ($Z$) and neutrons ($N$) in the nucleus. It is calculated by the formula: $A = Z + N$.
Nuclides and isotopes
Atoms of the same element (with the same $Z$) may contain a different number of neutrons, and thus have different masses.
- A nuclide is a specific type of atom with a clearly defined value of nuclear charge and mass number.
- Isotopes are nuclides of the same chemical element. They have the same charge, but different masses.
To designate nuclides, the chemical symbol of the element is used. The nuclear charge ($Z$) is written at the bottom left, and the mass number ($A$) at the top left. For example, silicon isotopes are denoted as ${}^{28}_{14}\text{Si}$, ${}^{29}_{14}\text{Si}$, and ${}^{30}_{14}\text{Si}$.
Features of atomic masses in the Periodic Table
In nature, most chemical elements exist as a mixture of several isotopes. Only 19 elements are represented by a single nuclide — it is precisely their atomic masses in reference books that are closest to integers.
For the remaining elements, the atomic mass indicated in the Periodic Table is a fractional number. This is an arithmetic mean that takes into account the mass of each natural isotope and its percentage abundance. For example, for chlorine, the value $35.5$ is used in calculations, since natural chlorine consists of a mixture of heavy and light isotopes (${}^{35}_{17}\text{Cl}$ and ${}^{37}_{17}\text{Cl}$).
In chemical reactions, different isotopes of the same element behave almost identically. The exception is hydrogen isotopes: due to the colossal difference in mass (by a factor of 2 and 3), they have been assigned their own symbols and names.
Briefly: answers to topical questions
Which hydrogen isotopes have their own names and chemical symbols?
Due to the significant difference in mass, unique names and symbols are assigned to hydrogen isotopes. There are three main hydrogen isotopes, differing in the number of neutrons in the nucleus:
- Protium (${}^1_1 ext{H}$) — ordinary hydrogen, the nucleus consists of 1 proton without neutrons;
- Deuterium (${}^2_1 ext{H}$ or $ ext{D}$) — heavy hydrogen, the nucleus contains 1 proton and 1 neutron;
- Tritium (${}^3_1 ext{H}$ or $ ext{T}$) — radioactive hydrogen, the nucleus consists of 1 proton and 2 neutrons.
Which elementary particles are united under the general term "nucleons"?
Under the general term "nucleons", the constituent parts of the atomic nucleus — protons and neutrons — are grouped together. Further research led to the discovery of these particles that make up the atomic nucleus.
What forces ensure the stability of the nucleus and prevent protons from mutual electrostatic repulsion?
The stability of atomic nuclei is ensured by a special attraction between protons and neutrons at very short distances, known as the strong nuclear interaction. The resulting nuclear forces exceed the electrical repulsion between like-charged protons by a factor of 200.
To remember the mass number relationship: A = Z + N.
Class and exam questions
What sign does the atomic nuclear charge have?
The nuclear charge is always positive because it contains positively charged protons and neutral neutrons.
How to find the number of electrons in an atom?
You need to look up the atomic number of the element in the Periodic Table. It equals the nuclear charge, which in turn equals the number of electrons in a neutral atom.
Why is the atomic mass in the Periodic Table a fractional number?
Because most elements in nature are a mixture of isotopes with different masses. The table indicates the average value that accounts for the abundance of each isotope.
What else to study on this topic
- Dependence of element properties on nuclear charge (Periodic Law)
- Reasons why Mendeleev initially arranged elements by mass
- Conditions for the stability of multi-charged nuclei
- Names and features of hydrogen isotopes
- Algorithm for calculating the number of neutrons in the nucleus of a specific isotope
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