Atomic Number
The atomic number of an element is a fundamental characteristic that represents the number of protons in its atomic nucleus. This number is crucial as it uniquely identifies each chemical element and determines its position on the periodic table. In a neutral atom, the atomic number also equals the number of electrons, influencing the element's chemical properties. Atoms are primarily composed of protons and neutrons, with the combined mass of these particles accounting for nearly all of the atom's mass.
The periodic table organizes elements based on their atomic numbers and chemical similarities, revealing patterns in their properties and behaviors. Elements in the same group share similar outer electron configurations, which leads to comparable chemical reactivity. For instance, alkali metals, with one outer electron, easily form positive ions. Understanding atomic numbers not only aids in classifying elements but also enhances comprehension of chemical reactions and bonding behaviors within the diverse realm of chemistry.
Atomic Number
FIELDS OF STUDY: Physical Chemistry; Organic Chemistry; Inorganic Chemistry
ABSTRACT
The atomic number of an element is based on the number of protons in the atom’s nucleus. The atomic number uniquely identifies each element.
The Nature of the Atomic Number
The atomic number of an element represents the exact number of protons in the nucleus of each atom of that element. Because protons and electrons have opposite electrical charges, the atomic number of an uncharged, neutral atom is also equal to the number of electrons surrounding the atomic nucleus. The number of electrons that surround a neutral atom of an element is a key determinant of that element’s specific chemical properties.
In addition to electrons, every atom is composed of a specific number of two subatomic particles, protons and neutrons, that comprise the atomic nucleus. A proton and a neutron are almost equal in mass, with the neutron having only slightly more mass. Between them, an atom’s protons and neutrons account for at least 99.98 percent of the atomic mass. Mass is a physical property that is defined relative to an acceptable standard. One atomic mass unit (u) is defined as one-twelfth of the mass of an atom of carbon-12, the most abundant isotope of carbon, which has six protons (according to carbon’s atomic number) and six neutrons; therefore, 1 u approximately represents the mass of one nucleon—that is, one proton or one neutron. (An isotope is an atom of a specific element that contains the usual number of protons in its nucleus but a different number of neutrons.)
The atomic weight of an element is determined by calculating the weighted average of the masses of all of that element’s isotopes. Because an element’s isotopes all have the same number of protons, they exhibit nearly identical chemical properties. Isotopes differ from one another in the number of neutrons in the atomic nucleus, which affects the atomic mass. Due to differences in the atomic mass, heavier isotopes react similarly but more slowly than lighter isotopes of the same element.
Atomic Numbers and the Periodic Table
Prior to the identification of the subatomic particles, scientists attempted to bring some order to their knowledge of the chemical elements. They found that certain elements exhibit similar chemical behaviors. These chemical similarities seemed to have a periodic relationship with the elements’ atomic masses; elements with similar chemical properties were found to have atomic masses that are either of nearly the same value or that increase at regular intervals. By arranging the known elements in tables according to the similarities in their chemical behavior, scientists developed the earliest versions of the periodic table. With the discovery of the existence of neutrons, protons, and electrons and the formulation of the octet rule of chemical bonding, the periodic relationships of the elements became more readily understood.
An uncharged atom has an equal number of protons and electrons and therefore has a neutral charge. The electrons occupy a region of space surrounding the atom’s nucleus that is about one hundred thousand times larger in diameter than the nucleus. Because chemical reactions and bonds take place at the level of the outermost electrons in an atom (the valence electrons), elements in the same group (the vertical columns of the periodic table) have similar distributions of their outermost electrons and therefore exhibit similar chemical behaviors. For example, all alkali metal elements (group 1) have a single outermost electron, so they all readily give up that electron to form a corresponding alkali metal ion bearing a single net positive charge. As the atomic number of the elements increases, so too do the number and distribution of the outermost electrons that they possess. The periods, the horizontal rows of the periodic table, rank the elements in their respective groups according to the highest energy level of their outermost electrons.
PRINCIPAL TERMS
- atomic mass: the total mass of the protons, neutrons, and electrons in an individual atom.
- electrical charge: a property of subatomic particles that causes them to exert a force on each other, either attractive (if their charges are of opposite signs) or repulsive (if they are of the same sign); by convention, a proton is assigned a charge of 1+ and an electron is assigned a charge of 1−.
- element: a form of matter consisting only of atoms of the same atomic number.
- periodic table: the chart representing the known elements by atomic number and electron distribution.
- proton: a fundamental subatomic particle with a single positive electrical charge, found in the atomic nucleus.
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