Atoms are composed of three primary subatomic particles: protons, neutrons, and electrons. The nucleus, or core of the atom, contains protons and neutrons. Electrons orbit this center.
Protons carry a positive electrical charge, and electrons carry a negative charge. Neutrons are electrically neutral. The identity of any element is determined by the number of protons in its nucleus.
The Defining Role of Protons and Neutrons
The number of protons within the nucleus is the atomic number (\(Z\)). Every atom of a specific element will always have the same atomic number. For example, any atom with three protons is Lithium, and any atom with six protons is Carbon.
The collective number of protons and neutrons is the mass number (\(A\)). This relationship is expressed as \(A = Z + N\), where \(N\) is the number of neutrons.
Atoms of the same element can exist with different numbers of neutrons; these variations are called isotopes. All Carbon atoms have six protons, but they can have six, seven, or eight neutrons. The existence of isotopes is why simply asking for “the element” with four neutrons is insufficient, as the number of protons can still vary.
Identifying Specific Isotopes with Four Neutrons
To identify the specific elements and their isotopes that contain exactly four neutrons, we must apply the relationship \(A = Z + 4\). Since the number of protons (\(Z\)) can be any positive integer, multiple elements can satisfy the condition of having four neutrons (\(N=4\)). Each element is identified by its proton count, and the specific isotope is named by its mass number (\(A\)).
The lightest element that can contain four neutrons is Lithium (\(Z=3\)). With four neutrons, this results in the mass number \(A=7\), creating the isotope Lithium-7. Beryllium (\(Z=4\)) combines with four neutrons to form Beryllium-8.
Moving up the periodic table, Boron (\(Z=5\)) forms the isotope Boron-9 with four neutrons. Carbon (\(Z=6\)) forms the isotope Carbon-10 with four neutrons. This list continues for elements with higher atomic numbers, provided they are stable or long-lived enough to be observed.
Stability and Lifespan of Four-Neutron Isotopes
Not all combinations of protons and neutrons result in a stable nucleus; most isotopes are radioactive and decay over time. Nuclear stability depends on a narrow range of neutron-to-proton ratios. Nuclei that fall outside this range are unstable and undergo radioactive decay to achieve a more favorable balance.
Of the four-neutron isotopes mentioned, Lithium-7 is the only one considered stable. This nucleus, with three protons and four neutrons, is the most common and abundant isotope of Lithium, making up about 92.5% of naturally occurring Lithium. Its neutron-to-proton ratio falls within the region of stability for light elements.
Beryllium-8, with four protons and four neutrons, is highly unstable. This isotope decays almost instantaneously into two alpha particles (Helium-4 nuclei). Boron-9 and Carbon-10 are also radioactive, having short lifespans as they are proton-rich and rapidly decay.