Chemical element

A chemical element is an identity shared by all chemical atoms whose atomic nuclei contain the same number of protons. That number is its atomic number, $Z$: six means carbon, eleven means sodium. The identity survives differences in neutron count, which distinguish isotopes, and differences in electron count, which distinguish neutral chemical atoms from ions. See the IUPAC Gold Book, chemical element.

The same name can also denote a pure chemical substance containing only that chemical element. These two uses answer different questions. Molecular oxygen, ${\mathrm{O}}_2$, is a chemical substance made entirely of oxygen; water, ${\mathrm{H}}_2\mathrm{O}$, is a chemical compound containing both hydrogen and oxygen. A water molecule contains three chemical atoms but only two chemical elements. Joining chemical atoms through chemical bonds changes their arrangement without changing their atomic numbers.

This distinction makes stoichiometry possible: ordinary chemical reactions rearrange chemical atoms, so one can count each chemical element before and after a transformation. Changing the proton count instead changes the chemical element and requires a nuclear reaction. The periodic table organizes these identities into a useful map, while atomic number bookkeeping keeps identity, isotope, and electric charge separate.

A stable samarium isotope used as the nuclear absorber in the Goldhaber experiment.

A chemical element of atomic number 59. Its trivalent ion has a $4f^2$ open-shell electron configuration. In LS coupling, Hund's rules select $L=5$, $S=1$, $J=4$, the ${}^3{H}_4$ term symbol. The corresponding free-ion Landé g-factor is $4/5$ and its effective paramagnetic moment is $(4/5)\sqrt{20}{\mu }_B\simeq 3.58{\mu }_B$.

An alkaline-earth element whose divalent ions can substitute for calcium in suitable crystal hosts. Radioactive strontium isotopes are relevant fission-product constituents.

The existence of different structural forms of the same chemical element. Diamond and graphite are crystalline examples for carbon; molecular oxygen and ozone illustrate that allotropy need not consist only of different packing arrangements of identical molecules.

The linear oxo species $\mathrm{Am}{\mathrm{O}}_2^{+}$ and $\mathrm{Am}{\mathrm{O}}_2^{2+}$ for Am(V) and Am(VI), respectively. The charge changes by one on a single-electron redox step.

Hafnium is the heavier group-4 congener of zirconium. Changes in alkyl-bond energetics can change both propagation and chain-transfer rates in metallocene polymerization.

Zirconium is a group-4 metal; Zr(IV) is formally $d^0$. Its cationic alkyl complexes can catalyze polymerization through alkene migratory insertion.

Neptunium is the chemical element with atomic number 93. Its isotope neptunium-239 occurs between uranium-239 and plutonium-239 in a neutron-capture and beta decay production chain.

A gadolinium isotope with a very large thermal-neutron absorption cross section. Its isotope-specific cross section is not the same as the isotope-averaged value for natural gadolinium.

A fissile material produced from uranium-238 by neutron capture followed by two beta-minus decays.

A chemical atom of an alkali metal.

Ground-state electronic angular momentum $J=1/2$ couples to nuclear spin $I=3/2$, giving $F=1,2$. For contact hyperfine interaction $A\mathbf{I}\cdot \mathbf{J}/{\hslash }^2$, their zero-field energies are $-5A/4$ and $3A/4$. The splitting is $2A$. Electronic weak-field contributions give $g_{F=1}\simeq -1/2$ and $g_{F=2}\simeq +1/2$; the small nuclear contribution refines these numbers.

Degradation associated with the slow transformation of white tin to lower-density grey tin. Nucleation, impurities and thermal history strongly affect its kinetics.

The diamond-cubic form of tin favoured below about $13{}^{\circ }\mathrm{C}$ at ambient pressure. Its lower density than white tin contributes to the damage associated with tin pest.

The metallic tetragonal form of tin stable near ordinary room temperature at ambient pressure.

$\mathrm{S}{\mathrm{n}}_5^{2-}$, a five-vertex trigonal-bipyramidal Zintl cluster. Removing five mainly s-rich lone pairs from its 22 valence electrons leaves 12 skeletal electrons.

The periodic table arranges chemical elements in increasing atomic number, across successive periodic table periods. Its columns are periodic groups, numbered 1–18. The layout places recurring outer electron configurations above one another, making similarities in chemical bonds and chemical reactions easier to recognize. Ordering is by proton count, not by mass; the mass number of a particular isotope does not determine its box. See the official IUPAC periodic table and numbering conventions.

For the first eighteen neutral chemical atoms in their ground states, a compact description counts electrons in successive electron shells. Thus carbon has $(2,4)$, meaning two electrons in the first electron shell and four in the second; the sum $2+4=6$ agrees with its atomic number. Sodium has $(2,8,1)$, whose sum is $11$. These are occupancy counts, not circular trajectories of electrons.

Reading each row from left to right gives:

The counting outline is short: the occupied electron subshells in this range are $1s$, $2s$, $2p$, $3s$, and $3p$, filled in that order for these neutral ground states. Each $s$ electron subshell holds up to two electrons and each $p$ electron subshell up to six, giving $2$, $2+6=8$, and $2+6=8$ along these three rows. This $2,8,8$ summary applies only through argon; it is not a general rule that each electron shell fills completely before the next begins, nor a claim that the third electron shell can hold only eight electrons. See Flowers et al., Chemistry 2e, electronic structure and electron configurations.

The recurring outer counts make the arrangement useful. Lithium and sodium each have one valence electron, whereas fluorine and chlorine each have seven valence electrons. Such similarities help anticipate chemical reactions, without making all properties identical throughout a periodic group. Helium is an instructive exception to an eight-electron shortcut: its two electrons complete the first electron shell, so it belongs above neon and argon in periodic group 18. Hydrogen shares column 1 with lithium and sodium but has its own distinct chemistry. See Flowers et al., Chemistry 2e, periodic-table organization.

A member of the actinide series of chemical elements. Their oxidation states, ionic sizes and radioactive isotopes matter in nuclear fuel and waste chemistry.

A member of the lanthanide element series, whose ionic-size and redox trends reflect the filling and incomplete shielding of the 4f shell.

A conventional measure of atomic extent. Covalent, metallic, ionic and van der Waals definitions differ, so one universal hard-sphere radius should not be assumed.

Relative reluctance of a heavier main-group ns electron pair to participate in the higher oxidation state. Retained pairs can nevertheless influence structure through mixing and stereochemical activity.

The reduction in size across the lanthanide series associated with incomplete screening by $4f$ electrons; it contributes to size similarities and contractions in subsequent $5d$ elements.

A periodic group is a vertical column of the periodic table. For example, fluorine and chlorine are in periodic group 17, with seven valence electrons in each neutral ground state chemical atom. A shared outer electron configuration supports similarities in chemical reactions, rather than equality of every property.

A periodic table period is a horizontal row of the periodic table, read in increasing atomic number. Periodic table period 2 runs from lithium, $Z=3$, to neon, $Z=10$: $10-3+1=8$ chemical elements. In these neutral ground states, the second electron shell gains electrons while the first retains two.

The chemical element with atomic number five.

An isotope of boron with 5 protons and 5 neutrons. Its nuclear spin concerns the coupled nuclear quantum state, not the electronic configuration.

The chemical element with atomic number nine.

The chemical element with atomic number thirteen.

The oxide with formula $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$. Crystal structure, porosity and surface chemistry distinguish its different material forms.

The chemical element with atomic number fifteen.

A molecular allotrope consisting of tetrahedral ${\mathrm{P}}_4$ molecules. Every vertex has three P–P bonds and one lone pair; its $60^{\circ }$ P–P–P angles produce substantial angle strain.

The chemical element with atomic number sixteen.

${\mathrm{S}}_4^{2+}$, a square-planar sulfur ring with a useful six-pi-electron aromatic description. Its 22 valence electrons are partitioned differently from a five-vertex Zintl cluster.

The chemical element with atomic number seventeen.

A chemical element in group 18 of the periodic table. Argon is a familiar example: its chemical inertness under many ordinary conditions makes it useful when a process needs a gas that does not readily react with the material being processed. Chemical inertness is a statement about conditions, rather than an assertion that no compound can exist.

The chemical element with atomic number eighteen. Argon is a colourless noble gas. It can provide a protective atmosphere around hot, reactive materials, for example in welding, where excluding oxygen limits oxidation. Commercial production separates it by distillation of liquid air.^[ref]

Its discovery shows why a small disagreement can matter more than a reassuringly repeatable result. Two preparations can each give consistent measurements while producing different mixtures. The Rayleigh-Ramsay argon density experiment investigated just such a discrepancy. Gas density links the weighing to a mean molar mass; a separate separation process tests which component accounts for it.

Argon-40 is a stable isotope with 18 protons and 22 neutrons. Radiogenic argon-40 accumulated from potassium-40 is a daughter product used in potassium-argon dating.

In his 1895 lecture, Lord Rayleigh compared gas masses in a globe under matched pressure and temperature. Removing oxygen from air left gas about half a percent heavier than nitrogen prepared entirely from chemical sources. Several preparation routes reproduced the difference; hydrogen contamination was a concern. An eight-month storage test re-established the filling conditions: simply reweighing a sealed globe could not test a change in molar mass. William Ramsay concentrated the residual gas by circulating the mixture over hot magnesium, which consumed nitrogen. A spectral test checked remaining nitrogen. Agreement of spectra from this route and the alternative spark-and-alkali separation supported a common residual gas.^[ref]

The density difference alone identifies neither a unique chemical element nor its concentration. A heavier contaminant, a lighter reference contaminant, or unmatched filling conditions can all shift a weighing. A separation process and an optical spectrum provide additional observations. This combination makes the experiment powerful: the anomaly motivates another operation whose result can be checked independently.

Consider an original simplified model containing only nitrogen and argon, with argon mole fraction $x$. Let their molar masses be $M_N$ and $M_A$. Adding the component masses gives the mixture's mean molar massFor an ideal gas mixture at the same pressure and temperature as a pure-nitrogen reference, the gas density ratio isThe inversion requires known identities, the binary-mixture assumption and $M_A\ne M_N$. A density ratio cannot uniquely recover an arbitrary multicomponent mixture. Nor should $x$ be interpreted as the argon fraction of untreated air: the model sample has only the two specified components.

Choose round model values $M_N=28\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ and $M_A=40\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$. For $x=0.010$, the calculated relative density excess isThus a one-percent mole fraction does not mean a one-percent gas density change. In a chosen $2.00\mathrm{L}$ globe at $100\mathrm{kPa}$ and $300\mathrm{K}$, the ideal gas law gives approximately $2.24508\mathrm{g}$ for pure nitrogen and $2.25471\mathrm{g}$ for this mixture: only $9.62\mathrm{mg}$ separates the weighings. These are model calculations with rounded molar masses, not historical measurements.

A direct control calculation explains why the filling conditions matter. At fixed pressure, pure nitrogen athas the same gas density as that mixture at $300\mathrm{K}$. A roughly $1.28\mathrm{K}$ mismatch can therefore imitate the entire stipulated composition signal. For small changes, the ideal gas law givesAn independently measured pressure and temperature, matched weighing procedure, and checks for residual vapour distinguish composition from these alternative causes. The model explains the need for controls; it does not supply the complete corrections for a real gas-weighing instrument.

Sodium is the chemical element of atomic number eleven. Carbon-fusion channels in carbon burning include ${}^{12}\mathrm{C}+{}^{12}\mathrm{C}⟶{}^{23}\mathrm{Na}+p$, producing sodium-23 and a proton. Its abundance in expelled stellar material depends on later reactions and mixing, rather than on this one production channel alone.

A calcium ion with one electron removed, leaving a closed 18-electron core and one valence electron. It is distinct from the divalent calcium ion used in much biological terminology. Its bound electronic levels and ionization continuum support the Ca II optical series.

In singly ionized calcium, $np{}^{2}P\to 4s{}^{2}S$ resonance doublets, $ns{}^{2}S\to 4p{}^{2}P$ doublets and $np{}^{2}P\to 3d{}^{2}D$ three-component groups approach the same ionization continuum from different lower terms. A three-line group need not have spin multiplicity three. The electric-dipole rule excludes $P_{1/2}\to D_{5/2}$.^[ref]

The divalent cation ${\mathrm{Ca}}^{2+}$. Changes in its free intracellular concentration participate in calcium signaling; total cellular calcium includes substantial bound and sequestered pools.

Calcium-40 is an even-even nucleus with the closed-shell values $Z=N=20$. Its extra stability illustrates nuclear shell corrections beyond a smooth semi-empirical mass formula.

Calcium hydroxide supplies calcium cations and hydroxide anions on dissolution in water. The ideal one-to-two stoichiometry relates molar solubility to the dissolved concentrations; solubility product measurement by titration explains how the hydroxide inventory is measured.

Potassium-40 is a long-lived radioactive isotope with 19 protons and 21 neutrons. It can decay to calcium-40 through beta-minus decay, or to argon-40 through electron capture and a rare beta-plus decay. The argon-producing branch supports potassium-argon dating.

The atomic number $Z$ is the number of protons in an atomic nucleus, and fixes the chemical element. It is a count, with no units. An atomic nucleus with six protons is always carbon, regardless of how many neutrons accompany them or how many electrons surround it. See the IUPAC Gold Book, atomic number.

Three independent bookkeeping questions become easy once their symbols are kept separate. Let $N$ count neutrons, $n_e$ count electrons, $A$ be the mass number, and $Q$ be the net electric charge. A proton carries electric charge $+e$ and an electron carries $-e$, where $e$ is the positive elementary charge; a neutron carries zero net electric charge. HenceThe reasoning is just counting: add protons and neutrons for $A$, and subtract negative electron contributions from positive proton contributions for $Q$. A neutral chemical atom has $Q=0$, so $n_e=Z$. For a positively charged ion, fewer electrons remain. Notation ${}_Z^A\mathrm{X}$ gives the mass number at upper left and atomic number at lower left, with $\mathrm{X}$ standing for the chemical element; a right superscript such as $+$ specifies the electric charge in units of $e$. See Flowers et al., Chemistry 2e, atomic structure and symbolism.

For neutral carbon-12, ${}_6^{12}\mathrm{C}$, the atomic number gives six protons, the subtraction $N=12-6=6$ gives six neutrons, and neutrality gives six electrons. Neutral carbon-13, ${}_6^{13}\mathrm{C}$, instead has $N=13-6=7$ neutrons, with the same six protons and six electrons. Thus these two isotopes occupy the same carbon box in the periodic table.

For neutral sodium of mass number $23$, ${}_{11}^{23}\mathrm{Na}$, the counts are eleven protons, $23-11=12$ neutrons, and eleven electrons. Removing one electron gives the ion ${}_{11}^{23}{\mathrm{Na}}^{+}$: $Q/e=1$, so $n_e=11-1=10$. Both its atomic number and its twelve neutrons stay unchanged. This is ionization, not conversion into the chemical element neon, even though a neutral neon chemical atom also has ten electrons.

This bookkeeping is useful whenever a symbol must be translated into a physical inventory. Mass number answers how many protons plus neutrons there are; electric charge answers the imbalance between protons and electrons; atomic number alone answers which chemical element it is. Keeping those questions distinct prevents an isotope or an ion from being mistaken for a different chemical element.

Beryllium is the chemical element with atomic number four. Its isotope beryllium-7 provides an important route to primordial lithium-7.

An isotope of beryllium with 4 protons and 6 neutrons. Its nuclear spin concerns the coupled nuclear quantum state, not the electronic configuration.

Beryllium-7 contains four protons and three neutrons. It forms in reactions including ${}^3\mathrm{He}+{}^4\mathrm{He}\to {}^7\mathrm{Be}+\gamma$ and becomes lithium-7 through electron capture. In the early universe its ionization state affects when capture occurs, so the final primordial lithium inventory includes the earlier beryllium inventory rather than just lithium present when nuclear burning ends.

Lithium is the chemical element with atomic number three. Its stable isotopes include lithium-6 and lithium-7. Lithium observed in old stellar atmospheres is an important but processing-sensitive test of Big Bang nucleosynthesis.

The Li3 molecule, with three valence electrons and a bound distorted triangular structure. The equilateral open-shell arrangement is subject to Jahn-Teller coupling; electron counting alone does not establish dissociation energetics.

The Li2 molecule. Its two valence electrons occupy a primarily 2s-derived bonding orbital; filled 1s-derived bonding and antibonding core levels do not give an additional net simple bond-order contribution.

Lithium-7 has three protons and four neutrons. In Big Bang nucleosynthesis it can form directly or through beryllium-7, which later undergoes electron capture. Competition between direct lithium production, lithium destruction and beryllium production produces a valley in the primordial lithium abundance versus baryon-to-photon ratio. Its measured stellar surface abundance need not equal its birth abundance because stellar transport and nuclear burning can remove lithium.

A nuclear species specified by atomic number, mass number, and nuclear energy state. Isotopes compare nuclides with the same atomic number; changing only the nuclear energy state does not create a different isotope.

Isotopes of a chemical element have the same atomic number $Z$, hence the same number of protons, but different neutron numbers $N$ and therefore different mass numbers $A=Z+N$. For example, every neon atomic nucleus contains ten protons: neon-20, neon-21, and neon-22 contain ten, eleven, and twelve neutrons, respectively. They occupy one chemical element box in the periodic table, rather than three boxes.

Three labels answer different questions. The chemical element fixes $Z$; the isotope specifies which neutron number accompanies that $Z$; the ion label specifies the net electric charge. Neutral ${}_{10}^{22}\mathrm{Ne}$ and the ion ${}_{10}^{22}{\mathrm{Ne}}^{+}$ both contain ten protons and twelve neutrons; they contain ten and nine electrons, respectively. Ordinary ionization changes the electron count, leaving the isotope unchanged. Changing the neutron number can change nuclear binding energy and the possibility of radioactive decay. Isotopic abundances can help distinguish material sources, while radioactive decay can additionally supply chronological information.

A mass spectrum makes the distinction experimentally visible: a single chemical element can give separate ion peaks at different mass-to-charge ratios. Its bulk atomic weight is a weighted mean governed by isotopic abundances, rather than the location of one compulsory peak. The historical mass spectrograph experiment and the synthetic neon mass spectrum below illustrate these complementary observations.