A formula unit is a specified compositional accounting entity for a chemical substance that need not consist of discrete molecules. For ordinary crystalline $\mathrm{NaCl}$, one formula unit counts one sodium ion ${\mathrm{Na}}^{+}$ and one chlorine ion ${\mathrm{Cl}}^{-}$; it does not identify an isolated pair within the crystal. Cohen et al., IUPAC Green Book, 3rd ed., §2.10.1(iii)(b), p. 51 defines a formula unit as a specified group of chemical atoms, rather than a unit of measurement. An amount of substance of one mole of $\mathrm{NaCl}$ formula units therefore corresponds to one mole of each kind of ion.

A molecular formula gives the number of chemical atoms of each chemical element in a molecule. For urea, ${\mathrm{CH}}_4{\mathrm{N}}_2\mathrm{O}$ counts one carbon, four hydrogen, two nitrogen and one oxygen chemical atom. It records composition rather than the arrangement of chemical bonds.

The chemical element with atomic number 26. Iron-group nuclear binding energy and silicon burning are central to the termination of ordinary energy-producing stellar fusion.

Manganese is the chemical element with atomic number 25 and symbol $\mathrm{Mn}$. It is a transition metal. It has many accessible oxidation states. Manganese-containing steel uses its materials chemistry, while manganese oxides and biological metal centers exploit redox behavior; naming the particular compound is necessary to identify which role applies.^[ref]

The chemical element with atomic number 28. Radioactive nickel-56 is produced in explosive burning; the nickel-56 decay chain contributes energy to supernova emission.

The chemical element with atomic number 27. Cobalt-56 is the intermediate radioactive nucleus in the nickel-56 decay chain.

The chemical element with atomic number 10. Neon burning uses photodisintegration and alpha capture, while neon-22 can provide neutrons through ${}^{22}\mathrm{Ne}(\alpha ,n){}^{25}\mathrm{Mg}$.

The chemical element with atomic number 12. Magnesium isotopes occur among products of carbon burning and alpha-capture neutron-source reactions.

The chemical element with atomic number 14. Oxygen burning produces silicon-group nuclei; subsequent silicon burning is a reaction network approaching an iron-group mixture.

The chemical element with atomic number 56. Enrichment in barium can reveal transferred slow neutron-capture process products, as in a barium star.

The chemical element with atomic number 82. Lead isotopes are important heavy products of the slow neutron-capture process in suitable neutron-exposure conditions.

The chemical element with atomic number 92. Its long-lived radioactive isotopes illustrate actinide production by the rapid neutron-capture process.

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.

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.

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.

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

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.

Potassium is the chemical element with atomic number 19 and symbol $\mathrm{K}$. It is a alkali metal. It commonly forms ${\mathrm{K}}^{+}$. The ion plays a major role in cellular electrical gradients, whereas elemental potassium is a reactive metal; nutritional or physiological statements about potassium normally concern its compounds and ions.^[ref]

Calcium is the chemical element with atomic number 20 and symbol $\mathrm{Ca}$. It is a alkaline earth metal. Its common ion is $\mathrm{C}{\mathrm{a}}^{2+}$. Calcium in calcium carbonate or a biological signaling system is present in chemically bound or solvated forms, rather than as elemental metal; this distinction is essential when connecting mineral and biological chemistry.^[ref]

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.