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.

The chemical element with atomic number seventeen.

The chemical element with atomic number sixteen.

The chemical element with atomic number fifteen.

The chemical element with atomic number thirteen.

The chemical element with atomic number nine.

The chemical element with atomic number five.

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.

Copper is the chemical element with atomic number 29 and symbol $\mathrm{Cu}$. It is a transition metal. Its high electrical conductivity supports wiring, and $+1$ and $+2$ are common oxidation states. Copper-containing alloys include brass and bronze, whose properties differ from those of the pure metal.^[ref]

Zinc is the chemical element with atomic number 30 and symbol $\mathrm{Zn}$. It is a metal. Its common ion is $\mathrm{Z}{\mathrm{n}}^{2+}$. Coating iron with zinc protects it through both a surface barrier and preferential zinc oxidation; zinc in an enzyme instead has a coordinated ionic role rather than behaving as a bulk metallic coating.^[ref]

Platinum is the chemical element with atomic number 78 and symbol $\mathrm{Pt}$. It is a transition metal. It resists many corrosive environments but can be highly active at its surface as a catalyst. This contrast shows that bulk chemical persistence and catalytic surface activity are compatible properties.^[ref]

Mercury is the chemical element with atomic number 80 and symbol $\mathrm{Hg}$. It is a metal. It is liquid near ordinary room temperature, making it a familiar exception to the usual solid state of metals. Mercury(I) commonly contains the dimeric cation $\mathrm{H}{\mathrm{g}}_2^{2+}$, while mercury(II) has different chemistry.^[ref]

Tungsten is the chemical element with atomic number 74 and symbol $\mathrm{W}$. It is a transition metal. Its very high melting point supports high-temperature applications. Electron emission from a heated tungsten filament connects thermal energy with the escape of electrons from a metal surface.^[ref]

Tin is the chemical element with atomic number 50 and symbol $\mathrm{Sn}$. It is a metal. Both $+2$ and $+4$ oxidation states are common. Its different solid allotropes have different structures, and tin-containing solders illustrate how alloy composition can change melting and mechanical properties.^[ref]

Nicotinamide adenine dinucleotide phosphate is a cofactor used in metabolism. Its oxidized and reduced forms, NADP+ and NADPH, carry electrons between coupled chemical reactions. In oxygenic photosynthesis, light-dependent reactions produce NADPH for the Calvin cycle, which returns NADP+ for reuse. ^[ref].

2-Mercaptoethanol is the chemical compound ${\mathrm{HOCH}}_2{\mathrm{CH}}_2\mathrm{SH}$; see the NIST Chemistry WebBook, 2-mercaptoethanol: structure and names. In the ribonuclease A refolding experiment, it breaks protein disulfide bonds, while the peptide bonds of the protein backbone remain intact. This separates a change in crosslinks from cutting the polypeptide chain.

Urea is the chemical compound with molecular formula ${\mathrm{CH}}_4{\mathrm{N}}_2\mathrm{O}$ and structure $({\mathrm{NH}}_2{)}_2\mathrm{CO}$. See the NIST Chemistry WebBook, urea: formula and structure. Concentrated urea is used to promote protein denaturation in the ribonuclease A refolding experiment.

An isocyanate bears an $\mathrm{R}-\mathrm{N}=\mathrm{C}=\mathrm{O}$ group. A molecule bearing two such groups can react with a diol to form polyurethane connections.

A diol has two hydroxy groups. Its two groups can serve as joining ends in polyaddition; the rest of its molecule can already be a long chain.

Ellman's reagent is 5,5'-dithiobis(2-nitrobenzoic acid), abbreviated DTNB, a disulfide used to quantify accessible thiols. Reaction releases the colored TNB species measured by absorbance at wavelength $412\mathrm{nm}$. The disulfide bond experiment above explains why a thiol measurement can become an indirect bridge count only after controlled reduction. ^[ref]