Lithium: lyndismálmur (mood-metal)

The best name for lithium is, in my opinion "laufsteinsmálmur", the "metal in a leafy stone", so-named because of the leafy flakes in the mineral in which it was originally found, nl. petallite (laufsteinn, laufspat). The metal was named after its MINERAL origin, because the only known alkali-metals known at the time of the discovery, were obtained from PLANT sources. The construction "Laufsteinsmálmur (leaf-stone-metal) is found upon the same idea as what the international name was found upon, its mineral, stony (Greek "lithos", stone), with this difference that the "leafy" nature of the stone must be specified, because "steinsmálmur" (metal form a stone), would make no sense.

Yet, there's another one, this time based on a unique property of the element: "lyndismálmur" ("lyndi" means "mental condition" and refers to lithium use, in the form of compounds, like lithium carbonate (kolsúr lyndismálmur, "carbacidic mood-metal" , for psychiatric conditions like bipolar disorder and schizophrenia.)

Plutonium and Nobelium

Plutonium is the homologue of Samarium in the lanthanide series, which was translated "samreskjumálmur" (samreskja is the Icelandic adaptation of Russian name "samarski", after other Old Icelandic adaptations of Russian toponyms: polotsk (Pallteskja) and Smolensk (Smáleskja).  The construction for plutonium becomes: samreskjublý (samarish lead).

Tungsten:
Another one for tungsten. The international name is "wolfram", named after the nickname  of the mineral scheelite "lupi spuma" (froth of the wolf), because it devoured tin in the smelter. It is, however not necessary make reference to a wolf: átfrauðmálmur (át- (eating away, corroding) + frauð (foam) + málmur (metal), or átfrauðglit, a construction in which "glit" stands for "metallic lustre".

Tungsten (Wolfram)

A new possibility is "Fenrisglit" (Fenrir (wolf in Old Norse Mythology) + glit (metallic lustre)
The other ones are "þungsteinn", "vargfróðumálmur", ylfi (i-shift of "úlfur") and baskamálmur, because it was discovered by two brothers of the Basque Royal Society.

Ginnungagufa og hjáfrumloft

A poetic name for hydrogen would be "ginnungagufa". The old mythical prefix "ginnunga-" occurs in "ginnungap", which stands for "the great void, the emptiness before heaven and earth was created. Only two other words occur in Old Icelandic writs that have this prefix:
1) Ginnunga-himinn (the "void" of the "ginnungagap", himinn = heaven) anhe great vault of heaven, 2) Ginnunga-vé ("vé" in the meaning of "holy place", the universe)
This examples show that "ginnungagufa" (gufa = steam, but also "gaseous substance" in general) could serve as a poetic name for the element, much mor elegant than "frumloft" ("frum-" (original) + "loft" (air, gaseous substance)
In de case of helium, the shortest compound would be "hjáfrumloft" (hjá- (additional, co-) and "frumloft" original gas,  a syllable less long than "frumeðalloft" (original noble gas). Hydrogen and helium are the primordial gases of the universe. Another possibility is "ifreðli", from "ifröðull", an eddaic name for the sun, in which "röðull" is i-shifted to "-reðli". A shorter possibility is just "reðli", from "röðull", which is also an poetic name for the sun in the Snorra Edda. "Röðull" is most probably related to the german "rad", because the sun was thought to be a disk.

Barium: Boloníumálmur

Apart from "þungjörðungur", derived from "þungjörð" ("heavy earth" or "baria", BaO), we can name the metal after the old name of barite (BaSO4): "bologna stone". Many element are named after countries or place-names, so we could call Barium "bologne-metal", Boloníumálmur ("Boloníia" is the form of the city's name use in Old Icelandic texts)

An update

Update:

Ruthenium: Hrærekssilfur (Ruthenium was called after an old name for Russia. Why not naming the element after the founder of the Russian nation, Rurik (Icelandic: "Hrærekur"). Many element were named after individuals.

Uranium: ýmisefni, aurgelmisblý (One could name uranium after the equivalent of Uranus in Nordic mythology: Ýmir (or "Aurgelmir"). The construction Aurgelmisblý sound-similar to uranium in the first syllable. Because the element was named after the planet Uranus and not after the god, after whom the planet was named, it would be appropriate to use Aurgelmir for the gas-giant too.

Tantalum: Tálvonarglit (Tálvonarlegur (tantalizing) + glit (metallic lustre)

Lathanides: "Lotuþribblingar" or "lotujarðmálmur": þribbil (element in group 3B: þrí- + B +-ill)

Hafium: Hafnarþungsteinn (hafnar- refering to Copenhagen, þunsteinn (wolfram)

Wolfram: Vargfróðumálmur (an alternative, after "wolfram" (the froth of a wolf, because tungsten-containing scheelite devoured tin in the smelter)

Titanium: Forguðamálmur (forguð= preguðir (pre-gods, the Titans were the Elder gods or "pre-gods" in Greek Mythology)

Krypton: stikuloft (Once krypton was used to exactly establish the length of a metre)

Nobelium: Elfráðsblý (the shortest construction (Elfráður (Alfred (Alfred Nobel) + blý (lead))

Europium: Eftnumálmur (Eftna = Europe (i-shit of "aftan" (evening), the "Evening" land, German "Abendland", Icel. kvöldlönd, Kvöldálfa, Kvoldmeginland, but the shortest construction is "Eftna", which is as short as the second name for Europa in the Snorra Edda (Enea). It's best to keep the name feminine like Evrópa. Other derivation from "aftan": Eftningur (European), Afneskur (European), Aftnesku bandalagið (European Union), or "Eftningabandalagið".

Sulphur: Múspellshallur (synonym, "Múspell" was the "Mordor" in Nordic Mythology.

monosyllabic term for cadmium possible in Icelandic: tink

Cadmium is the homolog of zinc (Icelandic: sink) two places to the left of "tin": sink + tin = TINK