diff --git a/README.md b/README.md
index f00c1b6..96e67e6 100644
--- a/README.md
+++ b/README.md
@@ -80,8 +80,7 @@ julia> elements[1:4]
  A nice interactive visualization of the periodic table, based on PeriodicTable.jl, can be found [here](https://github.com/lancebeet/julia-scripts#periodictabletoyjl).
 
 ### Data by
-The data used for this package has been pulled up in parts from [here](https://github.com/Bowserinator/Periodic-Table-JSON).
-Some information has been (and will be) added over time.
+The data used for this package has been pulled up in parts from [here](https://github.com/Bowserinator/Periodic-Table-JSON). Atomic weights are from the [IUPAC 2021 tabulation](https://iupac.qmul.ac.uk/AtWt/)
 
 ### Related packages
 [Mendeleev](https://github.com/Eben60/Mendeleev.jl) is [API compatible](https://eben60.github.io/Mendeleev.jl/#Compatibility-Issues) with the `PeriodicTable` and contains much broader range of data on chemical elements. [IsotopeTable](https://github.com/Gregstrq/IsotopeTable.jl), also inspired by `PeriodicTable`, provides data on various isotopes.
diff --git a/src/elements.jl b/src/elements.jl
index e296bb1..1bd533f 100644
--- a/src/elements.jl
+++ b/src/elements.jl
@@ -49,7 +49,7 @@ const _elements_data = [
             ypos=1),
     Element(name="Helium",
             appearance="colorless gas, exhibiting a red-orange glow when placed in a high-voltage electric field",
-            atomic_mass=4.0026022u,
+            atomic_mass=4.002602u,
             boil=4.222K,
             category="noble gas",
             cpk_hex="#d9ffff",
@@ -89,7 +89,7 @@ const _elements_data = [
             ypos=2),
     Element(name="Beryllium",
             appearance="white-gray metallic",
-            atomic_mass=9.01218315u,
+            atomic_mass=9.0121831u,
             boil=2742.0K,
             category="alkaline earth metal",
             cpk_hex="#c2ff00",
@@ -187,7 +187,7 @@ const _elements_data = [
             xpos=16,
             ypos=2),
     Element(name="Fluorine",
-            atomic_mass=18.9984031636u,
+            atomic_mass=18.998403163u,
             boil=85.03K,
             category="diatomic nonmetal",
             cpk_hex="#90e050",
@@ -207,7 +207,7 @@ const _elements_data = [
             ypos=2),
     Element(name="Neon",
             appearance="colorless gas exhibiting an orange-red glow when placed in a high voltage electric field",
-            atomic_mass=20.17976u,
+            atomic_mass=20.1797u,
             boil=27.104K,
             category="noble gas",
             cpk_hex="#b3e3f5",
@@ -227,7 +227,7 @@ const _elements_data = [
             ypos=2),
     Element(name="Sodium",
             appearance="silvery white metallic",
-            atomic_mass=22.989769282u,
+            atomic_mass=22.98976928u,
             boil=1156.09K,
             category="alkali metal",
             cpk_hex="#ab5cf2",
@@ -269,7 +269,7 @@ const _elements_data = [
             ypos=3),
     Element(name="Aluminium",
             appearance="silvery gray metallic",
-            atomic_mass=26.98153857u,
+            atomic_mass=26.9815384u,
             boil=2743.0K,
             category="post-transition metal",
             cpk_hex="#bfa6a6",
@@ -311,7 +311,7 @@ const _elements_data = [
             ypos=3),
     Element(name="Phosphorus",
             appearance="colourless, waxy white, yellow, scarlet, red, violet, black",
-            atomic_mass=30.9737619985u,
+            atomic_mass=30.973761998u,
             category="polyatomic nonmetal",
             cpk_hex="#ff8000",
             density=1.82g/cm^3,
@@ -370,7 +370,7 @@ const _elements_data = [
             ypos=3),
     Element(name="Argon",
             appearance="colorless gas exhibiting a lilac/violet glow when placed in a high voltage electric field",
-            atomic_mass=39.9481u,
+            atomic_mass=39.95u,
             boil=87.302K,
             category="noble gas",
             cpk_hex="#80d1e3",
@@ -390,7 +390,7 @@ const _elements_data = [
             ypos=3),
     Element(name="Potassium",
             appearance="silvery gray",
-            atomic_mass=39.09831u,
+            atomic_mass=39.0983u,
             boil=1032.0K,
             category="alkali metal",
             cpk_hex="#8f40d4",
@@ -410,7 +410,7 @@ const _elements_data = [
             xpos=1,
             ypos=4),
     Element(name="Calcium",
-            atomic_mass=40.0784u,
+            atomic_mass=40.078u,
             boil=1757.0K,
             category="alkaline earth metal",
             cpk_hex="#3dff00",
@@ -431,7 +431,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Scandium",
             appearance="silvery white",
-            atomic_mass=44.9559085u,
+            atomic_mass=44.955907u,
             boil=3109.0K,
             category="transition metal",
             cpk_hex="#e6e6e6",
@@ -451,7 +451,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Titanium",
             appearance="silvery grey-white metallic",
-            atomic_mass=47.8671u,
+            atomic_mass=47.867u,
             boil=3560.0K,
             category="transition metal",
             cpk_hex="#bfc2c7",
@@ -472,7 +472,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Vanadium",
             appearance="blue-silver-grey metal",
-            atomic_mass=50.94151u,
+            atomic_mass=50.9415u,
             boil=3680.0K,
             category="transition metal",
             cpk_hex="#a6a6ab",
@@ -493,7 +493,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Chromium",
             appearance="silvery metallic",
-            atomic_mass=51.99616u,
+            atomic_mass=51.9961u,
             boil=2944.0K,
             category="transition metal",
             cpk_hex="#8a99c7",
@@ -513,7 +513,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Manganese",
             appearance="silvery metallic",
-            atomic_mass=54.9380443u,
+            atomic_mass=54.938043u,
             boil=2334.0K,
             category="transition metal",
             cpk_hex="#9c7ac7",
@@ -533,7 +533,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Iron",
             appearance="lustrous metallic with a grayish tinge",
-            atomic_mass=55.8452u,
+            atomic_mass=55.845u,
             boil=3134.0K,
             category="transition metal",
             cpk_hex="#e06633",
@@ -554,7 +554,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Cobalt",
             appearance="hard lustrous gray metal",
-            atomic_mass=58.9331944u,
+            atomic_mass=58.933194u,
             boil=3200.0K,
             category="transition metal",
             color="metallic gray",
@@ -575,7 +575,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Nickel",
             appearance="lustrous, metallic, and silver with a gold tinge",
-            atomic_mass=58.69344u,
+            atomic_mass=58.6934u,
             boil=3003.0K,
             category="transition metal",
             cpk_hex="#50d050",
@@ -595,7 +595,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Copper",
             appearance="red-orange metallic luster",
-            atomic_mass=63.5463u,
+            atomic_mass=63.546u,
             boil=2835.0K,
             category="transition metal",
             cpk_hex="#c88033",
@@ -615,7 +615,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Zinc",
             appearance="silver-gray",
-            atomic_mass=65.382u,
+            atomic_mass=65.38u,
             boil=1180.0K,
             category="transition metal",
             cpk_hex="#7d80b0",
@@ -635,7 +635,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Gallium",
             appearance="silver-white",
-            atomic_mass=69.7231u,
+            atomic_mass=69.723u,
             boil=2673.0K,
             category="post-transition metal",
             cpk_hex="#c28f8f",
@@ -655,7 +655,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Germanium",
             appearance="grayish-white",
-            atomic_mass=72.6308u,
+            atomic_mass=72.630u,
             boil=3106.0K,
             category="metalloid",
             cpk_hex="#668f8f",
@@ -675,7 +675,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Arsenic",
             appearance="metallic grey",
-            atomic_mass=74.9215956u,
+            atomic_mass=74.921595u,
             category="metalloid",
             cpk_hex="#bd80e3",
             density=5.727g/cm^3,
@@ -693,7 +693,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Selenium",
             appearance="black, red, and gray (not pictured) allotropes",
-            atomic_mass=78.9718u,
+            atomic_mass=78.971u,
             boil=958.0K,
             category="polyatomic nonmetal",
             cpk_hex="#ffa100",
@@ -731,7 +731,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Krypton",
             appearance="colorless gas, exhibiting a whitish glow in a high electric field",
-            atomic_mass=83.7982u,
+            atomic_mass=83.798u,
             boil=119.93K,
             category="noble gas",
             cpk_hex="#5cb8d1",
@@ -751,7 +751,7 @@ const _elements_data = [
             ypos=4),
     Element(name="Rubidium",
             appearance="grey white",
-            atomic_mass=85.46783u,
+            atomic_mass=85.4678u,
             boil=961.0K,
             category="alkali metal",
             cpk_hex="#702eb0",
@@ -770,7 +770,7 @@ const _elements_data = [
             xpos=1,
             ypos=5),
     Element(name="Strontium",
-            atomic_mass=87.621u,
+            atomic_mass=87.62u,
             boil=1650.0K,
             category="alkaline earth metal",
             cpk_hex="#00ff00",
@@ -790,7 +790,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Yttrium",
             appearance="silvery white",
-            atomic_mass=88.905842u,
+            atomic_mass=88.905838u,
             boil=3203.0K,
             category="transition metal",
             cpk_hex="#94ffff",
@@ -810,7 +810,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Zirconium",
             appearance="silvery white",
-            atomic_mass=91.2242u,
+            atomic_mass=91.224u,
             boil=4650.0K,
             category="transition metal",
             cpk_hex="#94e0e0",
@@ -830,7 +830,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Niobium",
             appearance="gray metallic, bluish when oxidized",
-            atomic_mass=92.906372u,
+            atomic_mass=92.90637u,
             boil=5017.0K,
             category="transition metal",
             cpk_hex="#73c2c9",
@@ -850,7 +850,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Molybdenum",
             appearance="gray metallic",
-            atomic_mass=95.951u,
+            atomic_mass=95.95u,
             boil=4912.0K,
             category="transition metal",
             cpk_hex="#54b5b5",
@@ -870,7 +870,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Technetium",
             appearance="shiny gray metal",
-            atomic_mass=98.0u,
+            atomic_mass=97.0u,
             boil=4538.0K,
             category="transition metal",
             cpk_hex="#3b9e9e",
@@ -890,7 +890,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Ruthenium",
             appearance="silvery white metallic",
-            atomic_mass=101.072u,
+            atomic_mass=101.07u,
             boil=4423.0K,
             category="transition metal",
             cpk_hex="#248f8f",
@@ -910,7 +910,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Rhodium",
             appearance="silvery white metallic",
-            atomic_mass=102.905502u,
+            atomic_mass=102.90549u,
             boil=3968.0K,
             category="transition metal",
             cpk_hex="#0a7d8c",
@@ -930,7 +930,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Palladium",
             appearance="silvery white",
-            atomic_mass=106.421u,
+            atomic_mass=106.42u,
             boil=3236.0K,
             category="transition metal",
             cpk_hex="#006985",
@@ -950,7 +950,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Silver",
             appearance="lustrous white metal",
-            atomic_mass=107.86822u,
+            atomic_mass=107.8682u,
             boil=2435.0K,
             category="transition metal",
             cpk_hex="#c0c0c0",
@@ -970,7 +970,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Cadmium",
             appearance="silvery bluish-gray metallic",
-            atomic_mass=112.4144u,
+            atomic_mass=112.414u,
             boil=1040.0K,
             category="transition metal",
             cpk_hex="#ffd98f",
@@ -991,7 +991,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Indium",
             appearance="silvery lustrous gray",
-            atomic_mass=114.8181u,
+            atomic_mass=114.818u,
             boil=2345.0K,
             category="post-transition metal",
             cpk_hex="#a67573",
@@ -1011,7 +1011,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Tin",
             appearance="silvery-white (beta, β) or gray (alpha, α)",
-            atomic_mass=118.7107u,
+            atomic_mass=118.710u,
             boil=2875.0K,
             category="post-transition metal",
             cpk_hex="#668080",
@@ -1031,7 +1031,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Antimony",
             appearance="silvery lustrous gray",
-            atomic_mass=121.7601u,
+            atomic_mass=121.760u,
             boil=1908.0K,
             category="metalloid",
             cpk_hex="#9e63b5",
@@ -1050,7 +1050,7 @@ const _elements_data = [
             xpos=15,
             ypos=5),
     Element(name="Tellurium",
-            atomic_mass=127.603u,
+            atomic_mass=127.60u,
             boil=1261.0K,
             category="metalloid",
             cpk_hex="#d47a00",
@@ -1070,7 +1070,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Iodine",
             appearance="lustrous metallic gray, violet as a gas",
-            atomic_mass=126.904473u,
+            atomic_mass=126.90447u,
             boil=457.4K,
             category="diatomic nonmetal",
             cpk_hex="#940094",
@@ -1089,7 +1089,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Xenon",
             appearance="colorless gas, exhibiting a blue glow when placed in a high voltage electric field",
-            atomic_mass=131.2936u,
+            atomic_mass=131.293u,
             boil=165.051K,
             category="noble gas",
             cpk_hex="#429eb0",
@@ -1109,7 +1109,7 @@ const _elements_data = [
             ypos=5),
     Element(name="Cesium",
             appearance="silvery gold",
-            atomic_mass=132.905451966u,
+            atomic_mass=132.90545196u,
             boil=944.0K,
             category="alkali metal",
             cpk_hex="#57178f",
@@ -1128,7 +1128,7 @@ const _elements_data = [
             xpos=1,
             ypos=6),
     Element(name="Barium",
-            atomic_mass=137.3277u,
+            atomic_mass=137.327u,
             boil=2118.0K,
             category="alkaline earth metal",
             cpk_hex="#00c900",
@@ -1148,7 +1148,7 @@ const _elements_data = [
             ypos=6),
     Element(name="Lanthanum",
             appearance="silvery white",
-            atomic_mass=138.905477u,
+            atomic_mass=138.90547u,
             boil=3737.0K,
             category="lanthanide",
             cpk_hex="#70d4ff",
@@ -1168,7 +1168,7 @@ const _elements_data = [
             ypos=9),
     Element(name="Cerium",
             appearance="silvery white",
-            atomic_mass=140.1161u,
+            atomic_mass=140.116u,
             boil=3716.0K,
             category="lanthanide",
             cpk_hex="#ffffc7",
@@ -1188,7 +1188,7 @@ const _elements_data = [
             ypos=9),
     Element(name="Praseodymium",
             appearance="grayish white",
-            atomic_mass=140.907662u,
+            atomic_mass=140.90766u,
             boil=3403.0K,
             category="lanthanide",
             cpk_hex="#d9ffc7",
@@ -1208,7 +1208,7 @@ const _elements_data = [
             ypos=9),
     Element(name="Neodymium",
             appearance="silvery white",
-            atomic_mass=144.2423u,
+            atomic_mass=144.242u,
             boil=3347.0K,
             category="lanthanide",
             cpk_hex="#c7ffc7",
@@ -1248,7 +1248,7 @@ const _elements_data = [
             ypos=9),
     Element(name="Samarium",
             appearance="silvery white",
-            atomic_mass=150.362u,
+            atomic_mass=150.36u,
             boil=2173.0K,
             category="lanthanide",
             cpk_hex="#8fffc7",
@@ -1267,7 +1267,7 @@ const _elements_data = [
             xpos=8,
             ypos=9),
     Element(name="Europium",
-            atomic_mass=151.9641u,
+            atomic_mass=151.964u,
             boil=1802.0K,
             category="lanthanide",
             cpk_hex="#61ffc7",
@@ -1287,7 +1287,7 @@ const _elements_data = [
             ypos=9),
     Element(name="Gadolinium",
             appearance="silvery white",
-            atomic_mass=157.253u,
+            atomic_mass=157.25u,
             boil=3273.0K,
             category="lanthanide",
             cpk_hex="#45ffc7",
@@ -1307,7 +1307,7 @@ const _elements_data = [
             ypos=9),
     Element(name="Terbium",
             appearance="silvery white",
-            atomic_mass=158.925352u,
+            atomic_mass=158.925354u,
             boil=3396.0K,
             category="lanthanide",
             cpk_hex="#30ffc7",
@@ -1327,7 +1327,7 @@ const _elements_data = [
             ypos=9),
     Element(name="Dysprosium",
             appearance="silvery white",
-            atomic_mass=162.5001u,
+            atomic_mass=162.500u,
             boil=2840.0K,
             category="lanthanide",
             cpk_hex="#1fffc7",
@@ -1347,7 +1347,7 @@ const _elements_data = [
             ypos=9),
     Element(name="Holmium",
             appearance="silvery white",
-            atomic_mass=164.930332u,
+            atomic_mass=164.930329u,
             boil=2873.0K,
             category="lanthanide",
             cpk_hex="#00ff9c",
@@ -1367,7 +1367,7 @@ const _elements_data = [
             ypos=9),
     Element(name="Erbium",
             appearance="silvery white",
-            atomic_mass=167.2593u,
+            atomic_mass=167.259u,
             boil=3141.0K,
             category="lanthanide",
             cpk_hex="#00e675",
@@ -1387,7 +1387,7 @@ const _elements_data = [
             ypos=9),
     Element(name="Thulium",
             appearance="silvery gray",
-            atomic_mass=168.934222u,
+            atomic_mass=168.934219u,
             boil=2223.0K,
             category="lanthanide",
             cpk_hex="#00d452",
@@ -1406,7 +1406,7 @@ const _elements_data = [
             xpos=15,
             ypos=9),
     Element(name="Ytterbium",
-            atomic_mass=173.0451u,
+            atomic_mass=173.045u,
             boil=1469.0K,
             category="lanthanide",
             cpk_hex="#00bf38",
@@ -1426,7 +1426,7 @@ const _elements_data = [
             ypos=9),
     Element(name="Lutetium",
             appearance="silvery white",
-            atomic_mass=174.96681u,
+            atomic_mass=174.9668u,
             boil=3675.0K,
             category="lanthanide",
             cpk_hex="#00ab24",
@@ -1446,7 +1446,7 @@ const _elements_data = [
             ypos=9),
     Element(name="Hafnium",
             appearance="steel gray",
-            atomic_mass=178.492u,
+            atomic_mass=178.486u,
             boil=4876.0K,
             category="transition metal",
             cpk_hex="#4dc2ff",
@@ -1467,7 +1467,7 @@ const _elements_data = [
             ypos=6),
     Element(name="Tantalum",
             appearance="gray blue",
-            atomic_mass=180.947882u,
+            atomic_mass=180.94788u,
             boil=5731.0K,
             category="transition metal",
             cpk_hex="#4da6ff",
@@ -1488,7 +1488,7 @@ const _elements_data = [
             ypos=6),
     Element(name="Tungsten",
             appearance="grayish white, lustrous",
-            atomic_mass=183.841u,
+            atomic_mass=183.84u,
             boil=6203.0K,
             category="transition metal",
             cpk_hex="#2194d6",
@@ -1508,7 +1508,7 @@ const _elements_data = [
             ypos=6),
     Element(name="Rhenium",
             appearance="silvery-grayish",
-            atomic_mass=186.2071u,
+            atomic_mass=186.207u,
             boil=5869.0K,
             category="transition metal",
             cpk_hex="#267dab",
@@ -1529,7 +1529,7 @@ const _elements_data = [
             ypos=6),
     Element(name="Osmium",
             appearance="silvery, blue cast",
-            atomic_mass=190.233u,
+            atomic_mass=190.23u,
             boil=5285.0K,
             category="transition metal",
             cpk_hex="#266696",
@@ -1549,7 +1549,7 @@ const _elements_data = [
             ypos=6),
     Element(name="Iridium",
             appearance="silvery white",
-            atomic_mass=192.2173u,
+            atomic_mass=192.217u,
             boil=4403.0K,
             category="transition metal",
             cpk_hex="#175487",
@@ -1569,7 +1569,7 @@ const _elements_data = [
             ypos=6),
     Element(name="Platinum",
             appearance="silvery white",
-            atomic_mass=195.0849u,
+            atomic_mass=195.084u,
             boil=4098.0K,
             category="transition metal",
             cpk_hex="#d0d0e0",
@@ -1589,7 +1589,7 @@ const _elements_data = [
             ypos=6),
     Element(name="Gold",
             appearance="metallic yellow",
-            atomic_mass=196.9665695u,
+            atomic_mass=196.966570u,
             boil=3243.0K,
             category="transition metal",
             cpk_hex="#ffd123",
@@ -1609,7 +1609,7 @@ const _elements_data = [
             ypos=6),
     Element(name="Mercury",
             appearance="silvery",
-            atomic_mass=200.5923u,
+            atomic_mass=200.592u,
             boil=629.88K,
             category="transition metal",
             cpk_hex="#b8b8d0",
@@ -1649,7 +1649,7 @@ const _elements_data = [
             ypos=6),
     Element(name="Lead",
             appearance="metallic gray",
-            atomic_mass=207.21u,
+            atomic_mass=207.2u,
             boil=2022.0K,
             category="post-transition metal",
             cpk_hex="#575961",
@@ -1669,7 +1669,7 @@ const _elements_data = [
             ypos=6),
     Element(name="Bismuth",
             appearance="lustrous silver",
-            atomic_mass=208.980401u,
+            atomic_mass=208.98040u,
             boil=1837.0K,
             category="post-transition metal",
             cpk_hex="#9e4fb5",
@@ -1804,7 +1804,7 @@ const _elements_data = [
             ypos=10),
     Element(name="Thorium",
             appearance="silvery, often with black tarnish",
-            atomic_mass=232.03774u,
+            atomic_mass=232.0377u,
             boil=5061.0K,
             category="actinide",
             cpk_hex="#00baff",
@@ -1824,7 +1824,7 @@ const _elements_data = [
             ypos=10),
     Element(name="Protactinium",
             appearance="bright, silvery metallic luster",
-            atomic_mass=231.035882u,
+            atomic_mass=231.03588u,
             boil=4300.0K,
             category="actinide",
             cpk_hex="#00a1ff",
@@ -1843,7 +1843,7 @@ const _elements_data = [
             xpos=5,
             ypos=10),
     Element(name="Uranium",
-            atomic_mass=238.028913u,
+            atomic_mass=238.02891u,
             boil=4404.0K,
             category="actinide",
             cpk_hex="#008fff",
@@ -2047,7 +2047,7 @@ const _elements_data = [
             xpos=16,
             ypos=10),
     Element(name="Lawrencium",
-            atomic_mass=266.0u,
+            atomic_mass=262.0u,
             category="actinide",
             cpk_hex="#c70066",
             discovered_by="Lawrence Berkeley National Laboratory",
@@ -2081,7 +2081,7 @@ const _elements_data = [
             xpos=4,
             ypos=7),
     Element(name="Dubnium",
-            atomic_mass=268.0u,
+            atomic_mass=270.0u,
             category="transition metal",
             cpk_hex="#d1004f",
             density=29.3g/cm^3,
@@ -2129,7 +2129,7 @@ const _elements_data = [
             xpos=7,
             ypos=7),
     Element(name="Hassium",
-            atomic_mass=269.0u,
+            atomic_mass=270.0u,
             category="transition metal",
             cpk_hex="#e6002e",
             density=40.7g/cm^3,
@@ -2177,7 +2177,7 @@ const _elements_data = [
             xpos=10,
             ypos=7),
     Element(name="Roentgenium",
-            atomic_mass=282.0u,
+            atomic_mass=281.0u,
             category="unknown, probably transition metal",
             density=28.7g/cm^3,
             discovered_by="Gesellschaft für Schwerionenforschung",
@@ -2276,7 +2276,7 @@ const _elements_data = [
             xpos=16,
             ypos=7),
     Element(name="Tennessine",
-            atomic_mass=294.0u,
+            atomic_mass=293.0u,
             boil=883.0K,
             category="unknown, probably metalloid",
             density=7.17g/cm^3,
diff --git a/test/runtests.jl b/test/runtests.jl
index 4e65c80..d5c6394 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -86,3 +86,16 @@ end
 for g in [e for e in elements if e.phase=="Gas"]
     @test 5e-5u"g/cm^3" < g.density < 5e-2u"g/cm^3"
 end
+
+# Check atomic_mass against values from https://iupac.qmul.ac.uk/AtWt/
+a = [ 1.008, 4.002602, 6.94, 9.0121831, 10.81, 12.011, 14.007, 15.999, 18.998403163, 20.1797, 22.98976928, 24.305, 
+        26.9815384, 28.085, 30.973761998, 32.06, 35.45, 39.95, 39.0983, 40.078, 44.955907, 47.867, 50.9415, 51.9961, 
+        54.938043, 55.845, 58.933194, 58.6934, 63.546, 65.38, 69.723, 72.63, 74.921595, 78.971, 79.904, 83.798, 
+        85.4678, 87.62, 88.905838, 91.224, 92.90637, 95.95, 97.0, 101.07, 102.90549, 106.42, 107.8682, 112.414, 
+        114.818, 118.71, 121.76, 127.6, 126.90447, 131.293, 132.90545196, 137.327, 138.90547, 140.116, 140.90766, 
+        144.242, 145.0, 150.36, 151.964, 157.25, 158.925354, 162.5, 164.930329, 167.259, 168.934219, 173.045, 174.9668, 
+        178.486, 180.94788, 183.84, 186.207, 190.23, 192.217, 195.084, 196.96657, 200.592, 204.38, 207.2, 208.9804, 
+        209.0, 210.0, 222.0, 223.0, 226.0, 227.0, 232.0377, 231.03588, 238.02891, 237.0, 244.0, 243.0, 247.0, 247.0, 
+        251.0, 252.0, 257.0, 258.0, 259.0, 262.0, 267.0, 270.0, 269.0, 270.0, 270.0, 278.0, 281.0, 281.0, 285.0,
+        286.0, 289.0, 289.0, 293.0, 293.0, 294.0 ]
+@test all(map(z->isequal(ustrip(elements[z].atomic_mass), a[z]), eachindex(a)))
\ No newline at end of file