The discoveries of mendelevium, nobelium, and lawrencium followed. The reaction was further studied in 1988 to study the aqueous chemistry of lawrencium. The fourth element, either actinium or lawrencium, has only radioactive isotopes. Isotopes of lawrencium have also been identified in the decay of heavier elements. Lr is the lightest isotope of lawrencium to be produced to date. Lawrencium (Lr) is a synthetic element, and thus a standard atomic weight cannot be given. In 2015, the first ionization energy of lawrencium was measured, using the isotope Lr. In other tables, lutetium and lawrencium are the remaining group 3 members. They are commonly lanthanum and actinium, and less often lutetium and lawrencium. Sg, and lawrencium continues the trend with Lr being more stable than Rf. A lawrencium atom has 103 electrons, of which three can act as valence electrons. Very little is known about lawrencium, and none of its physical properties have been confirmed. Lawrencium, as an analogue of lutetium, would presumably display like characteristics.