"Uranium atoms" refers to the individual particles that make up the element uranium. They are tiny building blocks that have specific properties and can combine to form uranium compounds and materials. Full definition
They space themselves within the layers and alter the structure by causing the layers of uranium atoms above and below to draw closer to the oxygen. (pnnl.gov)
Instead, oxygen atoms pull in a bit of the negative charge from the nearest uranium atoms, from the next nearest, and from the next - next nearest. (pnnl.gov)