What are the trends in atomic size across transition metals?

Atomic size generally decreases across a period of transition metals due to increased nuclear charge without significant electron shielding.

In more detail, the atomic size or atomic radius refers to the distance from the nucleus to the outermost shell of an electron. As you move across a period in the periodic table from left to right, the atomic number increases, which means there are more protons in the nucleus. This increases the positive charge of the nucleus, which pulls the electrons closer and decreases the atomic size.

However, in the case of transition metals, there's a unique factor at play. The electrons in transition metals are added to an inner d-orbital, rather than the outermost shell. This means that these additional electrons do not contribute significantly to the shielding effect. The shielding effect refers to the phenomenon where inner shell electrons shield outer shell electrons from the full positive charge of the nucleus.

So, in transition metals, as you move from left to right, the nuclear charge increases with each additional proton, but the shielding effect remains relatively constant because the additional electrons are in an inner shell. This results in a stronger pull from the nucleus, drawing the outermost electrons closer and thus decreasing the atomic size.

It's important to note that this trend is not perfectly consistent. There are slight variations in atomic size among the transition metals due to other factors, such as electron-electron repulsion. However, the general trend is a decrease in atomic size across the period.