Consider the following atomic radii for the elements in Period 3 and Period 4 of

Question

Consider the following atomic radii for the elements in Period 3 and Period 4 of the Periodic Table. Explain why the atomic radii decrease going across a period (i.e., from left to right). With one exception (Ga), the atomic radius of an element in Period 4 is greater than the atomic radius of the element in the same Group in Period 3. Explain why the elements in Period 4 tend to have greater atomic radii than the elements in the same group in Period 3. In Period 4, the difference between the atomic radii of Ca and Ga is much larger than the difference between the atomic radii of all other adjacent pairs of elements. Explain.

Explanation / Answer

a]

Atomic radius decreases because you are adding protons to the nucleus as you move from the left to the right of the period. These additional protons pull with more force on the electrons outside the nucleus, drawing them all in closer, and making the radius of the atom smaller

b]

   In a group from top to bottom, the atoms get one addition energy level/ shell full of electrons.. which means those electrons can shield the strong positive pull of the nucleus, so the radius increases down the group..

c]

The reason d-orbitals make a difference is that electrons in d-orbitals do not screen nuclear charge as effectively as those in s and p orbitals. This is because of something called penetration .. essentially the mathematical shapes of d-orbitals prevent them from allowing electrons to penetrate very closely to the nucleus, compared with electrons in s or p-orbitals. In gallium, you have 10 electrons in the filled 3d-subshell, and each of these electrons is doing a slightly worse job (relatively speaking) of screening the nuclear charge than the electrons in the s and p orbitals. Therefore, the effective nuclear charge in gallium is very much higher than Ca ...SO thers is a a sharp decrease in atomic radius.

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