Chemistry — Periodic Trends: Atomic Radius & Ionisation Energy

What is atomic radius?

the size of an atom from its nucleus out to its outer electrons

What happens to atomic radius as you go down a group?

it increases

Why does atomic radius increase down a group?

each step adds a higher energy shell, so the outer electrons sit further out and are more shielded

What happens to atomic radius as you go across a period from left to right?

it decreases

Why does atomic radius decrease across a period?

more protons pull the outer electrons of the same shell in more tightly

What is ionisation energy?

the energy needed to remove the highest-energy electron from an atom

What happens to ionisation energy as you go down a group?

it decreases

Why does ionisation energy decrease down a group?

the outer electron is further out and more shielded, so it is easier to remove

What happens to ionisation energy as you go across a period from left to right?

it increases

Why does ionisation energy increase across a period?

more protons hold the outer electrons more tightly, so they are harder to remove

What is shielding?

when inner core electrons reduce the pull of the nucleus on the outer electrons

What is effective nuclear charge?

the net pull an outer electron actually feels, after accounting for shielding and distance

What three things affect how strongly an outer electron is held?

the number of protons, the shielding from inner electrons, and the distance from the nucleus

How are atomic radius and ionisation energy related?

they are inverse, so a bigger atom is easier to ionise and a smaller atom is harder

Why does a larger atom tend to have a lower ionisation energy?

its outer electron is far out and well shielded, so it is easily removed

Why does a smaller atom tend to have a higher ionisation energy?

its outer electron is held tightly close to the nucleus

Which direction on the table gives the largest atoms?

down and to the left

Which direction on the table gives the highest ionisation energy?

up and to the right

Down a group, valence electrons stay the same, so why does the radius grow?

the outer electrons occupy higher energy shells further from the nucleus

Across a period, why don't the added electrons increase the radius?

they fill the same outer shell while extra protons pull that shell inward

What is a simple summary of the two trends?

radius grows down and to the left, while ionisation energy grows up and to the right