Why does the First Ionisation Energy decrease down a group in the Periodic Table?

Two major factors control how tightly held the outermost electron is and therefore how much energy is required to remove it, which gives us the size of the Ionisation energy. The first of these factors is shielding. Electrons in orbitals/shells other than the orbital occupied by the outermost electron repel it slightly, reducing the amount of energy required to ‘pull’ away this outer electron. For example, if we consider Magnesium, which has an electronic arrangement of 1s2 2s2 2p6 3s2 our outermost electron is either of the two from the 3s sub-shell (these are both equivalent to each other) and the two full shells below the 3rd shell will act to shield the outermost electron from the attraction of the nucleus.

The second factor is the effective nuclear charge: the attraction to the positively charged nucleus felt by the negatively charged outermost electron. As the number of protons within the nucleus (i.e. the atomic number) increases, so does the effective nuclear charge.

It may also be worth mentioning the atomic radius (distance between the nucleus and the outermost electron) as this can often be worth a mark on some exam boards. Increased shielding will increase atomic radii, whereas an increase in effective nuclear charge will decrease atomic radii.

We can use these three properties to explain the trend in first ionisation energy:

1st IE decreases down the group: this is because the number of filled shells increases down the group, increasing shielding and the distance between the nucleus and the outermost electrons, for very similar effective nuclear charge. This means the outermost electron is more loosely held down the group and so less energy is required to remove it. Thus, 1st IE decreases down the group.

Daniel K. A Level Chemistry tutor, GCSE Chemistry tutor, GCSE Biology...

1 year ago

Answered by Daniel, an A Level Chemistry tutor with MyTutor

Still stuck? Get one-to-one help from a personally interviewed subject specialist


Marc L. A Level Chemistry tutor
View profile
£20 /hr

Marc L.

Degree: Chemistry (Masters) - Durham University

Subjects offered: Chemistry


“I'm enthusiastic because I love the wonder of chemistry, but I'm patient and understanding because I remember the drudgery of A-level. I'm compassionate and friendly as a tutor because that's what it takes to bridge the gap between the...”

Jay L. A Level Art tutor, GCSE Art tutor, A Level Chemistry tutor, GC...
View profile
£20 /hr

Jay L.

Degree: Mathematics, Operational Research, Statistics and Economics (MORSE) (Bachelors) - Warwick University

Subjects offered: Chemistry, Maths+ 4 more

Further Mathematics
-Personal Statements-

“Hi! I am currently a fresher at the University of Warwick, studying Maths, Operational Research, Statistics and Economics, or MORSE. I have always enjoyed the challenge of maths and would really like to help you experience joy and sa...”

Fabiha A. GCSE Biology tutor, A Level Biology tutor, GCSE Chemistry t...
View profile
£20 /hr

Fabiha A.

Degree: Biomedical Science (Bachelors) - Kings, London University

Subjects offered: Chemistry, Science+ 1 more


“About me I am currently studying Biomedical Science at King's College London because it combines my twomost loved subjects, Biology and Chemistry. My enthusiasm for all things science make me want to keep learning and also use my kno...”

About the author

Daniel K. A Level Chemistry tutor, GCSE Chemistry tutor, GCSE Biology...
View profile

Daniel K.

Currently unavailable: for regular students

Degree: Chemistry (MChem, BSc - Integrated Masters) (Masters) - Leeds University

Subjects offered: Chemistry, Biology


“Enthusiastic Chemistry Undergraduate looking to tutor at GCSE and A Level”

MyTutor guarantee

You may also like...

Posts by Daniel

What is meant by the 'First Ionisation Energy' of an element?

Why does the First Ionisation Energy decrease down a group in the Periodic Table?

Other A Level Chemistry questions

Why do transition metals form coloured solutions?

What is a buffer solution? How to make a buffer solution?

Why does a salt not conduct electricity in its solid state?

Why are transition metal complexes coloured?

View A Level Chemistry tutors


We use cookies to improve our service. By continuing to use this website, we'll assume that you're OK with this. Dismiss