Explain why Aluminium and Sulfur do not fit the expected trends of first ionisation energies of period 3?

Firstly, we should define the first ionisation energy

The first Ionisation Energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form a mole of gaseous ions, each with an 1+ charge.

The expected trend as we go across the period is that ionisation energy will increase as the nuclear charge increases and the number of shielding shells of electrons does not change. So the extra amount of protons means the nucleus holds the outer electrons more strongly so it requires more energy to remove an electron.

Aluminium has a lower ionisation energy than Magnesium. This is unexpected as Al has more protons. This can be explained by electron configurations.

Magnesium's electron config: 1s2s2 2p3s

Aluminium's electron config: 1s2s2 2p3s3p

Aluminium's outer electron is in a p orbital. This p sub-level is of higher energy than the s sub-level and so less energy is required to remove this electron.

Sulfur has a lower ionisation energy than phosphorous. This again is explained by their electron configurations

Phosphorous: 1s2s2 2p3s3p3

Sulphur: 1s2s2 2p3s3p4

A p sub-level has 3 sub-shells which can hold 2 electrons in each sub-shell. In phosphorous, the electrons are unpaired with one electron in each sub-shell with parallel spins. However, sulphur has 4 electrons so one of them must pair in a sub shell. As electrons are both negative particles, the paired electrons repel each other and so it is easier to remove the unpaired electron in phosphorous- so less energy is required.

Jake G. GCSE Chemistry tutor, A Level Chemistry tutor

7 months ago

Answered by Jake, a GCSE Chemistry tutor with MyTutor

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


£18 /hr

Ben B.

Degree: Medicine (Bachelors) - Bristol University

Subjects offered: Chemistry, Science+ 3 more

-Medical School Preparation-

“Hi - I'm currently in my 3rd year of studying Medicine at Bristol University and am taking an intercalated year out to conduct my own research into 3D printing applications in Cerebral Palsy. I have a real interest in science with ent...”

MyTutor guarantee

£18 /hr

Martin L.

Degree: Chemistry (Bachelors) - Warwick University

Subjects offered: Chemistry, Maths


“About Me: I am currently a third-year student studying Chemistry at the University of Warwick; and have had tutor experience at my secondary school. I take a patient approach to tutoring and believe that a lot of the concepts they tea...”

£18 /hr

Oliver P.

Degree: Biology (Masters) - Bristol University

Subjects offered: Chemistry, Science+ 5 more

Human Biology
Further Mathematics
Extended Project Qualification

“The tutor: I am a Biology MSci student at Bristol University and have a deep curiosity and passion for multiple fields of science and technology. I am patient and have a lot of experience teaching children in the age range of 11-...”

About the author

£18 /hr

Jake G.

Degree: Chemistry (Bachelors) - Warwick University

Subjects offered: Chemistry, Physics+ 1 more


“About me: Hi, I’m Jake and I’m a first year undergraduate at the University of Warwick studying Chemistry. As I only finished my A-levels last year, I still have expertise on the A-level syllabus and good exam technique. Due to my col...”

You may also like...

Other GCSE Chemistry questions

Crude oil is a fossil fuel. Describe how crude oil is separated into fractions.

What is the number of protons in an atom also called?

Balance the equation for the complete combustion of hydrogen.

Why can ammonium sulfate be described as a salt?

View GCSE 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