Water H2O

Water is something we can’t live without; roughly 70% of an adult human body is composed of water. I will be writing a followup post about the properties, but for now lets just focus on the molecular structure.

I made the model below to show its shape. The red centre is oxygen, the two whites are hydrogen, and the two empty red prongs represent two lone pairs.

The presence of two lone pairs means lots of repulsion between them, which pushes the two sets of bonded pairs closer together, so the two atoms of hydrogen are forced closer together to form a V shape.

Based on this we can predict the shape for hydrogen sulfide (H₂S), bent for the same reasons H2O is bent, see below:

and also the oxonium ion H₃O⁺

This oxonium ion is pyramidal as it is isoelectronic with ammonia, see previous post.

I highly recommend visiting Wolfram Alpha, simply type in a formula, name or equation and a treasure of information comes up.

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Chemistry Lesson 1: Periodic Trends

Click the link to download this lesson: Lesson 1: Periodic Trends

I wrote this to help remember the basics of periodic trends. These are basic but important for science A Level revision.

Click the link at the top and feel free to download the Word Document, leave a comment 🙂

Image courtesy of Wikipedia

Atomic radius

The distance from the atomic nucleus to the outermost stable electron orbital in an atom that is at equilibrium.

↓ Increases down a group (column of the periodic table)

←  Increases across a period (row of the periodic table) from right to left

Therefore atomic radii are largest in the bottom left corner of periodic table.

e.g. Ce is the largest, and Fr has a larger radius than He.

Why does atomic radius decrease as you go across a period? (from left to right) →

The effective nuclear charge increases → therefore attracting the orbiting electrons towards the nucleus and lessening the radius. Less distance between the electrons and the nucleus so the nuclei pull is stronger.

(See bottom of this post for more information on effective nuclear charge.)

Why does atomic radius increase as you go down a group? ↓

The addition of a new energy level (shell)

Ionisation Energy

 This is the energy required to