What do salts, car batteries, drain cleaner, and tin cans have in common? Answer: they are all examples of things you'll find in inorganic chemistry.

• This article is an introduction to inorganic chemistry.
• We'll start by defining what inorganic chemistry is and comparing it to organic chemistry.
• We'll then look at some of the key ideas in inorganic chemistry.
• Finally, we'll provide an overview of some of the topics you can expect to cover in the following articles.

What are inorganic compounds?

Before we go any further, let's first define inorganic compounds.

This might seem like a broad definition - it is! It encompasses all of the other elements in the world.

Take a look at the periodic table below. It shows carbon highlighted in pink. In inorganic chemistry, we look at compounds made from all of the other elements, from halogens to transition metals and everything in between.

The periodic table. Inorganic chemistry focuses on compounds that aren't carbon-based. Here carbon is shown in pink. Vaia Originals

Going back to one of our examples at the beginning, the batteries in diesel and petrol cars are made up of electrodes placed in solution. The positive electrode, the anode, is coated in lead dioxide whilst the negative electrode, the cathode, is made from a grid of a lead alloy filled with sponge lead. The solution connecting the two electrodes is commonly sulfuric acid. This is known as the electrolyte. On the other hand, electric cars contain batteries with a graphite anode, a mixed-metal oxide cathode, and a lithium-ion electrolyte. You'll study all sorts of substances like these in inorganic chemistry. In fact, inorganic chemistry plays a vital role in many areas of life. For example, we use inorganic chemistry to design and develop things like catalysts, paints, batteries, surfactants, cleaners, jewellery, and drugs.

A simplified diagram of the battery in an electric car. Anna Brewer Vaia Originals

The basics of inorganic chemistry

Let's now look at some of the basic ideas you'll come across in inorganic chemistry.

The periodic table

The periodic table as we know it today is based on one created by Russian chemist Dmitri Mendeleev. He used knowledge about the properties of elements to order them in rows and columns, and even left gaps for undiscovered elements.

The periodic table:

• Has columns known as groups and rows known as periods. Some notable groups include the alkali metals (group 1) and the halogens (group 7, also known as group 17).
• Shows something called periodicity. This means that it contains patterns that repeat every row.
• Is organised into blocks.
• Contains metals, non-metals and metalloids. Metals are generally considered to be elements that lose electrons to form positive ions, whereas non-metals gain electrons to form negative ions. Metalloids behave somewhere between the two.

The periodic table split into four blocks,
User:DePiep, CC BY-SA 3.0, via Wikimedia Commons

Ions

Cations are positive ions whereas anions are negative ions.

Oxidation states

You might have seen species such as $\mathrm{Fe}\left(\mathrm{II}\right)$ before. You can also find $\mathrm{Fe}\left(\mathrm{III}\right)$. What is the difference between the two?

Well, these species have different oxidation states.

Oxidation states are very useful in redox reactions, which you'll look at below. We represent them using superscript numbers or roman numerals. For example, $\mathrm{Fe}\left(\mathrm{II}\right)$ has an oxidation state of 2+ and can also be written as ${\mathrm{Fe}}^{2+}$. This means that it has lost two electrons compared to a neutral iron atom.

Redox

Redox is a term we use to describe oxidation-reduction reactions, which you'll come across in physical chemistry. However, they are important in inorganic chemistry too.

Lots of inorganic compounds are formed in redox reactions. Take a look at the example between zinc and copper sulfate:

$\mathrm{Zn}\left(\mathrm{s}\right)+{\mathrm{CuSO}}_4\left(\mathrm{aq}\right)\to {\mathrm{ZnSO}}_4\left(\mathrm{aq}\right)+\mathrm{Cu}\left(\mathrm{s}\right)$

We can show this as a redox reaction using oxidation states:

$\mathrm{Zn}+{\mathrm{Cu}}^{2+}\to {\mathrm{Zn}}^{2+}+\mathrm{Cu}$

Note the following:

• Zinc is oxidised because it loses electrons.
• Copper is reduced because it gains electrons.
• Zinc acts as a reducing agent because it reduces copper.
• Copper acts as an oxidising agent because it oxidises zinc.

Acids and bases

You also learn about acids and bases in physical chemistry, but they are relevant here as well.

Some elements and compounds are much better acids or bases than others, and you'll learn a bit more about that in inorganic chemistry.

Transition states

Imagine running a reaction in slow motion and taking a picture halfway through. If you zoom in closely, you might see that some of the original bonds in the reactants have broken but new bonds haven't quite formed, or that intermediate compounds have formed instead. This is an example of a transition state.

The transition state of a reaction. Anna Brewer, Vaia Originals

Types of compound

Organic chemistry is filled with molecules like alkenes, alcohols and amines. But in inorganic chemistry, you are much more likely to find salts, oxides and metalloids:

• A salt is an ionic compound formed when a positive ion bonds to a negative ion using electrostatic attraction. You might have come across salts before in physical chemistry, under Ionic Bonding.
• A mineral is a naturally occurring inorganic solid with a definite chemical composition and a crystalline structure. This means it contains a regular repeating arrangement of atoms.
• Oxides are compounds containing at least one oxygen ion with an oxidation state of 2+
• Nitrates are compounds containing the ${{\mathrm{NO}}_3}^{-}$ ion.
• Sulphates are compounds containing the ${{\mathrm{SO}}_4}^{2-}$ ion.
• Organometallic compounds cross the bridge between organic and inorganic chemistry. They are compounds containing at least one bond between a carbon atom in an organic compound, and a metal or metalloid.

Topics within inorganic chemistry

In inorganic chemistry, you’ll study a variety of topics, ranging from periodicity and group 2 metals to halogens and ions. Let’s explore some of them below.

Periodicity and trends

As we mentioned earlier, the periodic table shows periodicity: it contains patterns that repeat across every row in the table. This means that as you travel down a column in the periodic table, known as a group, you’ll find that the elements all react in a similar way. (Look at Periodicity and Trends to explore some of the trends in the periodic table, including atomic radii and ionisation energy.) Knowing these makes it a lot simpler to predict how an element will react. You’ll focus specifically on elements in period 3.

Groups 2 and 7

Group 2 contains the alkaline earth metals whilst group 7 (also known as group 17) contains the halogens, a family of nonmetals. In these two topics, you’ll explore their chemical and physical properties. For example:

• How does reactivity change as you move down the elements in group 2?
• Which group 2 compounds are soluble and which are not?
• Which halogen has the best oxidising ability?
• What are some uses of chlorine?

Transition metals

After that, you’ll visit transition metals.

Transition metals share four common properties:

• They have variable oxidation states.
• They are brightly coloured.
• They are good catalysts.
• They form complex ions. A complex ion is a compound formed when a transition metal bonds to other species, known as ligands, using coordinate or dative covalent bonds.

You’ll learn all about these in Transition Metals.

Reactions of ions in aqueous solution

Finally, in this topic you’ll delve deeper into ions and acidity.

• Why are some metals better acids than others?
• What is chelation?
• How is water replaced in ligand substitution reactions?

Further topics in inorganic chemistry

Inorganic chemistry doesn’t stop at the topics we explored above. Further examples include the group 1 metals, known as alkali metals, electrolysis, group 4, and the extraction of metals.