In order for an ionic bond to form, the beryllium has to let go of its electrons. It is too electronegative to do that.

Note: The trends in electronegativity in Group 2 are discussed on another page. That page looks at the way the electons are arranged in the beryllium-chlorine bond compared with the magnesium-chlorine bond.

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Beryllium forms 4-coordinated complex ions

Some simple background

Although beryllium doesn't normally form simple ions, Be2 , it does form ions in solution. In these, the beryllium ion becomes attached to four water molecules to give a complex ion with the formula [Be(H2O)4]2 .

The ion is said to be 4-coordinated, or to have a coordination number of 4, because there are four water molecules arranged around the central beryllium.

Many hydrated metal ions are 6-coordinated. For example, magnesium ions in solution exist as [Mg(H2O)6]2 .

The water molecules in these ions are attached to the central metal ion via coordinate bonds (dative covalent bonds). One of the lone pairs on each water molecule is used to form a bond with an empty orbital in the metal ion.

Each time one of these bonds is formed, energy is released, and the ion becomes more stable. It would seem logical for the metal ion to form as many bonds like this as it possibly can.

Note: If you aren't happy about coordinate bonding you must follow this link before you go on. You will find the bonding in hydrated metal ions discussed in some detail on that page.

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Why does beryllium only attach four water molecules?

The hydration of beryllium

The problem is that there has to be somewhere that the lone pairs on the water molecules can attach to. Beryllium has the electronic structure 1s22s2. It is helpful to draw this as an "electrons-in-boxes" diagram:

Note: If you aren't happy about orbitals you really ought to follow this link before you go on. You may want to explore further in that part of the site as well. Unless you understand exactly what this electrons-in-boxes diagram is about, you won't be able to make sense of what is coming up next.

When beryllium forms a 2 ion it loses the 2 electrons in the 2s orbital. That leaves the 2-level completely empty.

The 2-level orbitals reorganise themselves (hybridise) to make four equal orbitals, each of which can accept a lone pair of electrons from a water molecule. In the next diagram the 1s electrons have been left out. They aren't relevant to the bonding.

Each water molecule, of course, has two lone pairs of electrons. Only one of them is shown to avoid cluttering the diagram.

Notice that once four water molecules have bonded in this way, there isn't any more space available at the bonding level. All the empty orbitals from the original beryllium ion are being used.

The water molecules arrange themselves to get as far apart as possible - which is pointing towards the corners of a tetrahedron. The ion therefore has a tetrahedral shape.

The hydration of magnesium

You might think that magnesium would behave just the same, but at the 3-level there are 3d orbitals available as well as 3s and 3p.

When the magnesium ion is formed, it leaves empty 3s, 3p and 3d orbitals. When that ion is hydrated, it uses the 3s orbital, all three of the 3p orbitals and two of the 3d orbitals. These are reorganised to leave a total of six empty orbitals which are then used for bonding.

Why does magnesium stop at attaching six waters? Why doesn't it use the remaining 3d orbitals as well? You can't physically fit more than six water molecules around the magnesium - they take up too much room.

What about the other ions in Group 2?

As the ions get bigger, there is less tendency for them to form proper coordinate bonds with water molecules. The ions become so big that they aren't sufficiently attractive to the lone pairs on the water molecules to form formal bonds - instead the water molecules tend to cluster more loosely around the positive ions.

Where they do form coordinate bonds with the water, however, they will be 6-coordinated just like the magnesium.

Beryllium hydroxide is amphoteric

Amphoteric means that it can react with both acids and bases to form salts.

The other Group 2 hydroxides

The other hydroxides of the Group 2 metals are all basic. They react with acids to form salts. For example:

Calcium hydroxide reacts with dilute hydrochloric acid to give calcium chloride and water.

Beryllium hydroxide

Beryllium hydroxide reacts with acids, forming solutions of beryllium salts. For example:

But it also reacts with bases such as sodium hydroxide solution. Beryllium hydroxide reacts with the sodium hydroxide to give a colourless solution of sodium tetrahydroxoberyllate.

This contains the complex ion, [Be(OH)4]2-. The name describes this ion. Tetra means four; hydroxo refers to the OH groups; beryllate shows that the beryllium is present in a negative ion. The "ate" ending always shows that the ion is negative.

A simple explanation of what is happening

You need to think about where the beryllium hydroxide came from in the first place. It would probably have been made by adding sodium hydroxide solution to a solution of a beryllium salt like beryllium sulphate.

Remember that beryllium ions in solution exist as the hydrated ion, [Be(H2O)4]2 . The beryllium has such a strongly polarising effect on the water molecules that hydrogen ions are very easily removed from them.

The sodium hydroxide solution contains hydroxide ions which are powerful bases. If you add just the right amount of sodium hydroxide solution, you get a precipitate of what is normally called "beryllium hydroxide" - but which is a shade more complicated than that!

The product (other than water) is a neutral complex, and it is covalently bonded. All that has happened to the original complex ion is that two hydrogen ions have been removed from the water molecules.

You get a precipitate of the neutral complex because of the lack of charge on it. There isn't enough attraction between this neutral complex and water molecules to bring it into solution.

What happens if you add an acid to this?

The hydrogen ions that were originally removed are simply replaced. The precipitate dissolves as the original hydrated beryllium ion is re-formed.

What happens if you add a base?

Adding more hydroxide ions to the neutral complex pulls more hydrogen ions off the water molecules to give the tetrahydroxoberyllate ion:

The beryllium hydroxide dissolves because the neutral complex is converted into an ion which will be sufficiently attracted to water molecules.

Why doesn't this happen with, for example, calcium hydroxide?

Calcium hydroxide is truly ionic - and contains simple hydroxide ions, OH-. These react with hydrogen ions from an acid to form water - and so the hydroxide reacts with acids.

However, there isn't any equivalent to the neutral complex. Adding more hydroxide ions from a base has no effect because they haven't got anything to react with.

Note: This has been simplified to bring it into line with the sort of treatment you will meet for the acid-base behaviour of transition metal hydroxides. In particular, the structure of beryllium hydroxide is probably even more complicated than has been suggested above!

Purpose of the Workplace Study

Before this study began, we knew that people exposed to beryllium may develop two forms of beryllium disease, acute and chronic. These are lung diseases caused by exposure to beryllium. The acute form is a rare pneumonia-like disease that occurs shortly after very high exposures to beryllium. The chronic form may develop many years after being exposed to beryllium. Chronic beryllium disease is described in the fact sheet Steps to Protect Your Health.

Our study, however, focused on lung cancer. Some studies had previously linked lung cancer to beryllium exposure. However, this link was uncertain. Thus, we did our study to further examine this issue.

Description of the Workplace Study

We studied 9,225 workers employed at 7 beryllium processing plants between 1940 and 1969.

We based the study on work records that we got from the companies. We also obtained the death certificates of workers who had died. We counted the number of workers who died from each disease.

Then we calculated the number of deaths from each disease that we would expect to find in the workers, based on how often people die of these diseases in the U.S. population as a whole. If the number of deaths in the workers is higher than the expected number, then workplace exposures may be the cause.

What We Found

We found an increased risk of lung cancer in workers exposed to beryllium at all plants combined. We found 280 deaths, but expected 221.

The lung cancer excess was confined to workers hired in the 1940s and 1950s.

Among workers hired in the 1960s, the risk of lung cancer was noticeably lower than expected. We found 18 deaths from lung cancer and expected about 29.

We examined the effect of smoking and county of residence on lung cancer risk.

We concluded that these factors could not completely explain the increased lung cancer risk.

The study did not address the relationship of lung cancer to the degree of exposure or to specific types of beryllium compounds.

Another NIOSH Beryllium Study

Another NIOSH study, which looked at causes of death among 689 people who were reported to a national beryllium disease registry (the Registry Study), also found increased deaths from lung cancer. (203 people were included in both the workplace and the registry studies).

What Does This Mean?

The authors of the study and many scientists (including the International Agency for Research on Cancer) believe that most of the excess in lung cancer was due to beryllium exposure because:

* Smoking and county of residence did not completely explain the excess.

* Breathing in beryllium compounds causes lung cancer in some animals.

* The highest increase in lung cancer occurred many years after exposure began.

This is what we expected, since it takes a long time for cancers to develop.

Other scientists argue that part or all of the excess may have been due to other factors, including smoking or exposure to acid mists, uranium, and nickel. Chronic beryllium disease and lung cancer may develop many years after the last exposure to beryllium. Thus, you and your doctor should be aware that you might have an increased risk of developing these diseases.

What You Can Do to Protect Your Health

* You should not smoke. Cigarette smoking causes lung cancer and may make chronic beryllium disease worse. For ways to stop smoking, call the American Lung Association at 1-800-LUNG-USA or the American Cancer Society at 1-800-227-2345.

* If you have lung or breathing problems that don't go away, see your doctor. Take the fact sheet For Your Doctor with you.

If you, your doctor, or members of your family have any questions, or would like a copy of the technical reports, call at 1-800-356-4674.

Findings of the NIOSH Beryllium Study