Atoms and matter

Specific Learning Outcomes

By the end of this unit you should be able to:

Atoms and elements

The Greek philosophers were fond of something called 'thought experiments'. Here's one of them:

Suppose you have a rock, and you cut it in half. Then you cut the half in half, and in half again, and again, and so on.

Can you keep going forever?

They decided that the answer was no. They called this 'final' bit of rock that can't be cut in half atomos, meaning 'indivisible'. 

Therefore, they reasoned, the world is made of indivisible little bits they called atoms.

Another set of ideas was about 'what is everything made from?'. It was thought that everything was made of different combinations of earth, air, fire and water. They called these basic 'ingredients' elements. They were wrong about the earth, air, fire and water, but right about the idea that everything is made of the same basic ingredients.

Finding the basic ingredients

Metals such as copper and tin were probably discovered by accident when people tried to make pottery  from rocks that contained these metals. From this came the idea of 'purifying by fire', and the development of iron making.  Eventually, people came to realize that the true elements were things that could not be purified any further - iron, copper, zinc, tin, lead and gold were early examples. 

Some non-metals such as sulfur and charcoal (carbon)  were found to be the same: they were unable to be purified any further. The idea developed that these  purified substances were the 'true' elements, rather than earth, air, fire  and water. It followed that, if everything was made of atoms, then elements were made of all the same type of atom.

The next important discovery was that a fixed weight of one element would always combine only with a fixed weight of another - any variation from this and you had some left over of one element. This led to the idea of lighter and heavier atoms. The known elements could then be arranged in a list from lightest to heaviest atoms.

A Russian chemist, Dmitri Mendeleev, had the idea of arranging this list into a table,. In this table

This table showed regular 'periods' of chemical properties and eventually  led to the Periodic Table we know today.

The table below is the modern one . Mendeleev's original table was missing a lot of elements and this is one of the ways that chemists found new elements - by looking for gaps in the incomplete table and guessing the properties of the element. 

An example of a set of elements Mendeleev knew about with similar properties is the second to right (yellow) column which is known as the halogens. These are all found in sea salt.

Element names and symbols

Every element has a name and a symbol.

Names are different in different languages, but symbols are the same in every language.

This is why a student in Germany might wonder why saurstoff has a symbol O,  but to a kiwi boy it seeks logical that oxygen is O.  The German boy would be perfectly happy using Na for the element he calls natrium but the kiwi kid will wonder why sodium is Na. 

Elements that were earlier to be discovered are the most likely to have different names in different languages. Most elements discovered in the last 50 years are named after people e.g. Rutherfordium. These are usually the same in all languages.

Element symbols MUST be a capital (UPPERCASE) letter if they are single letter e.g. carbon C, It the element has two letters, the second letter must be lowercase e.g. cobalt Co. This is so you don't get it mixed up with a compound of carbon and oxygen, CO which is carbon monoxide.

Elements and atoms

Each element has a unique type of atom.

Atoms are made of three different sorts of particle: protons, neutrons and electrons. On the right is a diagram of a lithium atom.

It is the number of protons that defines what element an atom is: lithium has 3 protons. If you look at the periodic table, you will see the number 3 at the top above the symbol. This number is called the atomic number.

Every element has a different atomic number.  This means that every different element has a different number of protons.

A typical lithium atom contains 3 electrons (grey) orbiting 3 protons (red) and 4 neutrons (blue)

Structure of the atom

The central part of the atom is called the nucleus. It contains almost all the mass of the atom but occupies only a small part of the space. When the nucleus was first discovered by Ernest Rutherford's research team, there was no clear evidence that it contains two different sorts of particle, although Rutherford suspected it.

When the neutron was discovered by James Chadwick in 1932 the modern model of the atom was cemented in place,

The particlas found in the nucleus are protons and neutrons.  Protons provide the positive charge and are balanced by the negative charge of electrons which orbit outside the nucleus and have far lower mass.

Neutrons mass about the same as protons, but have no charge. The number of neutrons in an atom is usually about the same as the number or protons, but can vary. You can change the number of neutrons in an atom without changing what element it is as it is only the number of protons that determines the element.

In the example above, all three forms of hydrogen have only one proton. There can be zero, one or two neutrons in the nucleus with the proton. 

Hydrogen-1 (protium), with zero neutrons, makes up about 99.98% of hydrogen in an average sample (say, in your body. Hydrogen-2 (deuterium) makes up most of the remaining 0.02%. Tritium, or hydrogen-3, is radioactive and changes to other elements over time. Unless it is constantly topped up the amount of it gradually drops to zero. Tritium is created in the atmosphere by the effects of cosmic rays from space. Since this only happens in the atmosphere, we can use the amount of tritium to date groundwater and tell how long it was since it fell as rain.

Elements having different numbers of neutrons are called isotopes. Hydrogen is the only element for which we commonly use different names for the isotopes; usually chemists just refer to a number such as oxygen-18.

We have several shorthand ways of writing down the composition of isotopes. To the right is a picture of a shorthand way of writing the isotope of magnesium with 12 protons and 13 neutrons. We get the number of neutrons by subtracting the number of protons (atomic number) from the mass number

Since only magnesium has 12 protons , the "12" written above isn't really needed. This isotope of magnesium therefore could be written as 25Mg or as magnesium-25. Both these ways of writing it are common.

Natural magnesium is a mixture of magnesium-24 (with 12 neutrons), magnesium 25 (with 13 neutrons) and magnesium-26 (with 14 neutrons). All three types have 12 protons or they wouldn't be magnesium. The average mass number for a natural sample of magnesium is 24.306. This is known as the atomic weight.

For radioactive elements the nuclear reactivity of their isotopes can be important. Natural  uranium is mostly uranium-238. It is uranium-235 that produces the energy in nuclear bombs and reactors. .Uranium that contains extra U-235 is termed enriched. The process of enriching is complicated and expensive and is one of the reasons that nuclear power is fairly expensive.

Nuclear submarines don't have much space for their reactors and refueling them is very difficult. By using very highly enriched uranium these goals can be achieved - the extra expense of the enrichment is offset by the 20 years or so that the reactor will last without refueling.

Electrons

The electrons are arranged around the nucleus in "shells", and there are patterns to the way the electrons slot into the shells. These patterns are the cause of the 'periodic' part of the periodic table. The way that the electrons are arranged for a particular element is called the electron configuration.

Electron configuration rules

Neutral atoms have the same number of electrons as they have protons e.g. phosphorus has 15 protons and 15 electrons.

These electrons form 'shells' around the nucleus. Each shell can take a maximum number of electrons and they fill up from the first (innermost) shell outwards. The outermost shell is called the valence shell.

The number of electrons per shell is:

You will need to be able to write or draw electron configuration diagrams for the first 20 elements.

Below is a picture showing the order in which the first 20 electrons fill up:


In Year 9, you only have to fill up the shells with the right number of electrons; however, the order I have shown up where one pair fill up first in each shell is useful later on in chemistry because it will help you  explain the shapes of some molecules.. Below are some illustrations:

Boron - 5 protons in the nucleus and therefore 5 electrons. Two go into the first shell and three go into the second. We would write the electron configuration as 2, 3

Nitrogen - 7 electrons and electron configuration 2, 5

Oxygen - electron configuration 2, 6

Notice that I started to pair up the electrons in the second shell

Neon with 10 electrons, configuration 2, 8

With 8 electrons in the second shell the shell is now full.

Atoms are at their most stable when the valence shell is full. This is why neon occurs naturally only as an element made of single atoms. All the elements in the right hand column of the periodic table have full outer shells.  Helium has two electrons (a full first shell), neon and argon both have 8. Krypton and xenon also have full shells.

Having a full shell makes these gases unreactive. They are called the "Noble gases" - noble because they don't mix with common elements (and by comparison with gold, the 'noble' metal).

Formation of Ions

Chemical reactions occur because of interactions between electrons in the outer (valence) electron shells of atoms. None of the other electrons are involved. Therefore, the number of electrons in the outer shell are what determines the behaviour of elements.

Elements react to either gain (get extra) or lose (get rid of) electrons to reach a more stable number by either filling up their outer shell, or getting rid of a small number of electrons in their outer shell to "go down" to the next shell below (which will be full and stable).

For example, sodium has 11 electrons and its electron arrangement is 2, 8, 1

It would be more stable at either 2, 8, 0   or 2, 8, 8.  

To get to 2, 8, 0 it only has to lose one electron (it would be much harder to gain 7 and it never does this).

Once it has lost that one electron (for example, by giving it to a chlorine) it now has 11 protons but only 10 electrons. It has an overall +1 electric charge. We now call it the sodium ion and write it as  Na+ . The plus 1 is written as a superscript (but we don't write a number for 1, only for two or more),

Oxygen has 8 electrons and its electron arrangement is 2, 6

The easiest way for it to get to a stable number is to get two extra electrons to take it to 2, 8

When it does this it now has an overall charge of negative two, because it has 8 protons  but 10 electrons. We write it as O2- . Notice that we put the number before the minus sign; we do the same with positive ions e.g. Ca2+  for the calcium ion. 

Negative ions change their name from the element. The ion formed by oxygen is not called the oxygen ion, it is called the oxide ion. Negative ions made of one atome change their name to finish in -ide e.g. sulfide, oxide, fluoride, chloride, nitride, phosphide. There are other negative ions made of multiple atoms and they have different naming rules. For example, ions finishing in oxygen hafe -ate and the end of their name e.g. sulfur and oxygen together make an ion with the formula SO42-  which we call the sulfate ion. This is how we can tell from the name what kind of ion it is. You don't need to know how these multi-atom ions in Year 9, apart from naming them.