Elements and atoms
In the 17th century the chemist Robert Boyle wrote the modern definition of an element. This states than an ELEMENT is a substance that cannot be broken down into anything simpler. Antoine Lavoisier used this definition to explain how elements burn together in oxygen to form compounds. He published this new idea in a book in 1789 and most chemists were soon using it. The chemist John Dalton proposed that all elements were made up of tiny particles called atoms. Atoms could not be broken up and were identical in the same element. In 1802 he produced the first list of the RELATIVE ATOMIC MASS of 20 elements. The relative atomic mass was the mass of an atom of the element compared to the mass of hydrogen, the lightest element.
By 1817, a total of 49 elements had been discovered. The chemist Johann Dobereiner noticed that in groups of three similar elements (triads), the average of the masses of the lightest and heaviest atoms was very close to the atomic mass of the middle element. Lithium (7), Sodium (23) and Potassium (39) are an example of similar reactive metals. The average of the atomic masses of lithium and potassium are 23, which is the atomic mass of sodium. Most scientists thought that it was silly to look for patterns because each element was an individual with its own "personality". Other scientists found numerical patterns. The Frenchman de Chancourtois was the first to list all the known elements in order of their atomic mass. He arranged the elements in a spiral and found that similar elements, such as Li, Na and K appeared in a vertical line.
Discoveries of elements slowed in the mid-1800s, and most chemists still thought there was little point in looking for patterns. John Newlands however thought he could see a pattern in which every eighth element seemed to be similar, like the notes in a musical scale or "octave". He published his Law of Octaves with a table of the elements in 1866. At a meeting of the Chemical Society of London most members thought this idea was nonsense because, although it showed some patterns, he had also grouped together elements which were very different all together. A new burst of element discoveries was also taking place, making Newlands even more of a laughing stock.
The many colours in fireworks come from salts of different elements mixed with gunpowder. In 1859 Robert Bunsen and Gustav Kirchhoff used a special gas burner with a colourless flame to identify the exact colours of flames produced by each element. Within two years they found two new elements, caesium and rubidium. Other chemists used their method and soon more elements were being discovered. The Russian Chemist Dmitri Mendeleev was convinced that there was a pattern to the elements. He wrote everything he knew about each element on a separate card and arranged the cards in columns and rows. He found that if he left some gaps he could produce a repeating pattern in seven columns of elements. He published his PERIODIC TABLE of the elements in 1871. He also published predictions for the properties and atomic masses of elements that would fill the gaps.
Lines of discovery
New practical techniques produced a rush of new elements. In the 18th century it was apparatus for handling gases, then it was electrolysis, and in the 1860s it was Bunsen and Kirchhoff's flame colour apparatus. The light produced by an element in Bunsen's burner was passed through a prism, which split the light into a spectrum. Each element produced a different set of coloured lines - A LINE SPECTRUM. New elements had patterns of lines that were different from the known elements. Bunsen and Kirchhoff's spectroscopic method worked with very small samples of compounds so rare elements could be discovered.
In 1875, the first of Mendeleev's predicted elements was discovered using spectroscopy. In 1879 and 1866 two more followed. Chemists began to agree that Mendeleev's table represented a real pattern in the elements.
Mendeleev constructed his Periodic Table according to two rules: 1. Put the elements in order of their relative atomic mass, sometimes he swapped elements around to give a better pattern and said the data was wrong. 2. Put elements that are chemically similar in the same column. Properties included the formula of the oxides of the elements. Following these rules produced gaps, for example below aluminium. Gallium, a new element discovered in 1875, fitted this gap perfectly. The success of Mendeleev's predictions convinced scientists that the Periodic Table revealed a true pattern of relationships between elements and was a useful guide to the properties of elements.
No one can see inside atoms. They are so small that a million atoms can line up across a hair. Experiments show that atoms are made up of even smaller particles. There are ELECTRONS with a negative electric charge and almost no mass. Then there are PROTONS - positively charged particles that have the same mass as a hydrogen atom. Lastly, there are NEUTRONS with no charge but the same mass as the proton.
Particle Charge Mass Position in atom
Electron -1 Almost zero In shells around the nucleus
Proton +1 Same as hydrogen In nucleus at centre of atom
Neutron 0 Same as hydrogen In nucleus at centre of atom
An atom does not have an electric charge overall because the positive and negative charges cancel out. The number of electrons and protons must be the same. The number of protons in an atom is called the PROTON number. The number of electrons in an atom is the same as the proton number.
The modern Periodic Table lists the elements in order of their proton number.
Using the periodic table
The electrons in an atom are responsible for its chemical properties. All the atoms of an element behave in the same way, so they must have the same number of electrons and therefore the same number or protons. Each element has its own proton number which makes it different from every other element.
The PERIODIC TABLE is a useful guide to the particles in atoms. Once you find the element in the table you can give its proton number and the number of protons and electrons. The sum of the protons and neutrons in the nucleus of the atom gives the relative mass of the atom.
For example, aluinium is element number 13 in the Periodic Table and has a relative atomic mass of 27. This means its proton number is 13 so it has 13 protons in the nucleus and 13 electrons in the shells around it. The number of neutrons in the nucleus can be worked out:
Number of protons + Number of neutrons = relative atomic mass
13 + Number of neutrons = 27
So, the number of neutrons = 27-13, = 14
Bohn suggested that electrons are arranged in orbits or shells around the nucleus. The first shell, which is closest to the nucleus, can have up to 2 electrons. The next shell can take up to 8 electrons. The ELECTRON ARRANGEMENT of an atom can be written down quickly as a set of numbers seperated by full stops.
The electron arrangemenet of an oxygen atom is 2.6, so there are 2 electrons in the inside shell and then 6/8 in the next one out. The electron arrangement of a chlorine atom is 2.8.7. Chlorine has 2 electrons in the first shell, 8 in the second and 7 in the third. The arrangements can also be drawn in ring diagrams with a series of crosses symbolising the electrons.
The electron arrangement of an element can be worked out from its proton number. Electrons are put into the first shell first, then the second, then the third. In the first 20 elements, the maximum number of electrons in the third shell is 8.
The proton number of potassium is 19. With two electrons in the first shell and 8 in the second and third, there is one electron left over in the fourth shell. The electron arrangement is therefore 22.214.171.124. The horizontal rows in the Periodic Table are called PERIODS. From left to right across a period, the number of electrons in the outer shell increases from one element to the next. There are only two elements in the first period because the first shel can only have two electrons in it. The second and third period both have eight elements.
Shells and levels
The shell that an electron is in represents the ENERGY LEVEL of the electron. The closer the electron is to the nucleus, the lower the energy level of the electron. Electrons can only have the amount of energy corresponding to these energy levels. They cannot have an energy between the energy levels. This is how Niels Bohr explained why electrons stay in shells and do not fall into the nucleus.
In the modern Periodic Table, the elements are in order of their number or protons, called the PROTON NUMBER. Their electron arrangement is then used to put the elements into rows. The rows are called periods. This way or arranging the elements means that we can use the position of an element in the table to predict its properties.
In the Periodic Table metals such as magnesium, iron and gold are on the left side and in the mdidle. The non-metals such as oxygen, sulfer and bromine are on the right side.
As we move along a PERIOD from left to right, the elements change from metals to non metals.
The number of electrons in the outer shell increases from 1 to 8 along a period. So Period 1 has 1 electron in the outer shell, Period 2 has two electrons in the outer shell, etc.
Properties across a period
Sodium has a proton number of 11 and electron arrangement 2.8.1. As we move to the right each element has an additional proton and electron until we get to argon, which has the electron arrangement 2.8.8.
The first three elements, sodium, magnesium and aluminium, are metals. The MELTING POINT of these metals increases from left to right. The last four elements are non-metals. Two of them, phosphorus and slfer, are solids with low melting points. The last two, chlorine and argon, are gases. The element in the middle, silicon, has a very high melting point and is known as a semi-metal or metalloid.
Argon is an INERT element. This means that it does not react with any other elements. These changes in properties across a period are called TRENDS. The trends that we see in Period 3 also occur in the other periods. It is this repeating or periodic pattern that gives the table its name.
The chemical properties of an element are governed by its electron arrangement and in particular by the number of electrons in its outer shell. As we can work out the electron arrangement of an element from its position in the Periodic Table, we can use the table to predict the properties of any element. Any element with one, two, or three electrons in their outer shell are metals. Electrons with 5, 6 or 7 electrons in their outer shell are non-metals and any elements with 8 electrons in the outer shell are inert metals (such as Argon).
The colums in the Periodic Table are caleld GROUPS of elements. The group number is the number of electrons in the outer shell of an atom.
Some of the groups have been given names, for example, the first group on the left, Group 1, is known as the ALKALI METALS. Group 1 is made up of six elements: Lithium, sodium, potassium, rubidium, caesium and francium.
When Mendeleev designed his Periodic Table he put elements that had similar properties in the same group. This is still true with the modern Periodic Table. All the elements in a group have similar physical properties and chemical reactions with other elements and compounds.
The alkali metals have one electron in the outer shell of their atoms, which is why they are all in Group 1. They are soft and can be cut easily with a steel knife. As they are metals they conduct electricity and are malleable.
When freshly scratched or cut, the metal surface is shiny and silver in colour. In moist air the surface turns dull rapidly. This is because the metals corrode or TARNISH by oxygen in the air. To slow down the reaction the metals are kept in oil and out of contact with the air.
The reaction of the alkali metals with oxygen is speeded up by heating the metal. The metals burn with brightly coloured flames and form the metal oxide, for example:
Sodium + oxygen ---> Sodium oxide
If freshly cut samples of lithium, sodium and potassium are left in the air, the potassium tarnishes the quickest, followed by sodium and then lithium. This shows that the ALKALI METALS GET MORE REACTIVE FURTHER DOWN THE GROUP.
Other properties, such as melting point, boiling point and density, also show a gradual change or trend down the group..
The fact that all Group 1 metals have one electron in their outer shell explains the similarity in their properties. Going down the group the number of shells increases and the atoms get bigger. The outer electron is further and further away from the nucleus, in a higher and higher energy level.
Lithium, sodium and Potassium all have one electron in their outer shell. (2.1), (2.8.1), (126.96.36.199)
Group one reactions
A small piece of sodium in a bowl of cold water floats on the surface and moves around fizzing. The gas that is given off can be lit by a lighted splint. The piece of sodium eventually disappears and pH indicator added to the water turns blue. These observations show that hydrogen gas is given off an an alkaline solution is formed.
The word equation for the reaction of sodium with water is:
Sodium + Water ---> Hydrogen and Sodium Hydroxide
A similar reaction happens with other Group 1 metals but the reaction can be more violent. Alkali metals can explode in water. They are flammable, as is the hydrogen gas given off. The hydroxide formed can be corrosive and harmful. Group 1 metals must be kept away from water for safety.
The equation for the reaction of sodium and water can be written in symbols as:
2Na(s) + 2H20(l) ---> H2(g) + 2NaOH(aq)
This BALANCED symbol equation shows that two atoms of solid (s) sodium react with two molecules of liquid (l) water to form one molecule of hydrogen gas (g) and two formula units of sodium hydroxide in aqueous solution (aq).
Group one reactions
For the reaction of any group 1 metal with water, the formula of the metal hydroxide formed and the balanced equation are similar. Using M to stand for any Group 1 metal, we can write the general equation.
2M(s) + 2H20(l) ---> H2(g) + 2MOH(aq)
Down the group the reaction with water gets more violent. Lithium fizzes on the surface quite gently, sodium is noticeably faster and may explode if the piece is larger. Potassium always explodes with lilac-coloured flames, and rubidium explodes more violently.
Like sodium chlorine, the chlorides of other group 1 metals are white solids that dissolve in water, but only sodium chloride actually tastes pleasantly salty. Potassium chloride is often used as a substitute for sodium chloride in "low-sodium salt" but has a more bitter taste and is poisonous in large amounts.
When a piece of sodium is heated until it is burning and then put into a container of chlorine gas, it continues to burn with a yellow flame. A white smoke forms. The sodium continues to burn because it reacts quickly with the chlorine, making sodium chloride.
Sodium + Chlorine ---> Sodium chloride
Burning without oxygen
Sodium chloride is often called "salt" but it is in fact one of many different salts and should be known as "common salt". The other alkali metals also burn in chlorine gas to make similar salts. The formula of sodium chloride is NaCl. This shows that every sodium atom combines with a chlorine atom. The balanced chemical equation for the reaction of sodium with chlorine is:
2Na(s) + CL2 ---> 2NaCl(s)
The other alkali metals form chlorides with similar appearances and formulae and the balanced chemical equations are the same if the metal's symbol is substituted for sodium.
As in their reactions with oxygen and water, alkali metals further down the group react more violently with chlorine than those at the top. There is thus a pattern in the reactions of all the elements in a group. They all react in a similar way with other elements and compounds but the reactions get more violent as you replace the metal with one further down the group. The reactivity of the alkali metals with chlorine illustrates the similarity and the trend of the electron arrangement of these Group 1 elements. They each have one electron in the outer shell but as the atoms get larger down the group, this outer electron is further from the nucleus. The further the single outer electron is from the nucleus, the easier the atom finds it to combine with other elements, such as chlorine and oxygen, to form a stable compound; that is, its reactivity increases.
Iodine solution has been used to clean wounds since the early 1800s. Chlorine is added to water to make it fit to drink. Both chlorine and iodine kill microorganisms. If we look carefully at the properties of these and other Group 7 elements, we find other similarities.
The Group 7 elements form a vertical column in the Periodic Table on the right of the table amongst the on-metals. The elements are in the same group because they have similar chemical properties, but they appear to be quite different at normal temperature and pressure.
Fluorine (F) is a very pale yellow gas.
Chlorine (Cl) is a pale green gas.
Bromine (Br) is a dark red-brown liquid that evaporates easily to form a reddish brown gas.
Iodine (I) is a dark grey solid but on warming becomes a purple gas.
Astatine (At) is the most rare element on Earth and has never been seen in large enough quantities to describe its appearance.
There is no Group 7 element in the first Period on the Periodic Table.
Patterns in Group 7
An important similarity between all the halogens is that, as elements, the atom are joined together in pairs to form DIATOMIC MOLECULES. Diatomic means "two atoms". The formulae of the elements show this.
F2 Cl2 Br2 I2 At2
The formulae of the compounds of the HALOGENS are also similar. They each react with sodium to form a compound where an atom of the halogen combines with each atom of sodium, so the formula of sodium fluoride is NaF, sodium chloride is NaCl, sodium bromide is NaBr, sodium iodide is NaI and sodium astatide is NaAt. Note how the name of the element changes in the compound, with the ending becoming -ide instead of -ine. If you see the -ide ending, you know that the substance is a compound. In fact the name halogen means "salt maker". Like Group 1, the halogens show a trend or pattern in their properties as you go from the top of the roup, fluorine, to the bottom, iodine. There is so little astatine that its properties are uncertain. For example, the melting points of the elements increase in a fairly smooth curve that gets flatter down the group. All the HALOGENS have seven electrons in the outer shells of their atoms. The similaritity in the electron arrangement is the reason why the halogen elements have similar chemical properties. The patterns in the group can be explained by the increasing number of shells and size of the atoms down the group.
All the halogen elements are very reactive. They react with most elements and many compounds. This makes the elements and their vapours very hazardous, so they are classed as CORROSIVE and TOXIC. Experiments with the elements must be carried out in a fume cupboard by experienced chemists.
All the halogens react violently with alkali metals and other metals such as iron and form compounds known as HALIDES. For example,
Sodium + Bromine ---> Sodium bromide
Potassium + Iodine ---> Potassium iodide
Iron + Chlorine ---> Iron chloride
The halogens become less reactive down the group. Sodium will burn in chlorine but its reaction with iodine is less violent. The same pattern is seen in the reaction of iron with the halogens. Iron will burn when in contact with fluorine but needs to be heated to react with iodine. The difference in the reactivity of the halogens can be shown in DISPLACEMENT REACTIONS where one halogen will take the place of another in its compounds. Chlorine will displace bromine from potassium in bromide solution.
Chlorine + Potassium bromide ---> Potassium chloride + Bromide
However, bromide will not displace chlorine from its compounds. This shows that chlorine is more reactive than bromine. Similar tests show that the elements further down the group are less reactive.
In the case of non-metals, it is the smaller atoms that combine most readily with other elements. Going down the halogen group, the number of shells increases and the outer shell gets further from the nucleus. Therefore the reactivity of the halogens decreases down the group.
The equations show the similarities of the reaction. For example, with the alkali metals, the pattern is: 2M +X2 ---> 2MX
Here, M stands for the symbol of any Group 1 element and X for the symbol of any Group 7 element, for example, sodium Na (M), and chlorine Cl (X):
2Na + Cl2 ---> 2NaCl
In displacement reactions, it is the lower of the halogens that is displaced from its compounds as the element.
The compounds of the Group 1 metals and Group 7 non-metals, such as sodium chloride, have lots of similarities. They are colourless, crystalline solids. They have high melting points and when they are melted they will conduct an electric current.
When we think of an explanation for the properties of these compounds we have to make sure that all these facts can be accounted for. We can use our imagination to see how our ideas explain the facts.
An electric current is produced when charged particles move through a substance. Electric current flows through molten sodium chloride, so the molten salt must be made up of charged particles. The charged particles are called IONS. Ions are atoms that have gained a positive or negative charge.
We say that the compounds of Group 1 and Group 7 are IONIC COMPOUNDS because they are made up of ions.
When the salt is molten, the lamp lights up, showing that a current is passing through it. Ions are formed from atoms by gaining or losing electrons and have a charge. Atoms do not have an electric charge.
Positive and negative ions
An ionic compound is made up of positively charged ions and negatively charged ions. The positive ions are atoms that have lost electrons.
In sodium chloride the sodium ion is a sodium atom that has lost one electron, so it has a positive charge. Its symbol becomes Na+. The chloride ion is a chlorine atom that has gained one electron, so it has a negative charge. Its symbol becomes Cl-. Overall the sodium chloride has no charge because the positive sodium ions are balanced by the negative chloride ions.
We can use this explanation to predict that other Group 1 and Group 7 compounds are also made up of positive and negative ions. All the Group 1 metals will form ions with a 1+ charge and all the Group 7 elements will form ions with a 1- charge. Thus lithium iodide is made up of Li+ and I- ions.
When a metal atom forms a positive ion, it loses the electrons from its outer shell. Thus, a sodium atom, Na, with the electron arrangement 2.8.1 becomes a sodium ion, Na+ with the electron arrangement 2.8. Note that this electron arrangement is the same as that of a neon atom. When a non-metal atom forms a negative ion, it gains electrons in its outer shell. Thus, a chlorine atom, Cl, with the electron arrangement, 2.8.7 becomes a chloride ion, Cl-, with the electron arranegement 2.8.8. Together, the sodium and chloride ions form the compound sodium chloride.
Ionic compounds such as sodium chloride and copper sulfate form crystals which have a regular shape. In the crystals the ions pack together in a regular pattern which is repeated over and over again. This pattern is called a crystal LATTICE.
In the sodium chloride lattice, each positively charged sodium ion is surrounded by six negatively charged chlorine ions. Each chlorine ion is surrounded by six sodium ions. This pattern is repeated and builds up to form the cubic sodium chloride crystals.
In the solid state the ions are fixed in the lattice. When the ionic substance melts, the ions become free to move.
When an IONIC COMPOUND dissolves in water the ions leave the lattice. They are separated by water molecules and can move freely. When an electric current is passed into a liquid ionic compound or a solution, the charged ions move to the ELECTRODES (the ANODE and the CATHODE) and complete the circuit.
This is why an ionic compound conducts electricity when it is liquid or in solution, but not in a solid.