4.1.2.1The periodic table
The elements in the periodic table are arranged in order of atomic (proton) number, and so that elements with similar properties are in columns, known as groups. The table is called a periodic table because similar properties occur at regular intervals.
Elements in the same group have the same number of electrons in their outer shell — their outer electrons — and this gives them similar chemical properties. That single sentence explains almost everything else in this topic: the whole table is a map of electronic structure, and a group is just a set of elements that all end their filling in the same place.
Two things are asked for.
Explain how the position of an element is related to the arrangement of electrons in its atoms, and hence to its atomic number. The group number gives the number of outer electrons (Group 0 being the exception — a full outer shell). The period, or row, gives the number of occupied shells. Because the elements are ordered by atomic number and the electrons are filled from the inside out, position and electronic structure are the same information written two ways.
Predict possible reactions and probable reactivity from position. An element in Group 1 will react like the other Group 1 metals. An element between two known elements in a group will have properties between theirs.
Worked example
An element has the electronic structure 2,8,2. Give its group, its period, and one prediction about its reactions.
- Two electrons in the outer shell means Group 2.
- Three occupied shells means period 3.
- Adding the electrons gives 12 electrons, so 12 protons — atomic number 12, magnesium.
- A Group 2 metal loses its two outer electrons, so it will react to form positive ions with a 2+ charge.
Answer: Group 2, period 3; it will form 2+ ions and react as a metal.
- Periodic table
- an arrangement of the elements in order of atomic (proton) number, in which elements with similar properties are placed in columns called groups; it is periodic because similar properties recur at regular intervals
- Group
- a column of the periodic table; all its elements have the same number of electrons in their outer shell, which gives them similar chemical properties
4.1.2.2Development of the periodic table
Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights.
The early periodic tables were incomplete, and some elements were placed in inappropriate groups if the strict order of atomic weights was followed.
Mendeleev overcame some of the problems by leaving gaps for elements that he thought had not been discovered, and in some places changed the order based on atomic weights.
Elements with properties predicted by Mendeleev were discovered and filled the gaps. Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct.
You should be able to describe these steps. Two of them carry most of the marks, so make sure you can say why each mattered.
Why the gaps were the clever move: leaving a gap said that an undiscovered element existed and predicted what it would be like. When those elements were found and matched the predictions, that was evidence for the table — testing a prediction is what supports or refutes a new scientific idea, and a classification that predicts is doing more than one that only sorts.
Why isotopes fixed the ordering problem: relative atomic mass is an average across isotopes, so an element with a lot of heavy isotopes can have a higher atomic weight than the element after it. Ordering by atomic (proton) number instead removes the anomaly entirely, because the proton number always increases by one.
Common mistake
Mendeleev did not know about protons — that is the whole point of the story. He swapped pairs of elements because their properties demanded it, without being able to say why the atomic weights disagreed. The explanation only arrived with isotopes.
4.1.2.3Metals and non-metals
Elements that react to form positive ions are metals. Elements that do not form positive ions are non-metals.
The majority of elements are metals. Metals are found to the left and towards the bottom of the periodic table. Non-metals are found towards the right and top.
You are asked to explain the differences between metals and non-metals on the basis of these characteristic physical and chemical properties, and to explain how the atomic structure of metals and non-metals relates to their position in the periodic table.
The link is the number of outer electrons. Metals have few outer electrons, so losing them is the easy route to a full outer shell — that is why they form positive ions. Non-metals have nearly full outer shells, so gaining or sharing a small number of electrons is easier than losing many — that is why they form negative ions or covalent bonds.
The same argument explains how the reactions of elements are related to the arrangement of electrons in their atoms, and hence to their atomic number. This links forward to Group 0, Group 1, Group 7, and to bonding, structure and the properties of matter.
- Metal
- an element that reacts to form positive ions; metals are found to the left and towards the bottom of the periodic table
- Non-metal
- an element that does not form positive ions; non-metals are found towards the right and top of the periodic table
4.1.2.4Group 0
The elements in Group 0 of the periodic table are called the noble gases. They are unreactive and do not easily form molecules because their atoms have stable arrangements of electrons.
The noble gases have eight electrons in their outer shell, except for helium, which has only two electrons.
The boiling points of the noble gases increase with increasing relative atomic mass — that is, going down the group.
Worked example
Radon comes below xenon in Group 0. Predict its boiling point and say whether it is reactive.
- Radon has a greater relative atomic mass than xenon, and boiling point increases down the group.
- So radon's boiling point must be higher than xenon's −108 degrees C.
- A sensible prediction would be somewhere around −60 degrees C — above xenon, but still far below room temperature.
- Radon is in Group 0, so its atoms have a stable arrangement of electrons.
Answer: A boiling point above −108 degrees C, and unreactive because it has a stable arrangement of electrons in its outer shell.
You should be able to explain how properties of the elements in Group 0 depend on the outer shell of electrons of the atoms, and predict properties from given trends down the group. When a question hands you a table of data, read the trend off the data you are given rather than reciting a remembered figure — the marks are for using the trend.
- Noble gases
- the elements in Group 0, which are unreactive and do not easily form molecules because their atoms have stable arrangements of electrons — eight in the outer shell, except helium which has two
4.1.2.5Group 1
The elements in Group 1 of the periodic table are known as the alkali metals, and have characteristic properties because of the single electron in their outer shell.
In Group 1, the reactivity of the elements increases going down the group. Losing that single outer electron is what these reactions consist of, and the further down the group you go, the further that outer electron is from the nucleus and the more shells shield it — so it is lost more easily, and the element is more reactive.
You should be able to describe the reactions of the first three alkali metals with oxygen, chlorine and water. Two general equations cover most of them:
• metal + water → metal hydroxide + hydrogen, for example 2Na + 2H₂O → 2NaOH + H₂ • metal + chlorine → metal chloride, for example 2Na + Cl₂ → 2NaCl
You should also be able to explain how properties of the elements in Group 1 depend on the outer shell of electrons of the atoms, and predict properties from given trends down the group.
Common mistake
Group 1 reactivity increases down the group, Group 7 reactivity decreases. Both follow from the same cause — the outer shell gets further from the nucleus — but the direction is opposite because Group 1 loses an electron and Group 7 gains one. Learn the reason, not the two directions separately.
- Alkali metals
- the elements in Group 1, which have a single electron in their outer shell; reactivity increases down the group because the outer electron is further from the nucleus and more shielded, so it is lost more easily
4.1.2.6Group 7
The elements in Group 7 of the periodic table are known as the halogens, and have similar reactions because they all have seven electrons in their outer shell. The halogens are non-metals and consist of molecules made of pairs of atoms — Cl₂, Br₂, I₂.
In Group 7, the further down the group an element is, the higher its relative molecular mass, melting point and boiling point.
In Group 7, the reactivity of the elements decreases going down the group. Gaining one electron is what these reactions consist of, and the further down the group, the further the outer shell is from the nucleus and the more it is shielded — so an incoming electron is attracted less strongly and the element is less reactive.
You should be able to describe the nature of the compounds formed when chlorine, bromine and iodine react with metals and non-metals.
With metals, the halogens form ionic compounds — metal halides such as sodium chloride, in which the halogen has gained an electron to become a halide ion with a 1− charge.
With non-metals, the halogens form covalent compounds — molecules such as hydrogen chloride, in which pairs of electrons are shared.
A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt. Because reactivity decreases down the group, a halogen displaces any halogen below it and none above it.
Worked example
Chlorine solution is added to potassium bromide solution. State what you would see, and write the equation.
- Chlorine is above bromine in Group 7, so chlorine is more reactive than bromine.
- A more reactive halogen displaces a less reactive one from an aqueous solution of its salt, so chlorine displaces the bromide.
- The bromine released colours the solution orange.
- Chlorine takes the potassium; bromine is left as a molecule.
Answer: The colourless solution turns orange as bromine is displaced: Cl₂ + 2KBr → 2KCl + Br₂. Adding bromine to potassium chloride gives no reaction, because bromine is less reactive than chlorine.
Practical skill
Displacement reactions of the halogens
- Set out a grid of test tubes or a spotting tile, with a column for each halogen solution — chlorine, bromine and iodine water — and a row for each halide solution — potassium chloride, potassium bromide and potassium iodide.
- Add a few drops of each halogen solution to each halide solution.
- Record the colour of every mixture, including the ones where nothing happens.
- Compare each mixture with the colour of the halogen solution on its own, so that a colour change can be attributed to a displaced halogen rather than to the halogen added.
You should be able to explain how properties of the elements in Group 7 depend on the outer shell of electrons of the atoms, and predict properties from given trends down the group.
- Halogens
- the elements in Group 7, non-metals with seven electrons in their outer shell that exist as molecules of pairs of atoms; reactivity decreases down the group because the outer shell is further from the nucleus and more shielded, so an electron is gained less easily
Before you practise
- Elements are arranged in order of atomic (proton) number; a group is a column, and its members share the same number of outer electrons and so similar chemical properties.
- Mendeleev left gaps for undiscovered elements and changed the order in places; knowledge of isotopes later explained why ordering by atomic weight failed.
- Metals react to form positive ions and sit left and towards the bottom; non-metals do not form positive ions and sit right and towards the top.
- Group 0 noble gases are unreactive with stable arrangements of electrons — eight outer electrons, except helium with two; boiling point rises down the group.
- Group 1 alkali metals have one outer electron; reactivity increases down the group; with water they give a hydroxide and hydrogen.
- Group 7 halogens have seven outer electrons and exist as molecules made of pairs of atoms; relative molecular mass, melting point and boiling point rise down the group, but reactivity falls.
- A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt.