1.13–1.15Development 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.
1.16–1.17The 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
1.18Metals 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
1.19–1.20Electronic structure
The electrons in an atom occupy the lowest available energy levels — the innermost available shells. The electronic structure of an atom can be represented by numbers or by a diagram.
Sodium is 2,8,1: two electrons in the lowest energy level, eight in the second, and one in the third. You may answer in terms of either energy levels or shells; both are accepted.
The shells fill 2, then 8, then 8, which covers every one of the first twenty elements — and you should be able to represent all twenty in both forms.
Worked example
Give the electronic structure of an atom with atomic number 17, and say which group of the periodic table it is in.
- Atomic number 17 means 17 electrons.
- Fill from the inside: 2 in the first shell, leaving 15.
- 8 in the second shell, leaving 7.
- 7 in the third shell.
- The number of outer electrons gives the group.
Answer: 2,8,7 — seven outer electrons, so it is in Group 7. (It is chlorine.)
Common mistake
Draw the electrons in pairs around the shell rather than spread evenly, and count them before moving on. A diagram with nine electrons in the second shell is the commonest way to lose an easy mark.
- Electronic structure
- the arrangement of an atom's electrons in shells, filling the lowest available energy levels first — sodium is 2,8,1
Before you practise
- Mendeleev ordered the known elements by atomic weight, left gaps, and predicted the properties of elements not yet found.
- He was really ordering by atomic number, which had not been discovered — which is why a few pairs had to be swapped.
- The modern table is in order of increasing atomic number: rows are periods, columns are groups.
- The group number is the number of electrons in the outer shell; the period number is the number of shells.
- Metals are on the left and non-metals on the right — metals lose electrons to form positive ions, non-metals gain them.
- Write electronic configurations as 2,8,8 and check the total equals the atomic number.