How is the periodic table organised, and how do the properties of the groups change?
The arrangement of the periodic table by atomic number into groups and periods, the development by Mendeleev, the link between group number and outer electrons, and the properties and trends of Group 1, Group 7 and Group 0 elements.
A focused answer to the OCR Gateway GCSE Combined Science A work on the periodic table, covering the arrangement by atomic number into groups and periods, Mendeleev's development, the link between group and outer electrons, and the trends in Group 1, Group 7 and Group 0.
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What this topic is asking
OCR wants you to describe how the periodic table is arranged by atomic number into groups and periods, explain Mendeleev's contribution, link the group number to the outer electrons, and describe the properties and trends of Group 1, Group 7 and Group 0.
How the table is arranged
Because elements are arranged by atomic number, those with similar properties line up in the same group. The group number tells you the number of electrons in the outer shell (for the main groups), and this controls how the element reacts. Metals are on the left and centre, and non-metals on the right. Dmitri Mendeleev produced an early periodic table by arranging the known elements in order of atomic weight and grouping those with similar properties; crucially he left gaps for elements not yet discovered and even predicted their properties, which were later confirmed when those elements were found. This is why his table was accepted, and the modern table (ordered by atomic number) follows the same idea.
Group 1 and Group 7 trends
The alkali metals (such as lithium, sodium and potassium) are soft, have low densities, and react vigorously with water to make a metal hydroxide (an alkali) and hydrogen gas, for example sodium with water gives sodium hydroxide and hydrogen. The reactions get more violent down the group. The halogens (such as chlorine, bromine and iodine) exist as diatomic molecules (, , ), and a more reactive halogen will displace a less reactive one from a solution of its salt (for example chlorine displaces bromine from potassium bromide), which is a way of comparing their reactivity.
Group 0 and the structure of the table
The Group 0 noble gases (helium, neon, argon and so on) are colourless, unreactive (inert) gases that exist as single atoms. They are unreactive because they have a full outer shell of electrons (helium has 2, the rest have 8), which is a very stable arrangement, so they have no tendency to gain, lose or share electrons. Their boiling points increase down the group as the atoms get larger. Knowing the trends in these three groups lets you predict reactions and is a frequent exam theme that links straight to bonding and to predicting reactions.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksExplain why the Group 1 metals become more reactive going down the group, using ideas about electrons.Show worked answer →
A Chemistry Paper 4 structured question on Group 1 trends. Reward: Group 1 atoms all have one electron in their outer shell, and they react by losing that electron to form a ion. Going down the group, the atoms get larger because they have more electron shells, so the outer electron is further from the nucleus and is also shielded by more inner shells. This means the outer electron is held less strongly and is lost more easily, so the element is more reactive. Markers credit one outer electron lost to form a ion, the increasing distance and shielding down the group, and the link from "outer electron lost more easily" to "more reactive".
OCR 20214 marksDescribe two properties of the Group 0 noble gases and explain, in terms of electronic structure, why they are unreactive.Show worked answer →
A C2 question on Group 0. Reward two properties: the noble gases are colourless gases at room temperature, they are monatomic (exist as single atoms), they have very low boiling points (which increase down the group), and they are chemically unreactive (inert). The explanation: noble gases have a full outer shell of electrons (helium has 2, the others have 8), which is a very stable arrangement, so they have no tendency to gain, lose or share electrons and therefore do not easily react. Markers want two valid properties and the link from a full (stable) outer shell to being unreactive.
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