Chemistry: The Periodic Table and Separating Mixtures
The periodic table and separating mixtures, for 13+ Chemistry, covers how the modern periodic table is arranged into groups and periods, the properties of metals, non-metals and three important groups, and how chromatography separates and identifies soluble coloured substances.
Before you start
Make sure you're comfortable with these topics first:
Method
- Learn that the modern periodic table arranges all known elements in order of increasing atomic number (the number of protons in an atom), and that elements in the same vertical column, called a group, have similar chemical properties because they have the same number of electrons in their outer shell.
- Learn that a horizontal row is called a period, and that the period number tells you how many electron shells an atom of that element has occupied; electron shells fill with a maximum of 2 electrons in the first shell and up to 8 in the second and third shells.
- Learn that metals are found on the left and in the middle of the periodic table (the majority of elements) and are typically shiny, good conductors of electricity and heat, and malleable; non-metals are found on the right and are typically dull, poor conductors of electricity (graphite is an exception), and brittle when solid.
- Learn the properties of Group 1, the alkali metals: soft, low density (some float on water), and they react vigorously with water to produce a metal hydroxide and hydrogen gas; reactivity increases going down the group.
- Learn the properties of Group 7, the halogens: they exist as molecules of two atoms (diatomic), and reactivity decreases going down the group, the opposite trend to Group 1.
- Learn the properties of Group 0, the noble gases: colourless gases that are almost completely unreactive, because their outer electron shell is already full.
- Know the standard chromatography practical for separating soluble coloured substances, such as the dyes in an ink or a sweet coating: draw a pencil line (never pen, since ink would dissolve and move) near the bottom of chromatography paper, add a small, concentrated spot of the sample on the line, and stand the paper in a shallow layer of solvent that must stay below the pencil line; mark the position of the solvent front in pencil as soon as the paper is removed, since the front is hard to see accurately once the solvent has evaporated.
- Learn how to calculate an Rf value from a chromatogram: Rf equals the distance travelled by the substance divided by the distance travelled by the solvent, and know that a pure substance always has the same characteristic Rf value in a given solvent.
Worked example
A pupil runs a chromatogram to separate the dyes in a green sweet coating. The solvent front travels 8 cm up the paper. One of the dye spots travels 6 cm from the pencil line. Calculate the Rf value of this dye.
- Write down the formula: Rf equals the distance travelled by the substance divided by the distance travelled by the solvent.
- Identify the two distances from the chromatogram: the dye travelled 6 cm, and the solvent front travelled 8 cm.
- Substitute the values into the formula: Rf = 6 / 8.
- Calculate: 6 / 8 = 0.75.
- Final answer: the Rf value of the dye is 0.75. Rf is a ratio of two distances, so it has no unit, and its value is always between 0 and 1.
Practice questions
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Q1State the property used to arrange the elements in order in the modern periodic table.Show answer
Answer: Atomic number (the number of protons)
Q2State the name given to a horizontal row of the periodic table.Show answer
Answer: Period
Q3Name the group of the periodic table that contains elements which are almost completely unreactive because their outer electron shell is full.Show answer
Answer: Group 0 (the noble gases)
Q4State whether reactivity increases or decreases going down Group 1, the alkali metals.Show answer
Answer: Increases
Q5In a chromatography experiment, the solvent front travels 10 cm and a dye spot travels 4 cm. Calculate the Rf value of the dye.Show answer
Answer: 0.4 (4 / 10 = 0.4)
Q6Explain why a pencil, not a pen, must be used to draw the baseline on chromatography paper.Show answer
Answer: Pencil marks (graphite) are insoluble and will not move in the solvent, while ink from a pen would dissolve in the solvent and move up the paper, mixing with the sample and spoiling the result
Q7State one physical property that distinguishes a typical non-metal from a typical metal.Show answer
Answer: Any one of: non-metals are dull (metals are shiny), non-metals are poor conductors of electricity (most metals are good conductors), or non-metals are brittle when solid (metals are malleable)
Exam-style questions
Written in the style of a 13+ Common Entrance exam paper, with a full mark scheme.
A student notes the properties of four elements. Element A is shiny, conducts electricity, and is malleable. Element B is dull, does not conduct electricity, and is brittle. Element C is a colourless gas that does not react with any other substance tested. Element D is a soft solid that reacts vigorously with water, floating and fizzing on the surface. (a) State whether Element A is a metal or a non-metal, giving one reason. (b) State whether Element B is a metal or a non-metal, giving one reason. (c) Suggest which group of the periodic table Element C most likely belongs to. (d) Suggest which group of the periodic table Element D most likely belongs to.
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A student separates the dyes in a black ink using paper chromatography. Three spots (red, blue and yellow) appear on the chromatogram. The solvent front has travelled 12 cm from the pencil line. The red spot has travelled 3 cm, the blue spot has travelled 9 cm, and the yellow spot has travelled 10.8 cm. (a) Calculate the Rf value of the red dye. (b) Calculate the Rf value of the blue dye. (c) State which dye travelled closest to the solvent front, and give its Rf value. (d) A pure sample of a known yellow dye has a reference Rf value of 0.90 in the same solvent. State whether this supports the yellow spot on the chromatogram being the same dye, and explain why.
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An atom of sodium (Na) has an atomic number of 11, meaning it has 11 protons and 11 electrons, arranged in shells of 2, 8 and 1 electrons. (a) State which period of the periodic table sodium is found in, and explain your reasoning. (b) State which group of the periodic table sodium is found in, and explain your reasoning. (c) An atom of lithium has electron shells of 2 and 1. Predict, giving a reason, whether sodium is more or less reactive than lithium.
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Free printable worksheet
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This topic is chapter 8 of 13+ Science Workbook, the whole course as one free printable PDF.
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