On what property are the elements of the modern Periodic Table placed in order?

They are placed in order of increasing atomic number, that is, the number of protons in the nucleus of an atom of the element.

Why is atomic number, rather than relative atomic mass, used to order the elements?

Atomic number is a whole number that is fixed and unique for each element, whereas relative atomic mass depends on the mixture of isotopes present. Ordering by mass puts a few pairs of elements in the wrong place, while ordering by atomic number places every element in the group whose chemistry it shares.

What does the period number of an element tell you about its atoms?

It gives the number of the highest occupied principal energy level, in other words how many electron shells contain electrons. An element in Period 3 has electrons filling up to the third shell.

What do the elements in a group of the Periodic Table have in common?

They have the same number of electrons in their outer shell and the same type of outer sub-shell occupancy. Because chemical behaviour is governed largely by the outer electrons, the members of a group show similar chemical properties and a gradation in physical ones.

An element has 15 protons. Deduce its position in the Periodic Table and give its outer electron configuration.

With 15 protons the configuration is 1s² 2s² 2p⁶ 3s² 3p³. The highest occupied shell is the third, so it is in Period 3, and the outer shell holds five electrons, so it is in Group 5. The element is phosphorus and it lies in the p block.

WJEC AS-Level Chemistry 1410QS

Formulae and Equations

Topic overview Charges on common ions Building formulae for ionic and molecular compounds Assigning oxidation numbers Using oxidation numbers to follow redox changes Balancing equations and using state symbols Ionic and half equations

Basic Ideas About Atoms

Topic overview Radioactive decay and nuclear equations Alpha, beta and gamma radiation Half-life, dating and the uses of radioisotopes: half-life of radioactive decay Half-life, dating and the uses of radioisotopes: adverse consequences for living cells Ionisation energies and electronic structure Orbitals and the filling of sub-shells Atomic spectra and the energy of light: atomic emission spectrum of the hydrogen atom Atomic spectra and the energy of light: relationship between energy and frequency Atomic spectra and the energy of light: significance of the frequency

Chemical Calculations

Topic overview Relative masses and the mass spectrometer: relative atomic Relative masses and the mass spectrometer: how a mass spectrometer works Relative masses and the mass spectrometer: molecular ion The mole, the Avogadro constant and molar mass: relative mass terms and simple mass spectra The mole, the Avogadro constant and molar mass: moles and percentage composition by mass Empirical and molecular formulae Concentration, solubility and titration calculations: concentration in mol dm-3 and in g dm-3 Concentration, solubility and titration calculations: stoichiometry and the limiting reagent Concentration, solubility and titration calculations: titration calculations and estimating a titre Gas volumes and the ideal gas equation Yield, atom economy and experimental uncertainty: concepts of atom economy and percentage yield Yield, atom economy and experimental uncertainty: how to estimate the percentage error

Bonding

Topic overview Ionic bonds and covalent bonds: ionic bonding in terms of ion formation Ionic bonds and covalent bonds: covalent bonding and forces Electronegativity, polarity and in-between bonds: concepts of electronegativity and bond polarity Electronegativity, polarity and in-between bonds: forces Forces between molecules Hydrogen bonding Shapes of molecules and ions: VSEPR principle and its use Shapes of molecules and ions: bond angles associated with linear

Solid Structures

Topic overview Ionic crystals: sodium chloride and caesium chloride Giant covalent structures: diamond and graphite Simple molecular solids: iodine and ice Metals and the electron sea Using structure to explain physical properties

The Periodic Table

Topic overview How the Periodic Table is organised Oxidation and reduction as electron transfer Trends across periods and down groups Group 2: reactions, trends and the reasons behind them — reactions with oxygen, water and dilute acid Group 2: reactions, trends and the reasons behind them — trends down the group Group 2: reactions, trends and the reasons behind them — thermal stability of the carbonates Group 2: reactions, trends and the reasons behind them — oxides and hydroxides Identifying ions, making salts and gravimetric analysis: identifying cations with sodium hydroxide Identifying ions, making salts and gravimetric analysis: preparing a soluble salt from an acid Group 7: the halogens — physical properties and the trend in volatility Group 7: the halogens — reactions of the halogens with metals Group 7: the halogens — displacement reactions and the trend Group 7: the halogens — halide ions as reducing agents, uses of chlorine and the chlorate equilibrium

Simple Equilibria and Acid-Base Reactions

Topic overview Reversible reactions and dynamic equilibrium Le Chatelier's principle and shifting the position of equilibrium The equilibrium constant Kc Acids, bases, salts and neutralisation pH, strong acids and weak acids Acid-base titrations and their calculations

Thermochemistry

Topic overview Enthalpy changes and their definitions Calculating enthalpy changes from experimental measurements Determining enthalpy changes in the laboratory Hess's law and energy cycles Average bond enthalpies

Rates of Reaction

Topic overview Collision theory and the conditions that change rate Measuring rate and relating it to concentration Energy profiles and activation energy Temperature and the Boltzmann distribution Catalysts and how they work Following reactions in the laboratory

The Wider Impact of Chemistry

Topic overview Social and economic consequences of chemical manufacture and energy supply Environmental consequences of chemical processes and energy production Green chemistry and sustainable practice

Organic Compounds

Topic overview Drawing and reading organic structures Naming organic compounds Chain length, functional groups and physical properties Structural isomerism Electrophiles, nucleophiles, radicals and bond fission

Hydrocarbons

Topic overview Alkane bonding, combustion and fossil fuels: combustion reaction of alkanes and benefits Alkane bonding, combustion and fossil fuels: combustion calculations and acid rain Radical substitution of alkanes The C=C bond and why alkenes are reactive E–Z isomerism in alkenes Electrophilic addition, Markovnikov's rule and testing for alkenes: mechanism of electrophilic addition Electrophilic addition, Markovnikov's rule and testing for alkenes: Markovnikov's rule and the bromine water test Electrophilic addition, Markovnikov's rule and testing for alkenes: testing for a C=C bond Hydrogenation and addition polymers: conditions required Hydrogenation and addition polymers: nature of addition polymerisation

Halogenoalkanes

Topic overview Nucleophilic substitution by hydroxide ions Elimination to form alkenes Reactivity of the carbon–halogen bond and the silver nitrate test Uses of halogenoalkanes, CFCs and the ozone layer: uses of halogenoalkanes and CFCs Uses of halogenoalkanes, CFCs and the ozone layer: radical mechanism Uses of halogenoalkanes, CFCs and the ozone layer: replacements for CFCs Heating under reflux in the laboratory

Alcohols and Carboxylic Acids

Topic overview Two routes to ethanol: hydration and fermentation — hydration of ethene Two routes to ethanol: hydration and fermentation — fermentation Two routes to ethanol: hydration and fermentation — comparing the two routes on rate Classifying alcohols and dehydrating them Oxidising alcohols to aldehydes, ketones and acids Chemical tests for alcohols and carboxylic acids Carboxylic acids, bases and salts Making an ester and separating it by distillation: esterification Making an ester and separating it by distillation: carrying out the preparation Making an ester and separating it by distillation: separating and purifying Making an ester and separating it by distillation: yield

Instrumental Analysis

Topic overview Reading a mass spectrum Infrared spectra and functional groups NMR spectra and atomic environments Putting the three techniques together

Practical Skills and Techniques

Topic overview Measuring mass, volume, time and temperature Standard solutions and titration technique Indicators and acid–base titration curves Heating, refluxing, distilling and purifying: Heating Heating, refluxing, distilling and purifying: purifying Melting temperature, chromatography and pH measurement Electrochemical cells, rate methods and working safely: Electrochemical cells Electrochemical cells, rate methods and working safely: rate methods and working safely

Planning, Analysis and Evaluation

Topic overview Planning an investigation and controlling variables Recording observations and presenting data Graphs, gradients and processing data Errors, uncertainty, accuracy and precision Evaluating a method and suggesting improvements Research and referencing

Core Experiments and Their Chemistry

Topic overview Gravimetric and acid–base analysis Enthalpy and rate experiments: Enthalpy Enthalpy and rate experiments: rate experiments Redox titrations and electrochemical cells Equilibrium constants and following a titration with a pH probe Organic synthesis, separation and identification: Organic synthesis Organic synthesis, separation and identification: identification tests and chromatography

Mathematical Skills in Chemistry

Topic overview Units, standard form and proportion in chemical calculation: volume, equilibrium, rate and energy units Units, standard form and proportion in chemical calculation: standard form Units, standard form and proportion in chemical calculation: proportion Units, standard form and proportion in chemical calculation: checking answers, buffers, rate constants and units Significant figures, means and uncertainty in experimental data: significant figures in multiplication Significant figures, means and uncertainty in experimental data: relative atomic mass as a weighted mean Significant figures, means and uncertainty in experimental data: absolute and percentage uncertainty Symbols, rearranging equations and logarithms: chemical symbols and what each one asserts Symbols, rearranging equations and logarithms: rearranging an equation Symbols, rearranging equations and logarithms: substituting into equations for Kc, enthalpy and pH Symbols, rearranging equations and logarithms: logarithms, the Arrhenius equation and rearranging formulae Graphs: plotting, gradients, intercepts and tangents — reading a spectrum Graphs: plotting, gradients, intercepts and tangents — reaction order from data and plotting good graphs Graphs: plotting, gradients, intercepts and tangents — Arrhenius plots, zero-order graphs and tangents Graphs: plotting, gradients, intercepts and tangents — initial rates from tangents and interpreting graphs Shape, symmetry and three-dimensional structure: Shape Shape, symmetry and three-dimensional structure: symmetry

The Periodic Table

13 subtopics
39 things to know
AS Unit 1

What The Periodic Table covers

The Periodic Table is examined on AS Unit 1 of WJEC Chemistry, The Language of Chemistry, Structure of Matter and Simple Reactions. 243 recall questions across 13 subtopics.

Where it is assessed

AS Unit 1

The Language of Chemistry, Structure of Matter and Simple Reactions.

Check the WJEC specification ↗

The 13 subtopics

Each one has its own page. WJEC AS Chemistry students sit this topic in the first year.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

Argon has a relative atomic mass of 39.9 and potassium 39.1, yet argon comes first in the Periodic Table. Explain.

Key term

State the three blocks of the Periodic Table that the s, p and d labels refer to, and say what determines the label.

Key term

Write the electron configuration of a sodium atom and use it to place sodium in the Periodic Table.

Numbers

In the reaction between magnesium and chlorine, which element is oxidised and which is reduced?

Key term

Write half equations for the oxidation and the reduction taking place when calcium burns in oxygen.

Key term

Oxygen is usually assigned an oxidation number of −2, but not in hydrogen peroxide. What is it there, and why?

Key term

State and explain the trend in atomic radius across Period 3.

Key term

Why does atomic radius increase down a group?

Key term

Why does silicon have the highest melting temperature in Period 3?

Key term

Write the equation for the burning of magnesium in oxygen and describe what is seen.

Numbers

What is observed when calcium is burned in oxygen, and what is the equation?

Key term

Give the equation for barium burning in oxygen and state the colour of the flame.

Key term

State and explain the trend in reactivity down Group 2.

Numbers

Give the sums of the first two ionisation energies for magnesium, calcium and barium, and relate them to reactivity.

Key term

Nuclear charge increases down Group 1, so why does reactivity not decrease?

Key term

State and explain the trend in thermal stability of the Group 2 carbonates.

Key term

Does the thermal stability of the Group 2 hydroxides follow the same trend as the carbonates?

Key term

Describe an experiment to compare the thermal stability of two Group 2 carbonates.

Key term

State and explain the trend in solubility of the Group 2 sulfates.

Key term

Predict what happens to the pH of a saturated Group 2 hydroxide solution as you move down the group.

Key term

Why is a soluble barium salt, rather than a calcium or magnesium one, used with dilute acid to test for sulfate ions?

Key term

What is seen when sodium hydroxide solution is added to a solution containing magnesium ions, and what is the ionic equation?

Key term

What is seen when sodium hydroxide solution is added to solutions of magnesium ions and of barium ions, and how soluble is calcium hydroxide?

Key term

Why does the precipitate formed with hydroxide ions become less obvious as you move from Mg²⁺ to Ba²⁺?

Key term

Why must a precipitate be washed with distilled water before it is weighed in a gravimetric analysis?

Key term

Why is a slight excess of the precipitating reagent added in a gravimetric analysis?

Who and when

In a gravimetric analysis a student did not wash the precipitate before drying it. State the effect on the calculated result and explain why.

Numbers

Why must bromine always be handled in a fume cupboard?

Key term

Why does the enthalpy of vaporisation of the halogens increase down the group?

Key term

Write the equation for the reaction of sodium with chlorine and describe what is seen.

Key term

Write the equation for magnesium burning in chlorine and state the type of bonding in the product.

Key term

Describe what happens when chlorine is passed over heated iron wool, and give the equation.

Key term

Both sodium chloride and sodium iodide can be made by direct combination of the elements. Which forms more readily, and why?

Process

Rank the halide ions in order of reducing power and explain the order.

Key term

Why is fluorine the most powerful oxidising agent of all the elements?

Process

Chlorine and bromine both oxidise Fe²⁺ to Fe³⁺, but iodine does not. Explain, using electrode potentials.

Key term

Write the equation for the reaction of chlorine with aqueous sodium bromide and describe what is seen.

Key term

Explain, using oxidation numbers, why the reaction between chlorine and bromide ions is a redox reaction.

Key term

Give the appearance and state at room temperature of fluorine, chlorine, bromine and iodine.

Take The Periodic Table further

The same content, four other ways.

Questions from this topic

Each one with the model answer underneath.

1
On what property are the elements of the modern Periodic Table placed in order?
Model answer
They are placed in order of increasing atomic number, that is, the number of protons in the nucleus of an atom of the element.
2
Why is atomic number, rather than relative atomic mass, used to order the elements?
Model answer
Atomic number is a whole number that is fixed and unique for each element, whereas relative atomic mass depends on the mixture of isotopes present. Ordering by mass puts a few pairs of elements in the wrong place, while ordering by atomic number places every element in the group whose chemistry it shares.
3
Argon has a relative atomic mass of 39.9 and potassium 39.1, yet argon comes first in the Periodic Table. Explain.
Model answer
Argon has 18 protons and potassium has 19, so argon has the lower atomic number and is placed first. Argon is unusually heavy for its position because its most abundant isotope is argon-40, while potassium is dominated by potassium-39. Ordering by mass would place potassium among the noble gases, which its chemistry flatly contradicts.
4
What does the period number of an element tell you about its atoms?
Model answer
It gives the number of the highest occupied principal energy level, in other words how many electron shells contain electrons. An element in Period 3 has electrons filling up to the third shell.
5
What do the elements in a group of the Periodic Table have in common?
Model answer
They have the same number of electrons in their outer shell and the same type of outer sub-shell occupancy. Because chemical behaviour is governed largely by the outer electrons, the members of a group show similar chemical properties and a gradation in physical ones.
6
An element has 15 protons. Deduce its position in the Periodic Table and give its outer electron configuration.
Model answer
With 15 protons the configuration is 1s² 2s² 2p⁶ 3s² 3p³. The highest occupied shell is the third, so it is in Period 3, and the outer shell holds five electrons, so it is in Group 5. The element is phosphorus and it lies in the p block.

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Formulae and Equations

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The Periodic Table revision questions

Is The Periodic Table examined at AS in WJEC Chemistry?: Yes. It sits in AS Unit 1, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.
What does The Periodic Table cover in WJEC AS-Level Chemistry?: 13 subtopics: how the periodic table is organised, oxidation and reduction as electron transfer, trends across periods and down groups, group 2: reactions, trends and the reasons behind them — reactions with oxygen, water and dilute acid, group 2: reactions, trends and the reasons behind them — trends down the group, group 2: reactions, trends and the reasons behind them — thermal stability of the carbonates, group 2: reactions, trends and the reasons behind them — oxides and hydroxides, identifying ions, making salts and gravimetric analysis: identifying cations with sodium hydroxide, identifying ions, making salts and gravimetric analysis: preparing a soluble salt from an acid, group 7: the halogens — physical properties and the trend in volatility, group 7: the halogens — reactions of the halogens with metals, group 7: the halogens — displacement reactions and the trend and group 7: the halogens — halide ions as reducing agents, uses of chlorine and the chlorate equilibrium. Each one has its own page with questions and model answers.
How is The Periodic Table organised?: By the WJEC 1410QS specification, subtopic by subtopic, so it matches the order you are taught it in.

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