Patterns in the Periodic Table
SEAB syllabus topic 8: how the position of an element in the Periodic Table predicts its properties, how Group 1, Group 17 and Group 18 behave, what makes transition elements different, and how the reactivity series explains displacement, extraction and rusting, with an exam check for every outcome.
The Periodic Table arranges the elements in order of increasing proton number, so an element's group and period tell you its outer electrons and its properties.
Atoms of one group share an outer-electron count, so Group 1 metals, Group 17 halogens and Group 18 noble gases each follow a pattern in their properties.
The reactivity series places metals in order of how readily they form positive ions, and it predicts reactions with water, acid, oxides and other metal ions.
Arranging the Periodic Table
describe the Periodic Table as an arrangement of the elements in the order of increasing proton (atomic) number
The Periodic Table arranges the elements in order of increasing proton (atomic) number, with horizontal rows called periods and vertical columns called groups.
- Periodic Table
- Periodic Table: an arrangement of all the elements in order of increasing proton (atomic) number
- Proton (atomic) number
- Proton (atomic) number: the count of protons held in the nucleus of each atom of an element supports K324-2.2(c)
- Nucleon (mass) number
- Nucleon (mass) number: the sum of the protons and the neutrons that make up the nucleus of an atom supports K324-2.2(c)
- Period
- Period: a horizontal row of the Periodic Table, in which the elements have the same number of occupied electron shells
- Group
- Group: a vertical column of the Periodic Table, numbered 1 to 18 in the SEAB table, whose elements have similar properties
- Transition elements
- Transition elements: the block of metals in the middle of the Periodic Table, between Group 2 and Group 13
The first twenty elements
| Element | Proton number | Symbol |
|---|---|---|
| Hydrogen | 1 | H |
| Helium | 2 | He |
| Lithium | 3 | Li |
| Beryllium | 4 | Be |
| Boron | 5 | B |
| Carbon | 6 | C |
| Nitrogen | 7 | N |
| Oxygen | 8 | O |
| Fluorine | 9 | F |
| Neon | 10 | Ne |
| Sodium | 11 | Na |
| Magnesium | 12 | Mg |
| Aluminium | 13 | Al |
| Silicon | 14 | Si |
| Phosphorus | 15 | P |
| Sulfur | 16 | S |
| Chlorine | 17 | Cl |
| Argon | 18 | Ar |
| Potassium | 19 | K |
| Calcium | 20 | Ca |
Groups and periods
The SEAB glossary says “State implies a concise answer with little or no supporting argument”, so when asked how the elements are arranged, write increasing proton number and stop.
Depth: why and how
- Why is the table ordered by proton number and not by mass? The proton number defines the element, and ordering by it makes properties repeat in a pattern from one period to the next.
- How many electrons are in a neutral atom of sodium? A neutral atom has equal numbers of protons and electrons, so a sodium atom with proton number 11 has 11 electrons.
- How is the number of neutrons found? The number of neutrons is the nucleon number minus the proton number, so a fluorine atom with nucleon number 19 and proton number 9 has 10 neutrons.
- Why does argon come before potassium although argon has the larger relative atomic mass? Argon has relative atomic mass 40 and potassium 39, but argon has the smaller proton number, and the table follows proton number.
- Cross-link to The Particulate Nature of Matter: an atom has protons and neutrons in the nucleus and electrons arranged in shells.
Common mistakes
The elements are arranged in order of increasing relative atomic mass.
The elements are arranged in order of increasing proton (atomic) number.
Groups are the horizontal rows of the Periodic Table.
Groups are the vertical columns and periods are the horizontal rows.
The nucleon number decides where an element is placed.
The proton number decides where an element is placed, and the nucleon number is the number of protons plus neutrons.
Read the Periodic Table by proton number: periods are the rows and groups are the columns, numbered 1 to 18.
Exam check
Q1In which order are the elements arranged in the Periodic Table?
- Aincreasing relative atomic mass
- Bincreasing number of neutrons
- Cincreasing proton (atomic) number
- Dalphabetical order of their names
Show answer
MCQ answer key C (increasing proton (atomic) number): the modern Periodic Table follows increasing proton number, and the other orders do not match the table.
Q2What is a vertical column of elements in the Periodic Table called?
- Aa period
- Ba shell
- Ca group
- Da block of neutrons
Show answer
MCQ answer key C (a group): vertical columns are groups, whereas horizontal rows are periods.
Q3Sodium has proton number 11. How many protons and electrons are in a neutral sodium atom?
- A11 protons and 11 electrons
- B11 protons and 12 electrons
- C12 protons and 11 electrons
- D23 protons and 11 electrons
Show answer
MCQ answer key A (11 protons and 11 electrons): a neutral atom has equal numbers of protons and electrons, and both equal the proton number.
StructuredFluorine has proton number 9 and nucleon number 19.
- State the order in which the elements are arranged in the Periodic Table. [1]
- State what is meant by the term period. [1]
- State the number of electrons in a neutral fluorine atom. [1]
- Calculate the number of neutrons in an atom of fluorine. [2]
Total: 5 marks
Show mark scheme
(a) 1 mark
- Mark scheme: increasing proton (atomic) number
(b) 1 mark
- Mark scheme: a horizontal row of the Periodic Table
(c) 1 mark
- Mark scheme: 9 electrons, equal to the proton number
(d) 2 marks
- Mark scheme: number of neutrons = nucleon number − proton number
- Mark scheme: 19 − 9 = 10 neutrons
Position, proton number and electrons
describe how the position of an element in the Periodic Table is related to proton number and electronic configuration
The proton number gives the electronic configuration of an atom, the number of occupied shells gives the period, and the number of outer (valence) electrons gives the group.
- Electronic configuration
- Electronic configuration: the arrangement of the electrons of an atom in its shells, written as numbers such as 2,8,1 for sodium
- Electron shell
- Electron shell: an energy level around the nucleus that holds a limited number of electrons supports K324-2.2(b)
- Outer (valence) electrons
- Outer (valence) electrons: the electrons in the outermost occupied shell of an atom
- Shell capacity
- Shell capacity: for the first twenty elements the first shell holds up to 2 electrons and the second and third shells hold up to 8 electrons each, although the third shell can hold more electrons in later elements
- Period number
- Period number: the number of occupied electron shells in an atom of the element
- Group number
- Group number: found from the outer electrons: the same number for Groups 1 and 2, and that number plus 10 for Groups 13 to 18, except helium, which has 2 outer electrons and is in Group 18
Electronic configurations of the first twenty elements
| Element | Proton number | Electronic configuration | Group | Period |
|---|---|---|---|---|
| Hydrogen | 1 | 1 | 1 | 1 |
| Helium | 2 | 2 | 18 | 1 |
| Lithium | 3 | 2,1 | 1 | 2 |
| Beryllium | 4 | 2,2 | 2 | 2 |
| Boron | 5 | 2,3 | 13 | 2 |
| Carbon | 6 | 2,4 | 14 | 2 |
| Nitrogen | 7 | 2,5 | 15 | 2 |
| Oxygen | 8 | 2,6 | 16 | 2 |
| Fluorine | 9 | 2,7 | 17 | 2 |
| Neon | 10 | 2,8 | 18 | 2 |
| Sodium | 11 | 2,8,1 | 1 | 3 |
| Magnesium | 12 | 2,8,2 | 2 | 3 |
| Aluminium | 13 | 2,8,3 | 13 | 3 |
| Silicon | 14 | 2,8,4 | 14 | 3 |
| Phosphorus | 15 | 2,8,5 | 15 | 3 |
| Sulfur | 16 | 2,8,6 | 16 | 3 |
| Chlorine | 17 | 2,8,7 | 17 | 3 |
| Argon | 18 | 2,8,8 | 18 | 3 |
| Potassium | 19 | 2,8,8,1 | 1 | 4 |
| Calcium | 20 | 2,8,8,2 | 2 | 4 |
Reading the position of an atom
The SEAB glossary says “Describe requires candidates to state in words (using diagrams where appropriate) the main points of the topic”, so when you describe the position of an element, give its proton number, electronic configuration, group and period.
Depth: why and how
- How do you find the group and period from an electronic configuration? For chlorine with configuration 2,8,7, the last number gives 7 outer electrons, so the group is 7 + 10 = 17, and the three numbers show three shells, so the period is 3.
- Why is helium in Group 18 and not in Group 2? Helium has configuration 2, so its only shell is full, which matches the noble gases and not the metals of Group 2.
- Why does the configuration of an atom of the first twenty elements never show more than 8 electrons in the second shell? The second shell is full at 8 electrons, so extra electrons go into the next shell.
- Why does potassium start a new period? For the first twenty elements the third shell stops at 8 electrons, as in argon with configuration 2,8,8, so the next electron of potassium goes into a fourth shell and potassium has configuration 2,8,8,1.
- Cross-link to The Particulate Nature of Matter: electrons are arranged in shells (energy levels) around the nucleus.
Common mistakes
A chlorine atom is in Group 7 because it has 7 outer electrons.
In the SEAB table chlorine is in Group 17, because its 7 outer electrons plus 10 give the group number.
The period number is the number of outer electrons.
The period number is the number of occupied electron shells.
Helium has 2 outer electrons, so it is in Group 2.
Helium has a full first shell, so it is in Group 18 with the noble gases.
Shells give the period and outer electrons give the group, so write the electronic configuration first and read the position from it.
Exam check
Q1An atom has the electronic configuration 2,8,6. In which group and period is it?
- AGroup 6, Period 3
- BGroup 16, Period 3
- CGroup 16, Period 2
- DGroup 3, Period 6
Show answer
MCQ answer key B (Group 16, Period 3): the configuration has three shells, so the period is 3, and 6 outer electrons plus 10 gives Group 16.
Q2Which element is in Group 2 and Period 3?
- Asodium
- Bcalcium
- Cmagnesium
- Dberyllium
Show answer
MCQ answer key C (magnesium): magnesium has configuration 2,8,2, which has 2 outer electrons and three shells.
Q3Element X has proton number 15. What is its electronic configuration?
- A2,8,6
- B2,8,4
- C2,8,5
- D2,13
Show answer
MCQ answer key C (2,8,5): the first shell takes 2 electrons and the second shell takes 8, leaving 5 electrons in the third shell.
StructuredSulfur has proton number 16.
- Write the electronic configuration of a sulfur atom. [1]
- Deduce the group and the period of sulfur. [2]
- State the number of outer electrons in a chlorine atom. [1]
- Explain why an element with configuration 2,8,8 is in Group 18. [2]
Total: 6 marks
Show mark scheme
(a) 1 mark
- Mark scheme: 2,8,6
(b) 2 marks
- Mark scheme: Group 16, because 6 outer electrons plus 10 gives 16
- Mark scheme: Period 3, because the atom has three occupied shells
(c) 1 mark
- Mark scheme: 7 outer electrons
(d) 2 marks
- Mark scheme: its outer shell is full with 8 electrons
- Mark scheme: 8 outer electrons plus 10 gives Group 18, the group of the noble gases
Valence electrons and ionic charge
describe the relationship between number of outer (valence) electrons and the ionic charge of an ion for the first twenty elements
A metal atom loses its outer electrons to form a positive ion with a charge equal to the number of electrons lost, and a non-metal atom gains electrons to fill its outer shell, so the charge of its negative ion is 8 minus the number of outer electrons.
- Ion
- Ion: an atom, or a group of atoms, that has lost or gained electrons and so carries an electrical charge supports K324-3.1(a)
- Positive ion (cation)
- Positive ion (cation): an ion formed when an atom loses electrons, so it has more protons than electrons
- Negative ion (anion)
- Negative ion (anion): an ion formed when an atom gains electrons, so it has more electrons than protons
- Ionic charge
- Ionic charge: the charge of an ion, equal to the number of electrons lost (positive) or gained (negative)
- Noble gas configuration
- Noble gas configuration: a full outer shell of 8 electrons, or 2 electrons for helium, which ions of the first twenty elements usually have supports K324-3.1(a)
Ions of the first twenty elements
| Element | Outer electrons | Electrons lost or gained | Ion formed | Charge |
|---|---|---|---|---|
| Lithium | 1 | loses 1 | Li+ | 1+ |
| Sodium | 1 | loses 1 | Na+ | 1+ |
| Potassium | 1 | loses 1 | K+ | 1+ |
| Beryllium | 2 | loses 2 | Be2+ | 2+ |
| Magnesium | 2 | loses 2 | Mg2+ | 2+ |
| Calcium | 2 | loses 2 | Ca2+ | 2+ |
| Aluminium | 3 | loses 3 | Al3+ | 3+ |
| Nitrogen | 5 | gains 3 | N3– | 3− |
| Phosphorus | 5 | gains 3 | P3– | 3− |
| Oxygen | 6 | gains 2 | O2– | 2− |
| Sulfur | 6 | gains 2 | S2– | 2− |
| Fluorine | 7 | gains 1 | F– | 1− |
| Chlorine | 7 | gains 1 | Cl– | 1− |
Forming ions
Na → Na+ + e–A sodium atom loses its one outer electron and forms a sodium ion with a charge of 1+.
Mg → Mg2+ + 2e–A magnesium atom loses its two outer electrons and forms a magnesium ion with a charge of 2+.
Al → Al3+ + 3e–An aluminium atom loses its three outer electrons and forms an aluminium ion with a charge of 3+.
Cl + e– → Cl–A chlorine atom gains one electron and forms a chloride ion with a charge of 1-.
O + 2e– → O2–An oxygen atom gains two electrons and forms an oxide ion with a charge of 2-.
The number of outer electrons equals the group number only for Groups 1 and 2; for Groups 13 to 18 the group number is the number of outer electrons plus 10, except helium, which has 2 outer electrons and is in Group 18. supports K324-8.1(c)
The SEAB glossary says “Describe requires candidates to state in words (using diagrams where appropriate) the main points of the topic”, so describe the link as: metal atoms lose outer electrons to form positive ions, and non-metal atoms gain electrons to form negative ions.
Depth: why and how
- Why does a sodium ion have the same electronic configuration as neon? A sodium atom has configuration 2,8,1 and loses its outer electron, leaving 2,8, which is the configuration of neon.
- Why does a chloride ion have the same electronic configuration as argon? A chlorine atom with configuration 2,8,7 gains one electron to give 2,8,8, which is the configuration of argon.
- How can the charge of an ion be predicted from the group? Metal atoms in Groups 1, 2 and 13 (lithium, sodium, potassium, beryllium, magnesium, calcium and aluminium) form ions with charges 1+, 2+ and 3+, and non-metal atoms in Groups 15, 16 and 17 form ions with charges 3-, 2- and 1-.
- Why do hydrogen, carbon, boron, silicon and the noble gases not appear in the ion table? Carbon, boron and silicon usually share electrons instead of forming simple ions, hydrogen is a special case, and the noble gases already have full outer shells.
- Cross-link to Chemical Bonding and Structure: ions form by electron loss or gain and usually have the electronic configuration of a noble gas.
Common mistakes
A magnesium atom gains two electrons to form a magnesium ion, Mg²⁻.
A magnesium atom has 2 outer electrons, so it loses them and forms a magnesium ion with a charge of 2+.
Every ion has a charge equal to the number of outer electrons.
Only metal ions have a charge equal to the outer electrons lost; a non-metal ion has a charge of 8 minus the outer electrons, and it is negative.
A chlorine atom loses seven electrons to form a chloride ion.
A chlorine atom gains one electron to fill its outer shell and forms a chloride ion with a charge of 1-.
Metals lose their outer electrons to give positive ions, and non-metals gain electrons to give negative ions with a full outer shell.
Exam check
Q1Which ion is formed when a magnesium atom, electronic configuration 2,8,2, reacts?
- AMg2+
- BMg+
- CMg2–
- DMg6+
Show answer
MCQ answer key A (Mg2+): magnesium has 2 outer electrons, so it loses them and forms an ion with a charge of 2+.
Q2An atom of sulfur has electronic configuration 2,8,6. Which ion does it form?
- AS2+
- BS2–
- CS6–
- DS6+
Show answer
MCQ answer key B (S2–): sulfur has 6 outer electrons, so it gains 2 electrons to fill its outer shell and forms an ion with a charge of 2-.
Q3Aluminium has proton number 13. Which statement about its ion is correct?
- AIt has a charge of 3- and the electronic configuration of neon.
- BIt has a charge of 3+ and the electronic configuration of argon.
- CIt has a charge of 13+ and the electronic configuration of neon.
- DIt has a charge of 3+ and the electronic configuration of neon.
Show answer
MCQ answer key D (It has a charge of 3+ and the electronic configuration of neon.): an aluminium atom, configuration 2,8,3, loses its 3 outer electrons, leaving 2,8, which is the configuration of neon.
StructuredFluorine and calcium react to form an ionic compound.
- Write the electronic configuration of a fluorine atom. [1]
- State the charge of the ion formed by fluorine and explain why it has this charge. [2]
- State the charge of the ion formed by calcium. [1]
- Explain why a calcium ion has the same electronic configuration as an argon atom. [2]
Total: 6 marks
Show mark scheme
(a) 1 mark
- Mark scheme: 2,7
(b) 2 marks
- Mark scheme: the charge is 1-
- Mark scheme: a fluorine atom has 7 outer electrons and gains 1 electron to fill its outer shell
(c) 1 mark
- Mark scheme: 2+
(d) 2 marks
- Mark scheme: a calcium atom has configuration 2,8,8,2 and loses its 2 outer electrons
- Mark scheme: this leaves 2,8,8, the configuration of argon
Why elements in a group are alike
explain the similarities between the elements in the same group of the Periodic Table in terms of their electronic configuration
Atoms of the elements in one group share the same outer-electron count, so they lose, gain or share electrons in the same way and show similar chemical properties.
- Group similarity
- Group similarity: the similar chemical behaviour of elements in a group, caused by their equal numbers of outer electrons
- Outer-electron count
- Outer-electron count: the number of electrons in the outermost shell, which decides how an atom reacts
- Group 1
- Group 1: the alkali metals, each with 1 outer electron, which they lose to form ions with a charge of 1+
- Group 17
- Group 17: the halogens, each with 7 outer electrons, which gain one electron to form ions with a charge of 1-
- Group 18
- Group 18: the noble gases, each with a full outer shell, so they do not usually form ions or bonds
- Not identical
- Not identical: elements in a group are similar, not identical, because their atoms have different numbers of shells
Same group, same outer electrons
| Group | Electronic configurations | Outer electrons |
|---|---|---|
| Group 1 | lithium 2,1, sodium 2,8,1, potassium 2,8,8,1 | 1 |
| Group 2 | beryllium 2,2, magnesium 2,8,2, calcium 2,8,8,2 | 2 |
| Group 17 | fluorine 2,7, chlorine 2,8,7 | 7 |
| Group 18 | helium 2, neon 2,8, argon 2,8,8 | full outer shell |
The same type of change
Li → Li+ + e–Lithium loses its one outer electron.
Na → Na+ + e–Sodium loses its one outer electron in the same way.
K → K+ + e–Potassium also loses its one outer electron.
The SEAB glossary says “Explain may imply reasoning or some reference to theory, depending on the context”, so when you explain why elements in a group are alike, refer to their electronic configuration: the same number of outer electrons.
Depth: why and how
- Why do lithium, sodium and potassium form ions with the same charge? Each atom has 1 outer electron, and losing it gives an ion with a charge of 1+.
- Why are the halogens fluorine and chlorine alike in their reactions? Each atom has 7 outer electrons and needs one more electron to fill its outer shell, so each forms an ion with a charge of 1-.
- Why are elements in the same group not exactly the same? Their atoms have different numbers of shells, so details such as the speed of a reaction differ even though the type of change is the same.
- How does electronic configuration explain why helium, neon and argon are alike? Each has a full outer shell, so none needs to lose, gain or share electrons.
- Cross-link to Chemical Bonding and Structure: the type of ion or bond that an atom forms depends on its outer electrons.
Common mistakes
All the elements in one group have an equal number of electron shells.
The elements in one group have an equal count of outer electrons, and the elements in one period have an equal count of shells.
Elements in a group are alike because they have the same proton number.
Elements in a group are alike because their atoms have the same number of outer electrons, and each element has a different proton number.
All elements in a group react at the same speed.
Elements in a group undergo the same type of change, but their speed of reaction changes down the group.
Same group, same outer-electron count, so the same type of chemical change, and the SEAB answer must mention electronic configuration.
Exam check
Q1Why are lithium, sodium and potassium similar in their chemical properties?
- ATheir atoms each have one shell.
- BTheir atoms each have one outer electron.
- CTheir atoms each have the same proton number.
- DTheir atoms each have a full outer shell.
Show answer
MCQ answer key B (Their atoms each have one outer electron.): atoms of elements in one group have an equal count of outer electrons, and this decides how the atoms react.
Q2Fluorine and chlorine are in the same group. Which statement is correct?
- ABoth atoms have 6 outer electrons.
- BBoth atoms have the same number of shells.
- CBoth atoms have a full outer shell.
- DBoth atoms have 7 outer electrons.
Show answer
MCQ answer key D (Both atoms have 7 outer electrons.): fluorine, configuration 2,7, and chlorine, configuration 2,8,7, each have 7 outer electrons.
Q3Which pair of elements has atoms with the same number of outer electrons?
- Asodium and magnesium
- Boxygen and fluorine
- Cmagnesium and calcium
- Dlithium and beryllium
Show answer
MCQ answer key C (magnesium and calcium): magnesium, configuration 2,8,2, and calcium, configuration 2,8,8,2, each have 2 outer electrons and are in Group 2.
StructuredSodium and potassium are both in Group 1.
- Write the electronic configurations of sodium and potassium. [2]
- State the number of outer electrons in each atom. [1]
- Explain why sodium and potassium have similar chemical properties. [2]
- Name the type of ion formed by sodium and potassium atoms. [1]
Total: 6 marks
Show mark scheme
(a) 2 marks
- Mark scheme: sodium 2,8,1
- Mark scheme: potassium 2,8,8,1
(b) 1 mark
- Mark scheme: each atom has 1 outer electron
(c) 2 marks
- Mark scheme: they have the same number of outer electrons
- Mark scheme: so their atoms lose one electron in the same way to form ions with a charge of 1+
(d) 1 mark
- Mark scheme: positive ions (cations)
Metals and non-metals across a period
describe the change from metallic to non-metallic character from left to right across a period of the Periodic Table
Across a period from left to right the elements change from metals to non-metals, so metallic character decreases and non-metallic character increases.
- Metallic character
- Metallic character: how strongly an element behaves like a metal, for example by losing electrons to form positive ions
- Non-metallic character
- Non-metallic character: how strongly an element behaves like a non-metal, for example by gaining or sharing electrons
- Metal
- Metal: an element whose atoms tend to lose their outer electrons to form positive ions, found on the left and in the centre of the Periodic Table
- Non-metal
- Non-metal: an element whose atoms tend to gain or share electrons, found on the right of the Periodic Table
- Trend across a period
- Trend across a period: the gradual change in properties from one end of a period to the other, with the number of shells staying the same
- Gradual change
- Gradual change: the change from metallic to non-metallic character is gradual, so elements near the middle of a period show a mixture of metallic and non-metallic properties
Period 3 from metal to non-metal
| Element | Group | Outer electrons | Metal or non-metal |
|---|---|---|---|
| Sodium | 1 | 1 | metal |
| Magnesium | 2 | 2 | metal |
| Aluminium | 13 | 3 | metal |
| Silicon | 14 | 4 | in the middle, with a mixture of metal and non-metal properties |
| Phosphorus | 15 | 5 | non-metal |
| Sulfur | 16 | 6 | non-metal |
| Chlorine | 17 | 7 | non-metal |
| Argon | 18 | 8 | non-metal |
The trend on the Periodic Table
The SEAB glossary says “Describe requires candidates to state in words (using diagrams where appropriate) the main points of the topic”, so describe the trend as a change from metallic to non-metallic character from left to right across a period.
Depth: why and how
- How can you tell a metal from a non-metal by its position? Metals are on the left and in the centre of the table, and non-metals are on the right.
- Why does metallic character fall across Period 3? Going from sodium to chlorine the outer electrons increase from 1 to 7, and atoms with more outer electrons gain or share electrons instead of losing them.
- How do the oxides of sodium and sulfur differ? Sodium forms a basic oxide, whereas sulfur forms an acidic oxide, which shows the change from metal to non-metal.
- What happens to metallic character down a group? It increases, so lead in Group 14 is a metal but carbon in the same group is a non-metal.
- Cross-link to Acid-Base Chemistry: metal oxides are usually basic and non-metal oxides are usually acidic.
Common mistakes
Metallic character increases from left to right across a period.
Metallic character decreases from left to right across a period, because the elements change from metals to non-metals.
The change from a metal to a non-metal happens suddenly between two elements.
The change is gradual across a period, and elements near the middle show a mixture of properties.
Elements in Period 3 are all metals.
Sodium, magnesium and aluminium in Period 3 are metals, silicon lies in the middle with a mixture of properties, and phosphorus, sulfur, chlorine and argon are non-metals.
Left to right across a period the elements change from metals to non-metals, and the number of outer electrons rises along the way.
Exam check
Q1What is the trend in metallic character from left to right across a period?
- AIt decreases.
- BIt increases.
- CIt stays the same.
- DIt increases and then stays the same.
Show answer
MCQ answer key A (It decreases.): elements change from metals on the left to non-metals on the right, so metallic character decreases.
Q2Which element in Period 3 is a metal?
- Asulfur
- Bmagnesium
- Cchlorine
- Dphosphorus
Show answer
MCQ answer key B (magnesium): magnesium is on the left of Period 3 with 2 outer electrons, whereas the other three are non-metals on the right.
Q3Which statement about the elements from sodium to chlorine is correct?
- AThe number of outer electrons decreases and metallic character decreases.
- BThe number of shells increases and metallic character decreases.
- CThe number of outer electrons increases and metallic character increases.
- DThe number of outer electrons increases and metallic character decreases.
Show answer
MCQ answer key D (The number of outer electrons increases and metallic character decreases.): going across the period the outer electrons increase from 1 to 7 while the elements change from metals to non-metals.
StructuredSome of the elements in Period 3 are sodium, magnesium, aluminium, phosphorus, sulfur and chlorine.
- State how many electron shells each atom has. [1]
- State which of these elements are metals. [1]
- Describe how the metallic character of the elements changes from sodium to chlorine. [2]
- State the number of outer electrons in a sulfur atom and a sodium atom. [1]
Total: 5 marks
Show mark scheme
(a) 1 mark
- Mark scheme: three shells
(b) 1 mark
- Mark scheme: sodium, magnesium and aluminium
(c) 2 marks
- Mark scheme: metallic character decreases from left to right
- Mark scheme: the elements change from metals to non-metals
(d) 1 mark
- Mark scheme: sulfur 6 and sodium 1
You've read the free 28%. Unlock the rest of Chapter 8.
Still to read in this chapter:
- Valence electrons and metallic character
- Predicting Group 1 and Group 17
- Group 1: the alkali metals
- Group 17: the halogens
- Group 18: the noble gases
- Why noble gases are unreactive
- Transition elements
- Transition elements as catalysts
- Order of reactivity of metals
- Reactivity and positive ions
- Deducing the order from results
- Heating metal carbonates
- Obtaining metals from their ores
- Rusting of iron
- Sacrificial protection
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Notes version 1.0 · Last updated 2 Oct 2026 · Chapter 8 of 13