Learning Objective

In this lesson we will learn about the layout of the periodic table and how it relates to the electron configuration of elements.

Learning Outcomes

By the end of this lesson you will be able to:

  • Describe the general layout of the periodic table.

  • Identify different groups and periods on the periodic table.

  • Describe some common properties of elements in particular groups, including alkali metals, alkaline
    earth metals, halogens and noble gases.

  • Describe the relationship between the electron configuration of an element and its
    location on the periodic table.

  • Describe periodic trends in atom size and chemical reactivity.

 

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Introduction

  • The periodic table of the elements is a way of representing all of the known elements.
  • Elements are listed in order of increasing atomic number and arranged into groups and periods.
  • This layout reflects patterns in the atomic structure of the different elements.
  • It subsequently also reflects trends in physical and chemical properties of the elements.

 
periodic table of the elements

The Periodic Table of the Elements

(Image: Cepheus, Wikimedia Commons)

 

Atomic Number

  • The atomic number of an element is equal to the number of protons in the nuclei of each atom of that element.
    The element with the least number of protons in its atoms is hydrogen – hydrogen atoms contain one proton; therefore hydrogen has an atomic number of 1 and is the first element in the periodic table.
  • There are currently 118 known elements, covering atomic numbers 1-118.
  • Elements with atomic numbers greater than 94 are synthetic elements.
    These elements do not occur naturally, but have been synthesised artificially, usually in very small quantities.
    Synthetic elements have unstable nuclei and are thus radioactive.

 
atomic number periodic table

Hydrogen has the atomic number 1 – its atoms contain one proton.

 

Periods

  • As elements increase in atomic number, they are arranged into horizontal rows called periods.
    There are seven periods in the periodic table.
  • Periods 6 and 7 include two series of elements that are usually listed separately at the bottom of the periodic table.
    These elements are known as the lanthanides and actinides respectively.

 
periodic table period row

There are seven periods (rows) in the periodic table.

 

Groups

  • The vertical columns of the periodic table are called groups.
    There are 18 groups in the periodic table.
  • Groups are numbered 1 to 18.
    Groups 1-2 and 13-18 are sometimes referred to as main group elements.
    Groups 3-12 are known as the transition metals.
    In an older numbering system, Roman numerals were used for groups. The main groups were numbered I-VIII and the transition metal groups had their own numbering.

 
periodic table group column

There are 18 groups (columns) in the periodic table.

 

Arrangement of Groups and Periods

  • Groups and periods do not all contain the same numbers of elements.
    This gives the periodic table its unique shape, rather than a simple rectangle.
  • The layout of the periodic table arranges groups so that they contain elements with similar physical and chemical properties.
    In fact, as the periodic table was developed, many elements were predicted before they had even been discovered.
    Example 1
    Groups 1 elements (excluding hydrogen) are known as the alkali metals. They are all soft, highly reactive metals that react vigorously with water to form strong alkalis (basic solutions).
    Example 2
    Group 18 elements are known as the noble gases. They are all colourless, odourless gases that are very unreactive.

 

lithium metal group 1    sodium metal group 1    potassium metal group 1

Lithium, sodium and potassium are alkali metals, which are located in group 1 of the periodic table.

(Images: Dnn87, Wikimedia Commons)

 

Electron Configuration and the Periodic Table

  • Since chemical reactivity of elements is related to their electron configuration, there is a correlation between the electron configuration of an element and its location in the periodic table.
  • The number of the period that an element is in is the same as the number of electron shells in its atoms.
    All elements in a particular period will therefore have the same number of electron shells.
    For example, potassium (K), iron (Fe) and bromine (Br) are all in period 4. Therefore their atoms all contain four electron shells.
  • For main group elements, the number of the group that an element is in reflects the number of valence electrons (electrons in the outer shell) in its atoms.
    The number of valence electrons is equal to the last digit of the group number. (In the older group numbering system mentioned above, the number of valence electrons is equal to the group number.)
    All elements in a particular group will therefore have the same number of electron shells.
    For example, fluorine (F), chlorine (Cl) and iodine (I) are all in group 17 (group VII). Therefore their atoms all contain seven valence electrons.
  • Since the location of main group elements can be used to determine the number of electron shells and the number of valence electrons, it can also be used to determine the electron configuration, and vice versa.
    Example 1
    Magnesium is in period 3 and group 2 of the periodic table.
    Therefore magnesium atoms have three electron shells, and there are two electrons in valence shells.
    Since the maximum number of electrons in the first shell is two and the maximum number of electrons in the second shell is eight, magnesium has the electron configuration 2,8,2.
    Example 2
    Barium has the electron configuration 2, 8, 18, 18, 8, 2.
    This shows that barium atoms have six electron shells (because there are six numbers), and there are two electrons in valence shells (because the last number is 2).
    Therefore barium is in period 6 and group 2 of the periodic table.
  • For transition metals, lanthanides and actinides, electron configuration is complex and the number of valence electrons cannot be predicted from the location of the element in the periodic table, but is usually 1 or 2.

 
electron configuration periodic table

The electron configuration of elements is related to their location on the periodic table.

 

Formation of Ions

  • Since elements in the same main group have the same number of valence electrons, those that form ions will form ions with the same charge.
  • Elements in group 1 lose one valence electron to form +1 ions.
    Elements in group 2 lose two valence electrons to form +2 ions.
    Elements in group 3 lose three valence electrons to form +3 ions.
  • Elements in group 17 gain one valence electron to form -1 ions.
    Elements in group 16 gain two valence electrons to form -2 ions.
    Elements in group 15 gain three valence electrons to form -3 ions.
  • Subsequently, ionic compounds formed from elements in the same two groups will have the same ionic formula.
    For example, all ionic compounds formed from group 2 and group 17 elements have the same general formula AB2.
  • The charges of transition metal ions cannot be predicted from the periodic table, but they are always positive.
    They can also have more than one possible charge.
    Charges of transition metal ions can be determined from the chemical formula of compounds, or by looking up a valency table.

 
periodic table group valency ion charge

Elements in the same main group form ions with the same charge.

 

Groupings Within the Periodic Table

  • Different areas of the periodic table can be divided up into groupings of similar elements.
    These include alkali metals, alkaline earth metals, transition metals, post-transition metals, halogens and noble gases.

 

The Alkali Metals

  • The metals in group 1 are known as the alkali metals.
  • Alkali metals have the following properties:
    • They are shiny, silver metals.
    • They are soft (can be cut with a knife).
    • Although they are solids at room temperature, they have low melting and boiling points (all except lithium have a melting point less than 100°C).
    • They have low densities (lithium, sodium and potassium can float on water).
    • They are highly reactive (none occur naturally in their elemental form).
    • They have one valence electron, which they readily lose to form ions with a charge of +1.
    • They react vigorously with water to form strong alkalis (basic solutions) and hydrogen gas.
    • They form similar white salts when reacting with group 17 elements.

 
alkali metals group 1

The metals in group 1 are known as the alkali metals.

 

The Alkaline Earth Metals

  • The metals in group 2 are known as the alkaline earth metals.
  • Alkaline earth metals have the following properties:
    • They are silver-white metals.
    • They have relatively low densities.
    • They have relatively low melting and boiling points.
    • They are reactive (although not as reactive as alkali metals).
    • They have two valence electrons, which they lose to form ions with a charge of +2.
    • All except with beryllium react with water to form strong alkalis (basic solutions) and hydrogen gas.
    • They form similar white salts when reacting with group 17 elements.

 
alkaline earth metals group 2

The metals in group 2 are known as the alkaline earth metals.

 

The Transition Metals

  • The metals in groups 3-12 are known as the transition metals.
  • Transition metals generally have the following properties:
    • They are hard metals (group 11 is an exception).
    • They have high boiling points (except mercury, which is a liquid at room temperature).
    • They have high densities.
    • They have either one or two valence electrons and form positive ions.
    • Many can form ions with different charges (eg, copper can form Cu+ and Cu2+ ions).
    • They usually form coloured compounds.
  • Metals to the right of the transition metals are known as post-transition metals.
    They have similar properties to transition metals.

 
transition metals periodic table

The metals in groups 3-12 are known as the transition metals.

 

The Halogens

  • The elements in group 17 are known as the halogens.
  • Halogens have the following properties:
    • They are non-metals.
    • They have relatively low melting and boiling points (they range from gases to solids).
    • They produce toxic, coloured vapours.
    • They exist as diatomic molecules (molecules containing two atoms).
    • They have seven valence electrons, and readily attract electrons by forming ions with a charge of -1 or forming covalent compounds.
    • They are highly reactive (none occur naturally in their elemental form).
    • They react with hydrogen to produce toxic gases (hydrogen halides); these are covalent compounds, which dissociate into ions when dissolved in water, forming acids.
    • They react with metals to form salts, which are usually soluble.

 
halogens group 17

The elements in group 17 are known as the halogens.

 

The Noble Gases

  • The elements in group 18 are known as the noble gases (formerly, the inert gases).
  • The noble gases have the following properties:
    • They are non-metals.
    • They have very low melting and boiling points (all are gases at room temperature).
    • They have full valence shells (all have eight valence electrons, except helium, which has two).
    • They are chemically very unreactive (only xenon, krypton and argon have been found to form compounds).

 
noble gases group 18

The elements in group 18 are known as the noble gases.

 

Trends in the Periodic Table

  • Although elements in the same group have similar physical and chemical properties, there are gradual changes in properties going down groups.
    There are also gradual changes in physical and chemical properties going across periods.
    For example, there are general trends in size of atoms and reactivity of elements.

 

Size of Atoms

  • As you go down a group, an extra electron shell is added with each period.
    Therefore the size of atoms increases down a group.
  • As you go across a period, an extra proton and electron is added with each group.
    These electrons are added to the same shell.
    The extra protons and electrons create greater attraction between nuclei and electrons, pulling the electrons slightly closer to nuclei.
    Therefore the size of atoms decreases across a period.

 
periodic table trend atomic radius size

The size of atoms increases down a group and decreases across a period.

 

Chemical Reactivity

  • Since the size of atoms increases down a group, valence electrons become further away from the nucleus.
    Being further away from the nucleus means they are held less tightly.
  • Metals react by losing valence electrons.
    The easier it is to lose them, the more reactive metals are.
    Therefore, as you go down a group, metals become more reactive, as valence electrons are lost more easily.
  • Non-metals react by attracting electrons.
    The easier it is to attract them, the more reactive non-metals are.
    Therefore as you go down a group, non-metals become less reactive, as valence electrons are attracted less easily.
  • Because it is easier to gain or lose one electron than it is to gain or lose two or three electrons, the closer an atom is to having a full valence shell, the more reactive it is.
    Therefore, for metals, reactivity decreases towards the centre of the periodic table (excluding transition metals).
    Similarly, for non-metals, reactivity decreases towards the centre of the periodic table (excluding noble gases).

 
periodic table trend chemical reactivity

The reactivity of metals increases down a group; the reactivity of non-metals decreases down a group.

 

Summary

  • The periodic table of the elements lists all of the known elements, in a way that reflects periodic trends in their physical and chemical properties.
  • Elements are listed in order of increasing atomic number (proton number) and arranged into groups (columns) and periods (rows).
  • The layout of the periodic table arranges groups so that they contain elements with similar physical and chemical properties.
    These include alkali metals, alkaline earth metals, halogens and noble gases.
  • The layout also reflects patterns in the electron configuration of elements.
    Main groups contain elements with the same number of valence electrons, which therefore form ions with the same charge.
    Periods contain elements with the same number of electron shells.
  • Atom size increases down groups and decreases across periods.
  • For metals, reactivity increases down groups and decreases towards the centre of the periodic table (excluding transition metals).
    For non-metals, reactivity decreases down groups and decreases towards the centre of the periodic table (excluding noble gases).

 
periodic table molecule

(Image: Jynto, Wikimedia Commons)

 

(Header image: Tomasz Zajda, Adobe Stock)

 

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