Brief notes on Periodic Table
The Periodic Table
(Sec 4 topic)
Why should we study the Periodic Table? This Table is of utmost importance to scientists and technology users.
There are about 118 elements found in the modern Periodic Table (as in year 2023). Some heavier elements are made in the gigantic atomic reactors by scientists. Some elements are radioactive. The Japan nuclear disaster in 2011 was due partly to leaking radioactive materials and the earthquakes and tsunamis.
In the 18th century, many scientists have tried to organize the elements into patterns. One scientist came up with a spiral “ancient” Periodic Table.
Mendeleev, a shrewd and wise scientist, noticed patterns and trends in the elements that were discovered then and organized the elements into a “primitive” Periodic Table. He even made predictions about elements that were not yet discovered and he was very accurate about the properties of some of the “new” elements that were discovered after him.
The modern Periodic Table is a way of arranging elements in order of increasing proton (atomic) numbers (not in increasing atomic mass).
In the periodic table, the elements are divided into periods and groups.
A period is a horizontal row of elements. There are 7 periods of elements numbered from 1 to 7.
A group is a vertical column of elements. The Periodic Table has 8 groups of elements numbered from 1 to 18. Group 18 is also known as Group 0.(The metals between Groups 2 and 13 are known as transition elements.) There are other more elaborate numbering systems which you may learn in university in case you study chemistry in university.
Periods
The period number of an element is the same as the number of electron shells in the atom. Please note that in the electron shells, the electrons are further sub-divided into different energy levels known as sub-shells.
Period number = number of electron shells
For example,
The elements in period 1 have only one electron shell (hydrogen and helium). The elements in period 2 have two electron shells and so on.
Scientists have a theory that the universe began only with the hydrogen nuclei, which later combine to form helium and other heavier atoms.
Going across the period from left to right, elements change from metals to metalloids to non - metals and their properties also change.
So, from left to right across the period, there is a decrease in metallic properties and an increase in non-metallic properties of elements.
Some elements like silicon (an important element used in your computer’s chips), because of their positions, show the properties of both a metal and a non—metal. Such elements are known as metalloids. They have interesting properties which make them important for uses in the semi-conductor industry (eg. Foxconn which makes Apple products needs to know about Silicon)
A lot of the elements have multiple uses in various modern industries in the world. The universe and the earth are made up of the elements found in the Periodic Table. The human body contains more than billions of cells which are made of primarily carbon, hydrogen, oxygen, nitrogen, phosphorus and so on.
The group number of an element is the same as the number of valence electrons (electrons in the outermost shell).
Group number = number of electrons in outermost shell
For example,
All Group 2 elements (eg. Magnesium) have two valence electrons.
Similarly, elements in Group 14 have four valence electrons, elements in Group 15 have five valence electrons and so on.
The elements in the same group have the same number of electrons in the outermost shell.
Hence,
they have similar electronic configurations, and therefore, similar chemical properties. This makes the study of chemistry more organized and systematic.
they form ions or compounds with similar formulae
Going down a Group
As we go down the Group,
The atoms become bigger and heavier since the proton numbers become bigger. Do read up about atomic radius in Wikipedia or chemistry textbooks in your library.
The elements become more metallic, because they tend to lose electrons more easily.
For example, in Group 14, the element at the top of the Group is carbon which is a non- metal whereas the elements at the bottom (tin and lead), are metals.
The study of carbon compounds is in a branch of chemistry known as Organic Chemistry. There are millions of organic molecules known to mankind. The carbon atom is essential to the creation of animal and human life.
Predicting the properties of elements using the Periodic Table
Periodic trends
We can predict the properties of an element based on its location in the Periodic Table.
Position of an element in the Periodic Table helps a scientist to predict its properties.
For example,
Sulfur is found in period 3, Group 16. Hence,
Period number = number of electron shells = 3
Group number = electrons in outermost shell = 6
Hence, it may be deduced that its electronic configuration is 2, 8, 6.
Sulfur compounds are usually toxic to animals and mankind. Another element, Arsenic, is a toxic element. One NUS professor was poisoned in the 1990s due to someone placing arsenic in his coffee. Thank goodness, he recovered in hospital.
Going across the period from left to right there is an increase in the non - metallic properties. Therefore, it may also be predicted that its properties will be non - metaIlic and that it will form an acidic oxide (chapter of acids, bases and salts).
Group properties
Group I elements - Alkali metals
The first three elements in Group 1, lithium, sodium and potassium, are known as the alkali metals.
They are called alkali metals because they react with cold water to form alkalis.
The alkali metals have similar physical properties as follows:
They are soft enough to be cut easily. Other metals are usually hard. Group 1 metals are exceptional in their properties.
When freshly cut, they have a shiny surface which quickly tarnishes and oxidizes in air. So, sodium is stored in oil.
They have low densities and some metals float on water. Other metals, such as copper, have high densities.
They have low melting and boiling points. Other metals, such as iron, has a high melting point.
Group 1 metals are quite different from other metals. So it is a surprise that you are studying this Group of metals. Some Group 2 metals and more transition metals are more typical of metals. Do read up on Transition Metals in your textbook, especially for 2023 Sec 3 students.
Being in the same group, they have similar electronic structures, hence, similar chemical properties.
They react vigorously and some even violently with cold water to form alkalis and hydrogen gas. The resulting solution turns a red litmus paper blue. The general equation may be represented as
2M(s) + 2H2O(l) --> 2MOH(aq) + H2(g)
where M is the alkali metal. Note that alkali metals form ions with valency of one.
Alkali metals form compounds which
- are soluble in water
- are ionic
- have similar chemical formulae
Alkali metals react with oxygen to form basic oxides.
Trends on going down Group 1
Going down the Group, the melting points and boiling points of the alkali metals decrease as seen in the table below. This is very surprising!!! Do you know why?
Going down the Group, the reactivity of the alkali metals increases.
For example,
Reaction of alkali metals with water
As the size of the atom increases, it tends to lose its valence electron more readily to form an ion. An atom of potassium is bigger than sodium and lithium is smaller than sodium. Hence, potassium is the most reactive and reacts violently with water. Sodium reacts less violently and the reaction of lithium is relatively mild.
You can youtube on chemistry experiments on sodium, potassium with water.
BTW, How do you measure the size of an atom?
Lithium floats on water and reacts quickly. There is no flame.
2Li(s) + 2H2O(l) -- 2LiOH(aq) + H2(g)
Sodium reacts very quickly and melts. The molten sodium darts (dances and swirls and dashes [Pardon me for being lyrical!!!] ) around on the water surface and a yellow flame is seen.
2Na (s) + 2H2O(l) -- 2NaOH(aq) + H2(g)
Why is there a yellow flame? This is important for Combined Science students in their labs. Sodium compounds give a yellow flame in the flame tests.
Potassium reacts violently and melts. A lilac flame is seen.
2K(s) + 2H2O(l) -- 2KOH(aq) + H2(g)
Group 17 elements - Halogens
Halogens are the elements in Group 17 of the Periodic Table. Halogens such as chlorine, bromine and iodine belong to this group.
Halogens are very reactive non - metals.
- They exist as diatomic molecules, Cl2, Br2, I2 etc
- They have low melting and boiling points.
- They are coloured.
- They are generally not safe to be handled. However, Tincture of iodine can be used to dress minor wounds on boys who run about and cut themselves (just being sarcastic; boys will always be boys)
They are very reactive because each halogen has seven electrons in the outermost and needs only one more electron to achieve the stable electronic configuration of a noble gas. Hence, a halogen atom accepts one electron to form the negatively charged halide ion, such as Cl?, Br?, and I?
Halogens form salts called halides. For example, sodium chloride (aka table salt), potassium iodide are halides.
Trends on going down Group 17
The boiling points and melting points of halogens increase. Chlorine is a gas, bromine is a liquid and iodine is a solid at room temperature. Why is this so?
The colours of halogen become darker. Even A level students do not learn the reason why their colours become darker.
The reactivity of halogens decreases(!!!!!) down the group. Hence, a more reactive halogen can displace a less reactive halogen from its salt in aqueous solution.
Displacement reactions of halogens
The elements become less reactive down the group.
A displacement reaction is a reaction in which one element takes the place of another element in a compound. A crappy and lamey Eg. In one football match, Messi may replace Ronaldo in an imaginary football club. In this way, Messi has displaced Ronaldo, just like chlorine displacing sodium bromide solution to form sodium chloride and bromine. (I am only joking about the football club!!!! And dreaming!!!!) Use your imagination in learning chemistry. Lol……
For example, chlorine is more reactive than bromine. When chlorine water is added to a solution of sodium bromide, bromine is displaced and the solution becomes reddish brown.
Cl2(aq) + 2NaBr(aq) -- 2NaCl(aq) + Br2(aq)
Similarly, chlorine can displace iodine from a solution of iodide.
Cl2(aq) + 2NaI(aq) -- 2NaCl(aq) + I2(aq)
Bromine can also displace iodine from an iodide solution since iodine is less reactive than bromine.
Br2(aq) + 2NaI(aq) -- 2NaBr(aq) + I2(aq)
What observations do you expect? Observations are very important in chemistry.
There is a similar concept in the chapter about metals where a more reactive metal displaces a less reactive metal from its salts.
A less reactive halogen cannot displace a more reactive halogen from its halide. For example, bromine water cannot displace chlorine from a solution of potassium chloride.
**** Please note that these notes are incomplete. Please consult your school textbook for a fuller treatment.
Editor : Mr Chong.
(Sec 4 topic)
Why should we study the Periodic Table? This Table is of utmost importance to scientists and technology users.
There are about 118 elements found in the modern Periodic Table (as in year 2023). Some heavier elements are made in the gigantic atomic reactors by scientists. Some elements are radioactive. The Japan nuclear disaster in 2011 was due partly to leaking radioactive materials and the earthquakes and tsunamis.
In the 18th century, many scientists have tried to organize the elements into patterns. One scientist came up with a spiral “ancient” Periodic Table.
Mendeleev, a shrewd and wise scientist, noticed patterns and trends in the elements that were discovered then and organized the elements into a “primitive” Periodic Table. He even made predictions about elements that were not yet discovered and he was very accurate about the properties of some of the “new” elements that were discovered after him.
The modern Periodic Table is a way of arranging elements in order of increasing proton (atomic) numbers (not in increasing atomic mass).
In the periodic table, the elements are divided into periods and groups.
A period is a horizontal row of elements. There are 7 periods of elements numbered from 1 to 7.
A group is a vertical column of elements. The Periodic Table has 8 groups of elements numbered from 1 to 18. Group 18 is also known as Group 0.(The metals between Groups 2 and 13 are known as transition elements.) There are other more elaborate numbering systems which you may learn in university in case you study chemistry in university.
Periods
The period number of an element is the same as the number of electron shells in the atom. Please note that in the electron shells, the electrons are further sub-divided into different energy levels known as sub-shells.
Period number = number of electron shells
For example,
The elements in period 1 have only one electron shell (hydrogen and helium). The elements in period 2 have two electron shells and so on.
Scientists have a theory that the universe began only with the hydrogen nuclei, which later combine to form helium and other heavier atoms.
Going across the period from left to right, elements change from metals to metalloids to non - metals and their properties also change.
So, from left to right across the period, there is a decrease in metallic properties and an increase in non-metallic properties of elements.
Some elements like silicon (an important element used in your computer’s chips), because of their positions, show the properties of both a metal and a non—metal. Such elements are known as metalloids. They have interesting properties which make them important for uses in the semi-conductor industry (eg. Foxconn which makes Apple products needs to know about Silicon)
A lot of the elements have multiple uses in various modern industries in the world. The universe and the earth are made up of the elements found in the Periodic Table. The human body contains more than billions of cells which are made of primarily carbon, hydrogen, oxygen, nitrogen, phosphorus and so on.
The group number of an element is the same as the number of valence electrons (electrons in the outermost shell).
Group number = number of electrons in outermost shell
For example,
All Group 2 elements (eg. Magnesium) have two valence electrons.
Similarly, elements in Group 14 have four valence electrons, elements in Group 15 have five valence electrons and so on.
The elements in the same group have the same number of electrons in the outermost shell.
Hence,
they have similar electronic configurations, and therefore, similar chemical properties. This makes the study of chemistry more organized and systematic.
they form ions or compounds with similar formulae
Going down a Group
As we go down the Group,
The atoms become bigger and heavier since the proton numbers become bigger. Do read up about atomic radius in Wikipedia or chemistry textbooks in your library.
The elements become more metallic, because they tend to lose electrons more easily.
For example, in Group 14, the element at the top of the Group is carbon which is a non- metal whereas the elements at the bottom (tin and lead), are metals.
The study of carbon compounds is in a branch of chemistry known as Organic Chemistry. There are millions of organic molecules known to mankind. The carbon atom is essential to the creation of animal and human life.
Predicting the properties of elements using the Periodic Table
Periodic trends
We can predict the properties of an element based on its location in the Periodic Table.
Position of an element in the Periodic Table helps a scientist to predict its properties.
For example,
Sulfur is found in period 3, Group 16. Hence,
Period number = number of electron shells = 3
Group number = electrons in outermost shell = 6
Hence, it may be deduced that its electronic configuration is 2, 8, 6.
Sulfur compounds are usually toxic to animals and mankind. Another element, Arsenic, is a toxic element. One NUS professor was poisoned in the 1990s due to someone placing arsenic in his coffee. Thank goodness, he recovered in hospital.
Going across the period from left to right there is an increase in the non - metallic properties. Therefore, it may also be predicted that its properties will be non - metaIlic and that it will form an acidic oxide (chapter of acids, bases and salts).
Group properties
Group I elements - Alkali metals
The first three elements in Group 1, lithium, sodium and potassium, are known as the alkali metals.
They are called alkali metals because they react with cold water to form alkalis.
The alkali metals have similar physical properties as follows:
They are soft enough to be cut easily. Other metals are usually hard. Group 1 metals are exceptional in their properties.
When freshly cut, they have a shiny surface which quickly tarnishes and oxidizes in air. So, sodium is stored in oil.
They have low densities and some metals float on water. Other metals, such as copper, have high densities.
They have low melting and boiling points. Other metals, such as iron, has a high melting point.
Group 1 metals are quite different from other metals. So it is a surprise that you are studying this Group of metals. Some Group 2 metals and more transition metals are more typical of metals. Do read up on Transition Metals in your textbook, especially for 2023 Sec 3 students.
Being in the same group, they have similar electronic structures, hence, similar chemical properties.
They react vigorously and some even violently with cold water to form alkalis and hydrogen gas. The resulting solution turns a red litmus paper blue. The general equation may be represented as
2M(s) + 2H2O(l) --> 2MOH(aq) + H2(g)
where M is the alkali metal. Note that alkali metals form ions with valency of one.
Alkali metals form compounds which
- are soluble in water
- are ionic
- have similar chemical formulae
Alkali metals react with oxygen to form basic oxides.
Trends on going down Group 1
Going down the Group, the melting points and boiling points of the alkali metals decrease as seen in the table below. This is very surprising!!! Do you know why?
Going down the Group, the reactivity of the alkali metals increases.
For example,
Reaction of alkali metals with water
As the size of the atom increases, it tends to lose its valence electron more readily to form an ion. An atom of potassium is bigger than sodium and lithium is smaller than sodium. Hence, potassium is the most reactive and reacts violently with water. Sodium reacts less violently and the reaction of lithium is relatively mild.
You can youtube on chemistry experiments on sodium, potassium with water.
BTW, How do you measure the size of an atom?
Lithium floats on water and reacts quickly. There is no flame.
2Li(s) + 2H2O(l) -- 2LiOH(aq) + H2(g)
Sodium reacts very quickly and melts. The molten sodium darts (dances and swirls and dashes [Pardon me for being lyrical!!!] ) around on the water surface and a yellow flame is seen.
2Na (s) + 2H2O(l) -- 2NaOH(aq) + H2(g)
Why is there a yellow flame? This is important for Combined Science students in their labs. Sodium compounds give a yellow flame in the flame tests.
Potassium reacts violently and melts. A lilac flame is seen.
2K(s) + 2H2O(l) -- 2KOH(aq) + H2(g)
Group 17 elements - Halogens
Halogens are the elements in Group 17 of the Periodic Table. Halogens such as chlorine, bromine and iodine belong to this group.
Halogens are very reactive non - metals.
- They exist as diatomic molecules, Cl2, Br2, I2 etc
- They have low melting and boiling points.
- They are coloured.
- They are generally not safe to be handled. However, Tincture of iodine can be used to dress minor wounds on boys who run about and cut themselves (just being sarcastic; boys will always be boys)
They are very reactive because each halogen has seven electrons in the outermost and needs only one more electron to achieve the stable electronic configuration of a noble gas. Hence, a halogen atom accepts one electron to form the negatively charged halide ion, such as Cl?, Br?, and I?
Halogens form salts called halides. For example, sodium chloride (aka table salt), potassium iodide are halides.
Trends on going down Group 17
The boiling points and melting points of halogens increase. Chlorine is a gas, bromine is a liquid and iodine is a solid at room temperature. Why is this so?
The colours of halogen become darker. Even A level students do not learn the reason why their colours become darker.
The reactivity of halogens decreases(!!!!!) down the group. Hence, a more reactive halogen can displace a less reactive halogen from its salt in aqueous solution.
Displacement reactions of halogens
The elements become less reactive down the group.
A displacement reaction is a reaction in which one element takes the place of another element in a compound. A crappy and lamey Eg. In one football match, Messi may replace Ronaldo in an imaginary football club. In this way, Messi has displaced Ronaldo, just like chlorine displacing sodium bromide solution to form sodium chloride and bromine. (I am only joking about the football club!!!! And dreaming!!!!) Use your imagination in learning chemistry. Lol……
For example, chlorine is more reactive than bromine. When chlorine water is added to a solution of sodium bromide, bromine is displaced and the solution becomes reddish brown.
Cl2(aq) + 2NaBr(aq) -- 2NaCl(aq) + Br2(aq)
Similarly, chlorine can displace iodine from a solution of iodide.
Cl2(aq) + 2NaI(aq) -- 2NaCl(aq) + I2(aq)
Bromine can also displace iodine from an iodide solution since iodine is less reactive than bromine.
Br2(aq) + 2NaI(aq) -- 2NaBr(aq) + I2(aq)
What observations do you expect? Observations are very important in chemistry.
There is a similar concept in the chapter about metals where a more reactive metal displaces a less reactive metal from its salts.
A less reactive halogen cannot displace a more reactive halogen from its halide. For example, bromine water cannot displace chlorine from a solution of potassium chloride.
**** Please note that these notes are incomplete. Please consult your school textbook for a fuller treatment.
Editor : Mr Chong.
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