1. Periodic Trends Chemistry 5(C)
Knowing periodic trends will enable you to quickly determine the properties of elements by simply looking at their placement on the periodic table.
Periodic Trends
Chemistry 5(C)

2. Periodic Trends Learning objectives
Use the periodic table to identify and explain trends in
Atomic and ionic radii
Electronegativity
Ionization energy
After this lesson, you will understand how the periodic table is used to identify and explain trends in atomic and ionic radii, electronegativity, and ionization energy.

3. Periodic Trends
Periodic Trends – patterns of properties seen across periods and within groups on the periodic table
Atomic Radius – the size of an atom measured from the nucleus to the outermost electron
Ionic Radius – the size of an ion measured from the nucleus to the outermost electron
Electronegativity – the tendency of an atom to pull electrons towards itself when bonding with another atom
Ionization energy – the energy required to remove an electron from a neutral gaseous atom
Trends are shown most clearly and consistently in main group elements
Periodic trends are patterns of properties that repeat across the horizontal periods (or rows) and within the vertical groups (or columns) on the periodic table. They are called periodic trends since they repeat at regular intervals, called periods.
The four trends we’ll focus on are atomic radius, ionic radius, electronegativity, and ionization energy.
Atomic radius is the distance from the nucleus of an atom to its outermost electrons. The atomic radius is typically measured in picometers.
Ionic radius is similar to the atomic radius but it is measured for an ion instead of for a neutral atom.
Electronegativity is a measure of how likely atoms are to pull shared pairs of bonding electrons towards themselves. The units for electronegativity are Pauling units.
Ionization energy is a measure of how much energy is required to remove one electron from a neutral gaseous atom, forming a cation. Ionization energy is measure in kilojoules per mole. Additional ionization energies past the removal of the first electron can be calculated.
Periodic trends are shown most clearly and consistently in the main group elements.

4. Observed Atomic Radii Trend
Atomic radius increases moving down a group and decreases moving to the right across a period
Decreases
Increases
Atomic Radii
Increases
Atomic radius increases moving down a group and decreases moving to the right across the periodic table. The shading on this periodic table represents atomic radii values. Darker boxes represent larger atoms and lighter boxes represent smaller atoms. Generally, the atomic radius of each atom will increase moving towards the bottom left corner of the periodic table. Some transition metals are exceptions to this general trend.

5. Atomic Radii Down a Group
Atomic radius increases moving down a group
Each period represents an energy level
Elements further down on the periodic table have atoms with more energy levels
Each new energy level is further away from the nucleus
Electrons in inner energy levels contribute to shielding
Inner layers of electrons shield outer layers of electrons from the pull of the positively charged nucleus
As shown in the previous diagram the atomic radius increases moving down a group so that the largest atoms are towards the bottom of the periodic table. The reason that the atomic radius increases down a group is because each period on the periodic table represents an energy level for electrons to occupy. As you move down the periodic table, the period number increases, and an additional energy level is needed to hold the electrons in atoms of elements in that period. Each new energy level is like a layer of an onion. More energy levels mean that electrons in the outer energy levels are further away from the nucleus, increasing the atomic radius.
Energy levels also contribute to a concept known as shielding. The electrons in inner energy levels act as a shield, preventing the electrons in outer energy levels from being pulled in closer to the positively charged nucleus. Since the nucleus is positively charged, it attracts negatively charged electrons. However, at the same time, electrons in atoms repel each other, since they are all negatively charged. The more energy levels (or layers of electrons repelling each other) there are between the nucleus and the outermost electrons, the more shielding the atom exhibits and the larger the radius of the atom.

6. Atomic Radii Across a Period
Atomic radius decreases moving to the right across the periodic table
Each atom in a period uses the same energy level to hold its outermost electrons, while moving to the right increases the positive charge of the nuclei
Attraction of outer electrons to nuclei increases, drawing electrons in and decreasing the radius
As illustrated previously, the atomic radius decreases as you move to the right across a period. The reason for the decrease in size is that each atom in the a period uses the same energy level to hold its outermost electrons, while the positive charge of the nuclei increases as you move to the right across the period. Remember that when moving to the right across the period, each atom has an additional proton in its nucleus. Since protons are positively charged, they pull on the negatively charged electrons, drawing them in closer. Atoms furthest to the right in each period have the maximum amount of protons for that energy level and will be able to draw electrons in the closest, causing that atom to have the smallest atomic radius for that period.

7. Atomic Radii & Ionic Radii
Observed Ionic Radii Trend
Ion – charged atom formed by losing or gaining electrons in the outermost energy level
Ions generally increase in size moving down a group
Outer electrons are in higher energy levels
Atomic Radii & Ionic Radii
Increases
An ion is a charged atom formed when electrons are either lost or gained from the outermost energy level. The illustration shows both the radii of the atoms (shown in yellow and red) and ions (shown in green and purple) of elements in groups 1, 2, 16 and 17. Notice that positive ions are smaller than the atoms they originate from, while negative ions are larger than the atoms they originate from. As a result negative ions (which form from atoms of elements on the right side of the table) are generally larger than positive ions (which form from atoms of elements on the left side of the table).
The trend in the radii of atoms down a group is the same as the trend in radii of ions down a group. The size of ions generally increases when moving down a group in the periodic table. The reasoning behind this is the same as the reasoning behind the increase in radii of the atoms down a group. When moving down a group, each new period represents a new, higher energy level used to hold the electrons. Higher energy levels are further from the nucleus, resulting in larger atoms and ions.

8. Ionic Radii Trend – Cations
Group
Period
Cation – positively charged ion formed when an atom loses electrons
Radius will decrease when a cation is formed
Fewer electrons present decreases the size of the electron cloud
Cations generally decrease in size moving left-to-right across a period
Cations increase in positive charge moving left-to-right
Pull on electrons is stronger, decreasing radius
A cation is a positively charged ion formed when one or more (negatively charged) electron is lost. As shown in the illustration, when atoms (shown in yellow) lose one or more electrons to form positively charged cations (shown in green) the radius decreases because there are fewer electrons present in the electron cloud.
In general, as you more from left-to-right across a period, the size of a cation will decrease. Cations further to the right have more positive charge. Therefore, the pull on the electrons is stronger, resulting in more tightly held electrons and a smaller ionic radius.

9. Ionic Radii Trend – Anions
Group
Period
Anion – negatively charged ion formed when an atom gains an electron
Radius will increase when an anion is formed
More electrons are present increasing the size of the electron cloud
Anions generally decrease in size moving from left-to-right across a period
Moving left-to right, anions decrease in negative charge because elements have more protons
Electrons more tightly held
An anion is a negatively charged ion formed when an atom gains one or more electron. As shown in the illustration, when an atom (shown in red) gains one or more electron to form negatively charged anions (shown in purple) the radius increases. The size of the atom is smaller than the ion because there are more electrons present in the electron cloud.
In general, as you move from left-to-right across a period, the size of an anion will decrease. Anions further to the right on the table have decreased negative charges, because elements further to the right have larger numbers of protons, resulting in more tightly held electrons and smaller ionic radii.

10. Observed Electronegativity Trend
Electronegativity decreases moving down a group and increases moving to the right across a period
Electronegativity
Increases
Decreases
Increases
Electronegativity is a measure of how likely atoms are to pull shared pairs of bonding electrons towards themselves. The shading on the periodic table corresponds to the values. Darker boxes represent higher electronegativity values and lighter boxes represent lower electronegativity values. Electronegativity decreases moving down a group and increases moving to the right across a period. Generally electronegativity increases moving towards fluorine on the periodic table. Some transition metals are exceptions to this general trend. Also, notice that this trend does not include noble gases because they tend not to bond with other elements.

11. Electronegativity Trend
When atoms bond they do not always share electrons equally
Atoms with larger electronegativities pull bonding electrons closer to themselves
Atoms with greater electronegativities have smaller atomic radii
Atoms with smaller radii have less shielding
Positively charged nucleus is more exposed and can better attract shared electrons in the bond
Fluorine is the most electronegative element
When atoms bond, they do not always share electrons equally. If one atom has a larger electronegativity value, it will pull the shared bonding electrons closer to its end of the bond. The electrons will still be shared overall, they will just be closer to the atom with the larger electronegativity value.
You may have noticed that the electronegativity trend is opposite of the trend for atomic radius. An atom is likely to have a large value for electronegativity if it has a smaller radius. An atom with a small radius tends to have less shielding, or fewer energy levels of electrons between the nucleus and the outer electrons. Outer electrons are the electrons involved in bonding. Since the positively charged nucleus is more exposed in smaller atoms, it will better attract the shared electrons of the bond.
Since fluorine is the atom with the smallest radius that is not a noble gas, it is the most electronegative element.

12. Observed Ionization Energy Trend
Ionization energy decreases moving down a group and increases moving to the right across a period
Increases
Decreases
1st Ionization Energy He
Ionization energy is a measure of how much energy is required to remove one electron from a neutral gaseous atom, forming a cation. The shading in the illustration below shows that ionization energy increases to the right across a period and decreases down a group. This means the trend for ionization energy is opposite the trend for atomic radius.
The illustration shows the trend for the first ionization energy which is the removal of the first electron from an atom to form a cation. (Additional electrons can be removed from atoms and corresponding values for ionization energy can be calculated.)
While some exceptions to the ionization energy trend do occur, generally, the amount of energy required to remove an electron, increases moving towards helium on the periodic table.
Increases

13. Ionization Energy
Large ionization energy value means that electrons are difficult to remove
Requires a large amount of energy
Atoms with large ionization energy values have smaller radii
Fewer energy levels of electrons between the nucleus and the outer electrons
Positively charged nucleus strongly attracts electrons and a large amount of energy is required to remove them
Helium has the largest ionization energy
The values for ionization energy are related to how strongly an atom holds onto its electrons. When an atom has a large value for ionization energy, it is difficult to remove an electron from that atom so a large amount of energy is required. This is true for atoms that hold onto their electrons very tightly.
It is important to note that the trend for ionization energy is opposite the trend for atomic radius. Atoms with small radii have large ionization energy values. In an atom with a small radius, there are only a few energy levels between the nucleus and the outermost electrons, meaning the nucleus is not very well shielded and can strongly attract the electrons. The strong attraction between the nucleus and the electrons means that a lot of energy is required to remove electrons.
As shown in the previous illustration, helium will have the largest value for ionization energy because it has the smallest radius. Noble gases will have the largest values for ionization energy in their periods due to the fact that they are unreactive and their electrons remain in place.

14. Electronegativity & Ionization Energy Increase
Summary of Trends
Cation Radii Increase
Anion Radii Increase
Ionic Radii Increase
Atomic Radii Increase
Electronegativity & Ionization Energy Increase
The trend for atomic radius is the most important to remember since all the other trends can be determined by looking at the atomic radius. The atomic radius increases towards the bottom left corner of the periodic table.
Remember that like atomic radius, ionic radius increases as you move down a group. As you move the left across a period, the radii of cations will increase and the radii of anions will increase.
The trends for electronegativity and ionization energy are opposite the trend for radius so the elements with the largest electronegativities and ionization energies will be at the top right corner of the periodic table. Remember however, that noble gases are not typically included in the electronegativity trend since they do not usually form bonds. Therefore, fluorine is the most electronegative element, while helium has the highest ionization energy value.

15. Periodic Trends Learning objectives
Use the periodic table to identify and explain trends in
Atomic and ionic radii
Electronegativity
Ionization energy
This concludes our presentation on periodic trends. You now know how to identify and explain the trends for atomic and ionic radii, electronegativity, and ionization energy.