Periodic Trend
Understanding
- Atomic radius, ionic radius, ionization energy, electron affinity, electronegativity
- Trends in metallic and non-metallic behavior
- Oxides change from basic through amphoteric to acidic across a period
Application
- Prediction and explanation of the metallic and non-metallic behavior of an element based on its position in the periodic table
- Discussion of the similarities and differences in the properties of elements in the same group, with reference to alkali metals (group 1) and halogens (group 17)
- Construction of equations to explain the pH changes for reactions of Na2O, MgO, P4O10, and the oxides of nitrogen and sulfur with water
Effective Nuclear Charge
Before talking about different trends of the periodic table, one essential concept to understanding is the nuclear effective charge. The effective nuclear charge is the actual amount of positive (nuclear) charge experienced by an electron in an element.
Atomic Radius
- Atomic radius increases down the group: This is because the number of electron shell or energy level increases, so there are more electron shields, resulting in a decrease of the effective nuclear charge.
- Atomic radius decreases across a period: This is because the number of energy level stays the same, but effective nuclear charge increases.
Ionic Radius
Ionic radius is the distance between the nucleus and outmost electrons in an ion. Metals usually form positive ions since they tend to loose electrons for stability and non-metals usually form negative ions because they tend to gain eight valence electrons.
- The ionic radius of a positive ion, known as a cation is smaller than the atomic radius of the element
- The ionic radius of a negative ion, known as an anion is larger than the atomic radius of the element.
- Down the group, ionic radius decreases: This is because the number of energy level increases, so the effective charge decreases
- Ionic radius decreases across a period for non-metals: This is because the number of energy level increases, so the value of effective charge increases despite repulsions between electrons.
- Across a period, ionic radius decreases for metals: This is because the number of energy level increases, so the value of effective charge increases despite repulsions between electrons.
Ionization Energy
In chemistry, ionization energy, ionization energy is the minimum energy required to remove the most loosely bound electron of an isolated neutral gaseous atom or molecule.
- Across the period, ionization energy increases: An element's energy level stays the same and atomic radius decreases since the effective charge decreases, so ionization energy increases
- Down the group, ionization energy decreases: Atomic radius increases since an element's energy level increases down a group, causing more electron shield and the effective nuclear charge decreases that decreases ionization energy.
Electron Affinity
The electron affinity of an atom or molecule is defined as the amount of energy released when an electron is attached to a neutral atom or molecule in the gaseous state to form a negative ion.
- Electron affinity increases across a period: The is because the value of effective charge increases (the number of proton increases), so there is a greater attraction force from the nuclear, causing electron affinity to increase.
- Electron affinity decreases down a group: This is because the number of energy level increases, so there are more electron shields since valence electrons are farther from the nuclear, causing electron affinity to decrease.
Electronegativity
Electronegativity refers to the ability of an atom to attract shared electrons in a covalent bond. More electronegative elements hold the bond electrons “tighter” or closer to themselves that generates different dipole force. (Topic4: Bonding)
- Electronegativity increases across a period: This is because the number of energy level stays unchanged, but the effective nuclear charge increases, causing those elements to be more electronegative by producing a stronger attraction force.
- Going down a group, electronegativity decreases: This is because energy level increases, causing more electron shields that decreases the effective nuclear charge
Periodic Trend Summary
Group 1, Alkali Metals
Physical properties
- They are all good conductors of electricity and heat
- The have low density
- They have grey shiny surfaces when freshly cut with a knife
Chemical Properties
- They are very reactive metals
- They form ionic compounds with non-metals