Communication is an important approach to learning because being able to communicate your understanding well is important for success in all academic disciplines. Being able to master the “explain” command term in science demonstrates conceptual understanding and demonstrates advanced critical thinking.
This approach to teaching section 3.1 of the IB Chemistry syllabus uses sentence frames to help focus explanation writing. The lesson uses the following scaffold.
- Objectives. Written using command terms (actions / verbs)
- Important understandings
- Definition of key terms
- Identifications of the characteristics of a strong explanation
- The sentence frames. Setting up the explanation.
- Evaluation of mentor text examples. Class discussion
- The task
- Standards based assessment of communication
Students need to know that vertical and horizontal trends in the periodic table exist for atomic radius, ionic radius, ionization energy, electron affinity and electronegativity and that there are discontinuities in the increase across a period for first ionisation energy.
Students should be able to explain the physical properties of an element based on its position in the periodic table.
Every substance has a set of properties; unique traits or characteristics, which are used to identify it, like a fingerprint. With today’s rapid advances in technology new substances are continually being synthesized and discovered so understanding the unique properties of a substance helps chemists recognize it when it appears in a new place, identify its structure and bonding and group it with other similar substances.
The periodic table is one of a chemist’s most useful tools because it can be used to describe and explain the patterns in the properties of elements. First ionization energy, atomic radii, ionic radii, electronegativity and melting point are examples of physical properties that experience periodicity, a repeating pattern or trend on the periodic table.
A physical property is a characteristic that does not change the chemical composition of the substance. The factors affecting the periodic trends in physical properties are
- Nuclear charge
- Relative sharing
- Number of shells
- Electron-electron repulsion
- Core charge / shielding
They give rise to the following assumptions:
- As the distance between the positive protons in nucleus and the outer shell electrons increases the electrostatic attraction between them decreases.
- As the nuclear charge increases, the attractive force between the protons and the electrons increases.
- The outer-shell electrons are ‘shielded’ from the full nuclear charge of the protons by the inner-shell electrons.
- The core charge, the real nuclear charge / attraction felt by the outer-shell electrons is the nuclear charge minus the number of inner shell electrons.
- As the charge on an ion increases the electrostatic attraction between the protons in the nucleus and the outer shell electrons increases
- Electrons in a shell repel each other.
Two main factors determine how tightly an outer-shell electron is held. The force of electrostatic attraction between the positive protons in nucleus and the electron is directly related to their charges and inversely related to the distance between them. As the size of atoms increases, the attractive force on the outer-shell electron decreases. As the nuclear charge increases, the attractive force increases. There is, however, a complication. The outer-shell electrons are ‘shielded’ from the full nuclear charge by the inner-shell electrons. The concept of core charge is used to allow for this shielding. The effective nuclear charge felt by the outer-shell electrons, the core charge, may be found by subtracting the number of inner-shell electrons from the nuclear charge. For example within group 1:
Sodium: 11 protons and 10 inner-shell electrons = core charge of +1.
Potassium: 19 protons and 18 inner-shell electrons = core charge of +1.
Rubidium: 37 protons and 36 inner-shell electrons = core charge of +1
The core charge remains constant within a group. This means that within a group, the only factor affecting the electrostatic attraction between outer-shell electrons and the nucleus is the distance of the outer-shell from the nucleus. As the atomic number increases within a group (going down the group), the attractive force between the nucleus and the outer-shell electrons decreases.
Definition of key terms
First ionization energy is defined as the energy required to remove the first outer or valence shell electron from an atom in its gaseous state. The first ionization energy of sodium can be represented by the equation
Atomic radii is the distance from the nucleus to the electrons in the outer shell. The atomic radius is measured by taking half the distance between the nuclei of two bonded atoms.
Ionic radii is the distance from the nucleus to the electrons in the outer shell. The ionic radius is measured by taking half the distance between the nuclei of two bonded atoms
Electronegativity is the relative measure of an atoms ability to attract a shared pair of electrons in a bond.
Electron affinity is the energy released when one mole of gaseous non-metal atoms gains one mole of electrons to form a negatively charged ion.
Characteristics of communicating a strong explanation
- Use of subject specific terminology
- Chemistry terms are used correctly, the language is detailed and the voice is objective and not open to interpretation.
- Relevance and conciseness
- The response is focused (statements are well clarified and not generalized), easy to follow from beginning to end, concise, and does not repeat ideas
- Diagrams and symbols are used to electron configuration are used to enhance interpretation (e.g. electron configurations, orbital diagrams).
- Sentences starters and connectors are used effectively.
A strong explanation has two parts:
The set-up or description of the trend / property followed by an explanation / reason for the trend.
Question starters for describing a physical property or trend. For example:
- The trend in … increase / decreases across / down …
- … is larger / smaller than …
Question starters for explaining a physical property or trend. For example:
- This is because ….
- The reason for this is …
Sentences connectors to help connect the description to the explanation. For example
- Because, Since, Therefore, Consequently, As a result, Then, If, Thus, Due to, For example
Evaluation of mentor texts
Question. Explain why the magnesium ion is smaller than the magnesium atom
Mentor text 1
The magnesium ion is smaller than the magnesium atom. This is because when the magnesium loses electrons it forms an ion and loses the 4s2 ones. After they are lost, the magnesium ion has only three layers, rather than 4, so the radius is smaller.
Mentor text 2
The magnesium ion is smaller than the magnesium atom. This is because the magnesium ion has lost its two outer shell electrons and now has two shells occupied compared to the three occupied shells by the magnesium atom. As a result it decreases the radius of the magnesium ion. Therefore the net attractive force experienced by magnesium is greater (12p attracting 10e) decreasing the radius of the ion.
To what extent do mentor texts 1 and 2 meet the standard for communication with regard to the use of the correct language and terms, conciseness and clarity? What makes you say this?
Explain the following trend in physical properties
- Atomic radius across the third period and down group 1 and 17
- Ionic radius across the third period and down group 1 and 17
- Electronegativity across the third period and down group 1 and 17
- First ionization energy across the third period and down group 1 and 17. Across the third period there are discontinuities in first ionization energy between Mg and Al and P and S.
- Electron affinity across the third period and down group 1 and 17
Standards based assessment of communication
|Aspect||7 – 6
|5 – 4
Meets the standard
|3 – 2
Approaching the standard
Does not meet the standard
|Subject specific terminology||The use of subject specific terminology is always appropriate and correct. Any errors do not hamper understanding.||The use of subject specific terminology is mostly appropriate and correct.||There are errors in the use of subject specific terms.||There are many errors in the use of subject specific terminology and they considerably hamper understanding.|
|Relevance and conciseness||The report is relevant and concise thereby facilitating a ready understanding of the question.||The report facilitates an understanding of the question.||The understanding of is obscured.||The response can’t be understood.|
|Structure and clarity||The response is well structured and clear and presented in a coherent way.||The response is structured and clear.||The response is not well structured and is unclear.||The response is presented in an incoherent or disorganized way.|