Period 3 Elements A Level Chemistry

[Yvone Brem]

Note: This video is designed to help the teacher better understand the lesson and is NOT intended to be shown to students. It includes observations and conclusions that students are meant to make on their own.

Students will again focus on the first 20 elements. Students will first look at a diagram and animation to understand the basic pattern of the arrangement of electrons on energy levels around an atom. Students will be given cards with information about the electrons and energy levels for each of the first 20 atoms. They will again try to correctly match the cards with each element.

Students can try playing the Periodic Table Game, Game #2. This is an online version of the periodic table card game from this lesson that you can assign as class work or homework after students have played the game in the classroom.

Review with students that in lesson two they focused on the number of protons, neutrons, and electrons in the atoms in each element. In this lesson, they will focus on the arrangement of the electrons in each element.

Explain to students that electrons surround the nucleus of an atom in three dimensions, making atoms spherical. They can think of electrons as being in the different energy levels like concentric spheres around the nucleus. Since it is very difficult to show these spheres, the energy levels are typically shown in 2 dimensions.

Tell students that this energy level model represents an oxygen atom. The nucleus is represented by a dot in the center, which contains both protons and neutrons. The smaller dots surrounding the nucleus represent electrons in the energy levels. Let students know that they will learn more about electrons and energy levels later in this lesson.

Show students that you have 80 cards (4 for each of the first 20 elements). Before distributing the cards, explain that each card contains information about electrons and energy levels for the first 20 elements of the periodic table.

After all cards have been placed at the 20 different atoms, select two or three atoms and review whether the cards were placed correctly. This review will help reinforce the concepts about the structure of atoms and help students determine the number of protons and electrons in each atom.

Give each student a Periodic Table of Energy Levels activity sheet. This table contains energy level models for the first 20 elements. The electrons are included only for the atoms at the beginning and end of each period.

Note: In the energy level diagrams, the electrons are spread out evenly in the level. Some books show them spread out this way and some show them in pairs. The pairing of electrons is meant to represent that pairs of electrons are in separate orbitals within each energy level. At the middle school level, it is not necessary for students to learn about electron orbitals. This information is offered so that it is clearer to you why electrons are often shown in pairs in energy level diagrams. An orbital defines a region within an energy level where there is a high probability of finding a pair of electrons. There can be a maximum of two electrons in each orbital. This is why the electrons are often shown in pairs within an energy level.

Note: Students may wonder why an energy level can hold only a certain number of electrons. The answer to this is far beyond the scope of a middle school chemistry unit. It involves thinking of electrons as 3-dimensional waves and how they would interact with each other and the nucleus.

A certain number of electrons go into a level before the next level can have electrons in it. After the first energy level contains 2 electrons (helium), the next electrons go into the second energy level. After the second energy level has 8 electrons (neon), the next electrons go into the third energy level. After the third energy level has 8 electrons (argon), the next 2 electrons go into the fourth energy level.

Note: The third energy level can actually hold up to 18 electrons, so it is not really filled when it has 8 electrons in it. But when the third level contains 8 electrons, the next 2 electrons go into the fourth level. Then, believe it or not, 10 more electrons continue to fill up the rest of the third level. Students do not need to know this.

Tell students that the vertical columns in the periodic table are called groups or families. Ask students to compare the number of electrons in the outermost energy level for the atoms in a group. Students should realize that each atom in a group has the same number of electrons in its outermost energy level. For instance, hydrogen, lithium, sodium, and potassium all have 1 electron on their outer energy level. Let students know that these electrons in the outermost energy level are called valence electrons. They are the electrons responsible for bonding, which students will investigate in the next lesson.

Students will see that although potassium reacts more vigorously than sodium, the reactions are similar. Have students look at the periodic table to see where sodium and potassium are in relation to one another.

Students will see that this reaction is different from the sodium and the potassium. Have them locate calcium on the periodic table and point out that it is in a different group than sodium and potassium.

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In teaching the properties of the heavier elements, mainly the 6th and 7th period elements, including the lanthanides and actinides, discussion of relativistic effect appears to be important. In fact, without introducing the relativistic effect controlling the properties of the heavier elements, it becomes difficult to rationalize their properties. But unfortunately, in most of the inorganic chemistry text books, this aspect has been grossly overlooked and very often, properties of the heavier elements have been explained in terms of the effects of lanthanide contraction (in general, f-contraction). For example, by considering the effects of lanthanide contraction, inert pair effect observed for the heavier congeners of the p-block elements has been explained in most of the inorganic chemistry text books. But, it gives the incomplete chemistry. Besides the lanthanide contraction effect on the post-lanthanides, relativistic effect is quite important to understand the reason behind the inert pair effect noted for the heavier p-block congeners. In fact, relativistic effect itself contributes significantly to cause the lanthanide and actinide contractions.

This self explanatory tutorial review aims to answer all the probable questions related to the relativistic effect on the properties of elements across the periodic table. It will play the role of a chemistry teacher to the students to satisfy the needs of the students. In fact, the corresponding author has presented and used this review material in class rooms and refresher courses many times to benefit the students and college teachers and they have learnt and understood the subject clearly. The authors do believe that this tutorial review will be immensely helpful in chemistry education on the present important topic at the university level.

Aim: To illustrate by giving examples of the different special properties of the heavier congeners (i.e. the elements in the 6th and 7th periods) in the s-, p- and d-block elements due to the relativistic effect.

The electron velocity (v) decreases with the increase of n (principal quantum number) and the relativistic effect becomes more important for the smaller values of n and higher values of Z (i.e. heavier elements) as the electron velocity is directly proportional to Z and inversely proportional to n.

Relativistic contraction of orbitals (rrel = r/γ) is more important in the heavier elements for the orbitals with the lower n and l values where the velocity of the electron is higher and the relativistic contraction of the orbitals follows the order: s (l = 0) > p (l = 1) > d (l = 2) > f (l = 3). In fact, relativistic contraction of the s- and p- orbitals is more important and this orbital contraction is called the direct relativistic effect (Burke & Grant, 1967; Das et al., 2023; Pyper, 2020). Figure 1 compares the RDF (radial distribution function) curves of the relativistic and corresponding non-relativistic 3s-orbitals to illustrate the relativistic contraction of orbitals for a hydrogen like atom with Z = 80. These contracted s- and p-orbitals effectively shield the d- and f- orbital electrons and consequently, they experience the less electrostatic attraction by the positive nuclear charge and they actually undergo expansion and this relativistic expansion of the d- and f- orbitals is called the indirect relativistic effect that appears as the consequence of direct relativistic effect. Thus the order of relativistic orbital contraction (direct relativistic effect) is: 6s > 5s > 4s and ns > np (n = 4, 5, 6) and the order of relativistic expansion of the orbitals (indirect relativistic effect) is: 5d > 4d > 3d; 5f > 4f.

In teaching the properties of the heavier elements, specially the 6th and 7th period elements, including the lanthanides and actinides, discussion of relativistic effect appears to be important. In fact, without introducing the relativistic effect controlling the properties of the heavier elements, it becomes difficult to rationalize their properties. To answer the important questions listed in Section 2, it requires the discussion of relativistic effect to control the properties of the heavier elements in PT. But unfortunately, in most of the inorganic chemistry text books, this aspect has been grossly overlooked. Here these aspects have been illustrated in the next section.

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