NJC Physical Periodicity student version 2023

National Junior College SH1 H2 Chemistry 1 Physical Periodicity of Elements 1 Introduction to Periodic Table Success criteria  I can recognize the variations in the valence electronic configuration across the Period and down a Group. Mankind had always been seeking to discover new chemical elements since centuries ago. As of January 2021, a total of 118 elements have been identified. Studying these hundreds of elements with different physical and chemical properties will surely be a daunting task, unless there is a systematic and organized way to do it. Much of what we will cover in this topic is credited to the Russian chemist Dmitri Ivanovich Mendeleev (1834-1907), whose seminal hypothesis that the chemical and physical properties of the elements vary in a periodic way laid the foundations of the modern Periodic Table. The Periodic Table is a table of elements arranged by order of their atomic numbers in such a way that the periodic properties (chemical periodicity) of the elements are made clear. In fact, the Periodic Table is a masterpiece of organised chemical information. The standard form of Periodic Table includes periods (horizontal rows) and groups (vertical columns). The elements in the same period have th e same number of core electrons as that of the noble gas from previous period. Elements in the same group have the same number of valence electrons and hence similar chemical properties. Content Periodicity of physical properties of the elements: variation with proton number across the third period (sodium to chlorine) and down the group of: (i) atomic radius and ionic radius (ii) ionisation energy (iii) electronegativity Learning outcomes Trends and variations in atomic and physical properties For elements in the third period (sodium to chlorine), and in Group 2 (magnesium to barium) and Group 17 (chlorine to iodine) candidates should be able to: (a) describe and explain qualitatively the trends and variations in atomic radius, ionic radius, first ionisation energy and electronegativity: (i) across a Period in terms of shielding and nuclear charge (ii) down a Group in terms of increasing number of filled electronic shells, shielding and nuclear charge References 1. Hill, G. & Holman, J. (1992). Chemistry in Context, 6th edition. New York: Prentice Hall. 2. Ramsden, E. N. (1994). A Level Chemistry, 3rd edition. UK: Stanley Thornes (Publishers) Ltd. 3. Chemistry for Advanced Level, Peter Cann, Peter Hughes 4. A-level Chemistry, E.N. Ramsden 5. Chemistry: The Molecular Nature of Matter and Change, Martin S. Silberberg

National Junior College SH1 H2 Chemistry 2 Physical periodicity is the study of periodic trends which indicate the tendencies of certain elemental characteristics to change as one progresses along a row or column of the Periodic Table. The valence (outermost) shell electronic configuration of the elements can be used to determine the location of the element in the Periodic Table.  The principal quantum number, “n”, of the valence shell indicates the period of the element.  The number of valence electrons indicates the group of the element. (Note that elements with 3-8 valence electrons are in Group 13 to 18) Group Valence shell electronic configuration 1 ns1 2 ns2 13 ns2 np1 14 ns2 np2 15 ns2 np3 16 ns2 np4 17 ns2 np5 18 ns2 np6 n = period in which the element is found in the Periodic Table For example, an element with the electronic configuration of 1s22s22p63s23p63d104s24p5 is located at Period 4 of the Periodic Table and is in Group 17. The element is Br. Checkpoint 1: Write the electronic configurations of (i) Li and Na and (ii) F and Cl. Observe their relative positions on the Periodic Table. Underline the valence electronic configurations. (i) Li : 1s1s2 2s1s1 Na : 1s21s22s22p63s12s22p63s1 (ii) F : 1s 1s2 2s22p5 Cl : 1s221s2 2s2 2p6 3s2 3p5 2p63s23p5

National Junior College SH1 H2 Chemistry 3 2 Fundamental Considerations in Physical Periodicity Success criteria  I am able to discuss the effect of nuclear charge, shielding effect and number of filled electronic shells on the nuclear attraction. Apart from the valence electrons, we will now look at other factors which d etermine the characteristic physical properties of an atom. 2.1.1 Nuclear charge Nuclear charge is the total charge of all the protons in the nucleus. The larger the number of protons in the nucleus of an atom, the greater the nuclear charge. Hence, across a period or down a group, nuclear charge increases. 2.1.2 Shielding effect Valence electrons in an atom do not only ‘feel’ attraction from the protons in the nucleus, but also repulsion from inner shells electrons. The ‘feel’ of repulsion from inner-shell electrons is called shielding effect. The inner-shell electrons shield the outer electron from the full charge of the nucleus. Hence, shielding effect increases as the number of inner shell electrons increases. 2.1.3 Number of filled electronic shells (distance of electrons from nucleus) Increase in number of filled electronic shell results in: 1) Increase in distance between the valence electrons and the nucleus 2) Increase in shielding effect Hence attractive force between the valence electrons and the nucleus is weaker as the valence electrons is further away from the nucleus. 2.2 Nuclear attraction (“net attraction electrons experienced from the nucleus”) Nuclear attraction refers to “attraction between the nucleus and the electrons (nuclear charge) – “repulsion between electrons (shielding effect) + distance of electrons from nucleus (no. of filled electronic shells)” Nuclear attraction = “nuclear charge – (shielding effect + distance of electrons from nucleus” *calculations are not required Note: Electrons in the same electronic shell exert negligible shielding effect on each other.

National Junior College SH1 H2 Chemistry 4 3 Periodicity Trend Success criteria  I can describe and explain qualitatively the trends and variations in atomic radius, ionic radius, first ionisation energy and electronegativity: (i) across a Period (same valence electronic shell) in terms of shielding effect and nuclear charge; (ii) down a Group (increasing number of filled electronic shell) in terms of distance away from nucleus, shielding effect and nuclear charge.  I can predict and explain the variations in ionic radius for species with either same number of electrons (e.g. isoelectronic species, Na+, Mg2+) or same number of protons (e.g. Na vs Na+) using proton to electron ratio to account the nuclear attraction for the outermost electrons. 3.1 Atomic radius (atomic size) The atomic radius of an element is a measure of the size of its atoms, usually the mean distance from the nucleus to the boundary of space occupied by the valence electrons. Since the boundary is not a well -defined physical entity, internuclear distance is being measured instead. Depending on the type of bonding and structure the atoms are involved, there are three commonly known atomic radius, namely metallic radius, covalent radius and van der Waals radius. The metallic radius is defined as half the internuclei distance between the two adjacent metal ions in the metallic lattice. The covalent r