ASRJC 2024 H2 Chem Atomic Structure Notes

2024 JC1 H2 Atomic Structure 2024/Anderson Serangoon JC/Chemistry 1 Anderson Serangoon Junior College H2 Chemistry ATOMIC STRUCTURE Content 1. The nucleus of the atom: neutrons and protons, isotopes, proton and nucleon numbers 2. Electrons: electronic energy levels, ionisation energies, atomic orbitals, extranuclear structure Learning Outcomes Students should be able to: (a) identify and describe protons, neutrons and electrons in terms of their relative charges and relative masses (b) deduce the behaviour of beams of protons, neutrons and electrons in an electric field (c) describe the distribution of mass and charges within an atom (d) deduce the numbers of protons, neutrons and electrons present in both atoms and ions given proton and nucleon numbers (and charge) (e) (i) describe the contribution of protons and neutrons to atomic nuclei in terms of proton number and nucleon number (ii) distinguish between isotopes on the basis of different numbers of neutrons present (f) describe the number and relative energies of the s, p and d orbitals for the principal quantum numbers 1, 2 and 3 and also the 4s and 4p orbitals (g) describe the shapes of s, p and d orbitals (h) state the electronic configuration of atoms and ions given the proton number (and charge) (i) (i) explain the factors influencing the ionisation energies of elements (see the Data Booklet) (ii) explain the trends and variations in ionisation energies across a period and down a Group of the Periodic Table (see The Periodic Table lecture) (j) deduce the electronic configurations of elements from successive ionisation energy data (k) interpret successive ionisation energy data of an element in terms of the position of that element within the Periodic Table References 1. Chemistry the Central Science (8th edition). Brown, LeMay, Bursten 2. A–level Chemistry. E.N Ramsden 3. Understanding Advanced Physical Inorganic Chemistry. Jeanne Tan, Chan Kim Seng 4. Chemistry for Advanced Level. Peter Cann, Peter Hughes 5. Cambridge International AS and A Level Chemistry. Lawrie Ryan and Roger Norris Additional Activities – Learning Packages on SLS 1. Protons, Neutrons and Electrons 2. Energy Levels of Orbitals in an Atom 3. Representing Electronic Configuration 4. Ionisation Energy

2024 JC1 H2 Atomic Structure 2024/Anderson Serangoon JC/Chemistry 2 INTRODUCTION How has the understanding of atomic structure developed and evolved?  The atomic model has changed over time when new evidence surfaced through the various discoveries relating to the structure of an atom. http://bit.ly /1NT65ai Source: https://www.tes.com/lessons/e_Cw_–QvqG1a9w/atomic–structure From John Dalton’s atomic theory, the development of atomic theory had come a long way and our understanding of the nature of matter and the structure of atoms were deepened with the works of scientists such as J. J. Thompson, Ernest Rutherford, James Chadwick, Niels Bohr, Louis de Broglie, Erwin Schrödinger and Werner Heisenberg, just to name a few. Atomic model proposed by John Dalton Atomic model proposed by J. J. Thomson Atomic model proposed by Ernest Rutherford Source: The history of Atom (http://thehistoryoftheatom.weebly.com/john–dalton.html)

2024 JC1 H2 Atomic Structure 2024/Anderson Serangoon JC/Chemistry 3 PART 1: SUB–ATOMIC PARTICLES [SDL] 1.1 Three Fundamental Sub–atomic Particles Proton Neutron Electron Position within atom In nucleus In nucleus Around nucleus Actual Mass / g 1.673 x 10–24 1.675 x 10–24 9.109 x 10–28 Mass / amui 1.007 1.009 5.485 x 10–4 Relative masses 1 1 1/1840 Charge +1.602 x 10–19 C 0 –1.602 x 10–19 C Relative charge +1 0 –1 • Electrons are attracted to the protons in the nucleus by electrostatic forces of attraction. • The protons and neutrons in the nucleus are drawn very close together by nuclear forces. • These forces are only effective over a very short range because they do not pull the outer electrons into the nucleus. i. An atomic mass unit (amu) is defined as 1/12 the mass of one atom of carbon –12. It has the value of 1.6606 x 10 –24 g. 1.2 Representation of a Nuclide • ‘Nuclide’ refers to any species with a specified number of protons and neutrons. • The nuclide of an element, X, is represented by: where  Example 1 Determine the number of protons, neutrons and electrons of the following: proton 17 18 19 29 16 neutron 35–17 = 18 40–18 = 22 40–19 = 21 63–29 = 34 32–16 = 16 electron 17 18 19 29–1 = 28 16+2 = 18 lC35 Ar40 K40 +Cu63 −232S A X Z Z is atomic number / proton number • number of protons in the nucleus of an atom • unique for each element (thus, helping us to identify the element) • isotopes of an element have the same proton number A is the mass number / nucleon number • sum of protons and neutrons in the nucleus of an atom • isotopes of an element have different mass numbers due to different number of neutrons

2024 JC1 H2 Atomic Structure 2024/Anderson Serangoon JC/Chemistry 4 1.3 Isotopes • Isotopes are atoms of an element that have the same number of protons but different number of neutrons and hence different nucleon numbers (or mass number). • The different mass numbers are known as isotopic masses (e.g. chlorine has two isotopes, chlorine–35 and chlorine–37.) • They have the same number of electrons and thus, they show similar chemical properties. This is because in a chemical reaction, it is the electrons that are transferred between atoms. The nucleus remains intact. • They have different number of neutrons and thus, different masses. Hence they show different physical properties (e.g. density). • Not all isotopes are stable. Unstable isotopes undergo radioactive decay, sometimes producing a different isotope. o An example of an unstable isotope that undergoes radioactive (alpha) decay is uranium–238. In this process, the uranium atom ( U92 238 ) transmuted into an atom of thorium ( Th90 234 ) and, at the same time, gives off an alpha particle ( He2 4 ). U92 238 → Th90 234 + He2 4 + energy  Example 2 Species No. of protons No. of neutrons No. of electrons A 16 18 16 T 17 18 17 O 17 20 17 M 17 18 18 (a) Select a pair of isotopes. [Ans: T & O] (b) Which of the species are not from the same element as the rest? [Ans: A] (c) With reference to the Periodic Table, identify the species A, T, O and M by writing the symbols and show the nucleon numbers and charges. [Ans: 34S, 35Cl; 37Cl; 35Cl–]

2024 JC1 H2 Atomic Structure 2024/Anderson Serangoon JC/Chemistry 5 1.4 Behaviour of Sub–atomic Particles in an Electric Field When a beam comprising of protons, electrons and neutrons is passed through an electric field, the expected deflections are as shown below. • Angle of deflection, θ, is proportional to the charge of the particle, but inversely proportional to its mass 𝑞 𝑚 for a proton, H+ 1 1 = 1 1 = 1 𝑞 𝑚 for an electron = 1 1 1840 = 1840 Thus, the angle of deflection for electrons is 1840 times more than protons (θ e > θ p ). • Particles with smaller mass and/or higher charge will be deflected more. For your information • The greater the charge of the particle, the greater is the attractive force exerted on it from the oppositely charged plate, and the greater is the deviation from its original direction of motion • If two particles are moving at the same speed but one is more massive than the other, the heavier particle has a greater kinetic energy. Thus, more energy must be exerted on the heavier particle to cause it to deflect. Since the applied electric field is e xerting the