2021 Atomic Structure Lecture Notes Tutors

St Andrew’s Junior College JC1 H1 Chemistry 2021 1 St Andrew’s Junior College H1 Chemistry 2021 Lecture Notes 2 Atomic Structure Assessment Objectives: Candidates 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 and p orbitals [note : knowledge of wave functions is not required] (h) state the electronic configuration of atoms and ions given the proton number (and charge) (i) explain the factors influencing the ionisation energies of elements (see the Data Booklet) (see also topic: The Periodic Table) (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 The Periodic Table Assessment Objective: For elements in the third period (sodium to chlorine), and i n Group 2 (magnesium to barium) and Group 17 (chlorine to iodine) candidates should be able to: (a) recognise variation in the electronic configuration across a Period and down a Group (b) describe and explain qualitatively the general 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 electronic shells, shielding and nuclear charge Content: 1. Atomic Structure 2. Principal Quantum Numbers, Subshells and Orbitals 3. Electronic Configurations 4. Periodic Properties 5. Experimental Determination of Ionisation Energies

St Andrew’s Junior College JC1 H1 Chemistry 2021 2 1 Atomic Structure 1.1 History of the Atomic Model

St Andrew’s Junior College JC1 H1 Chemistry 2021 3 https://www.youtube.com/watch?v=xazQRcSCRaY The development and revision of the atomic model over time is due to new evidence found through advances in theory and technology. This does not necessarily mean that the past atomic models are inaccurate, they are still useful for explaining science in a simpler manner. 1.2 The Fundamental Sub-Atomic Particles of Matter (a) identify and describe protons, neutrons and electrons in terms of their relative charges and relative masses (c) describe the distribution of mass and charges within an atom  An atom is the smallest particle of an element that retains its chemical properties.  An atom consists of a dense positively charged nucleus made up of protons and neutrons, collectively known as nucleons.  The nucleus is surrounded by negatively charged electrons occupying regions in space around it (known as atomic orbitals).  The electrons are attracted by protons via electrostatic forces. Proton Neutron Electron Electron Cloud Relative charges and masses of sub-atomic particles Particle Charge / C Relative charge Rest mass / kg Relative mass Location in atom Proton ( 1 1p) +1.60 x 10−19 + 1 1.67 x 10−27 1 In nucleus Neutron ( 1 0n) 0 0 1.67 x 10−27 1 In nucleus Electron ( 0 1 e) −1.60 x 10−19 − 1 9.11 x 10−31 1/1840 Around nucleus

St Andrew’s Junior College JC1 H1 Chemistry 2021 4 1.3 Behaviour of beams of protons, neutrons and electrons in an electric field (b) deduce the behaviour of beams of protons, neutrons and electrons in an electric field Note: Deflection only occurs when the charged particles enter the electric field. Both p and e− move in straight path as they leave the electric field.  The angle of deflection is dependent on the relative masses and charges of the particles. Angle of deflection  charge mass  The particle with a lighter mass and/or higher charge will have a greater angle of deflection, ie deflect more.  Angle of deflection β > α as electrons have lighter mass than protons.  Self- Check: Q1 1.4 Number of protons, neutrons and electrons in 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  An atom of an element can be represented by X A Z Symbol of the element Mass number Proton number  The proton number (atomic number) is unique for each element . It gives the number of protons in the nucleus of an atom. + ― n Neutrons are not deflected (Neutral) Beam of protons, electrons, and neutrons Protons are positively charged and are deflected towards the negative potential p Electrons are negatively charged and are deflected towards the positive potential e− β α Nucleon number Atomic number Angle β > Angle α

St Andrew’s Junior College JC1 H1 Chemistry 2021 5 The nucleon number (mass number) gives the total number of protons and neutrons in the nucleus of an atom. Eg. Hଵ ଵ and C଺ ଵଶ In a neutral atom, the number of protons = the number of electrons. When an atom gains electrons, a negatively charged ion or an anion is produced (number of protons<number of electrons). When an atom loses electrons, a positively charged ion or a cation is produced (number of electrons<number of protons).  Self- Check: Q2 to Q5 1.5 Isotopes (e) (ii) distinguish between isotopes on the basis of different numbers of neutrons present  Isotopes are atoms of the same element with the same number of protons (same proton number) but different number of neutrons (different nucleon numbers). For example, Isotopes of carbon Isotopes of Hydrogen 12 6 C 13 6 C 1 1H 2 1 H 3 1H Number of nucleons 12 13 1 2 3 Number of protons 6 6 1 1 1 Number of neutrons 6 7 0 1 2  Relative atomic mass of an element is often not a whole number. This arises due to the presence of isotopes occurring in different percentage abundance. (Refer to Topic 1: The Mole Concept and Stoichiometry)  Self- Check: Q6

St Andrew’s Junior College JC1 H1 Chemistry 2021 6 2 Principal Quantum Numbers, Subshells and Orbitals  In an atom or ion, the electrons are arranged in different shells, which consist of one or more subshells.  Each subshell contains one or more orbitals. 2.1 Principal Quantum Number, n (Shell)  Electrons are arranged in shells with certain main energy levels around the nucleus.  Each shell is described by a number known as the principal quantum number, n. The value of n ranges from n = 1 to n =   The first shell (n=1) is closest to the nucleus and it is of the lowest energy level. When the value of n increases, the electron s are in a shell that is further away from the nucleus and of greater energy. Hence, the electron is less tightly bound to the nucleus.  Some examples from n = 1 to n = 4 are listed below: Principal