Atomic structure

• Mass in the nucleus, chemistry in the electrons. 4s fills before 3d and empties before it. Count the electrons removed before the first big jump.

• 1.1.1 I can understand — understand that atoms are mostly empty space surrounding a very small, dense nucleus that contains protons and neutrons; electrons are found in shells in the empty space around the nucleus
• 1.1.2 I can identify — identify and describe protons, neutrons and electrons in terms of their relative charges and relative masses
• 1.1.3 I can understand — understand the terms atomic and proton number, mass and nucleon number
• 1.1.4 I can describe — describe the distribution of mass and charge within an atom
• 1.1.5 I can describe — describe the behaviour of beams of protons, neutrons and electrons moving at the same velocity in an electric field
• 1.1.6 I can determine — determine the numbers of protons, neutrons and electrons present in both atoms and ions given atomic or proton number, mass or nucleon number and charge
• 1.1.7 I can state — state and explain qualitatively the variations in atomic radius and ionic radius across a period and down a group
• 1.2.1 I can define — define the term isotope in terms of numbers of protons and neutrons
• 1.2.2 I can understand — understand the notation ˣᵧA for isotopes (the mass or nucleon number x written as a superscript and the atomic or proton number y as a subscript, both to the left of the element symbol A), where x is the mass or nucleon number and y is the atomic or proton number
• 1.2.3 I can state — state that and explain why isotopes of the same element have the same chemical properties
• 1.2.4 I can state — state that and explain why isotopes of the same element have different physical properties, limited to mass and density
• 1.3.1 I can understand — understand the terms: shells, sub-shells and orbitals; principal quantum number (n); ground state, limited to electronic configuration
• 1.3.2 I can describe — describe the number of orbitals making up s, p and d sub-shells, and the number of electrons that can fill s, p and d sub-shells
• 1.3.3 I can describe — describe the order of increasing energy of the sub-shells within the first three shells and the 4s and 4p sub-shells
• 1.3.4 I can describe — describe the electronic configurations to include the number of electrons in each shell, sub-shell and orbital
• 1.3.5 I can explain — explain the electronic configurations in terms of energy of the electrons and inter-electron repulsion
• 1.3.6 I can determine — determine the electronic configuration of atoms and ions given the atomic or proton number and charge, using either of the following conventions: e.g. for Fe: 1s²2s²2p⁶3s²3p⁶3d⁶4s² (full electronic configuration) or [Ar]3d⁶4s² (shorthand electronic configuration)
• 1.3.7 I can understand — understand and use the electrons in boxes notation, e.g. for Fe: [Ar] followed by five 3d boxes holding ↑↓, ↑, ↑, ↑, ↑ and one 4s box holding ↑↓
• 1.3.8 I can describe — describe and sketch the shapes of s and p orbitals
• 1.3.9 I can describe — describe a free radical as a species with one or more unpaired electrons
• 1.4.1 I can define — define and use the term first ionisation energy, IE
• 1.4.2 I can construct — construct equations to represent first, second and subsequent ionisation energies
• 1.4.3 I can identify — identify and explain the trends in ionisation energies across a period and down a group of the Periodic Table
• 1.4.4 I can identify — identify and explain the variation in successive ionisation energies of an element
• 1.4.5 I can understand — understand that ionisation energies are due to the attraction between the nucleus and the outer electron
• 1.4.6 I can explain — explain the factors influencing the ionisation energies of elements in terms of nuclear charge, atomic/ionic radius, shielding by inner shells and sub-shells and spin-pair repulsion
• 1.4.7 I can deduce — deduce the electronic configurations of elements using successive ionisation energy data
• 1.4.8 I can deduce — deduce the position of an element in the Periodic Table using successive ionisation energy data

What this item practises. Choosing a transformation, choosing axes and a scale, and drawing a conclusion from a pattern rather than from a single value. The same skills apply to any Paper 5 question that gives a table of results in an unfamiliar context.

• “3d comes before 4s, so potassium is 1s²2s²2p⁶3s²3p⁶3d¹.” Correct 4s fills before 3d, and empties before it. The 4s sub-shell is lower in energy than 3d in the neutral atom, so potassium is [Ar]4s¹ and calcium is [Ar]4s². Once 3d is occupied, 4s is the outer, higher-energy sub-shell, so Fe²⁺ is [Ar]3d⁶, not [Ar]3d⁴4s². This one rule carries the whole of topic 28.
• “The electron has no mass.” Correct Its relative mass is 1/1836 (about 1/1840): negligible in a mass calculation, but not zero. A beam of electrons is deflected about 1836 times as far as a beam of protons at the same velocity precisely because that small mass is real.
• “Fe³⁺ has 23 protons.” Correct Ions keep their protons. Only electrons are lost or gained. Fe³⁺ has 26 protons, 30 neutrons (for iron-56) and 23 electrons.
• “Isotopes have different chemical properties because they have different masses.” Correct Isotopes have the same chemical properties because they have the same number of electrons in the same arrangement, and chemical reactions involve electrons only. The physical differences the syllabus asks for are limited to mass and density.
• “First ionisation energy is the energy needed to remove an electron from an atom.” Correct The energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions. The mole, the gaseous state and the 1+ charge are all part of the definition, and X(g) → X⁺(g) + e⁻ is the equation that goes with it.
• “The third ionisation energy of aluminium is Al(g) → Al³⁺(g) + 3e⁻.” Correct That equation is the sum of the first three. The third ionisation energy removes one electron from the 2+ ion: Al²⁺(g) → Al³⁺(g) + e⁻. One electron per equation, (g) on every species.
• “Aluminium's first ionisation energy is lower than magnesium's because it has more shielding from inner shells.” Correct The inner-shell shielding is the same (both have 1s²2s²2p⁶ inside). The difference is that aluminium's outer electron is in the higher-energy 3p sub-shell and is shielded by the 3s electrons as well. That is the Group 13 dip; the Group 16 dip (sulfur below phosphorus) is spin-pair repulsion in the first doubly occupied 3p orbital, not “a nearly full shell”.
• “The group is the number of electrons before the biggest jump.” Correct Count the electrons removed before the first big jump (a ratio much larger than the ratios either side of it, often 4 or more). In a complete set there is a second big jump later, into the 1s shell, and it can be as large as the first or larger; it tells you nothing about the group.
• “An orbital is the path the electron travels around the nucleus.” Correct An orbital is a region of space around the nucleus in which there is a high probability of finding an electron. It has a shape (s spherical, p two lobes), not a route, and it holds at most two electrons of opposite spin.
• “An ion has a different number of protons from its atom.” Repair Only electrons are lost or gained. The proton number is fixed by the element.
• “Neutrons are deflected slightly because they have mass.” Repair Deflection in an electric field needs a charge. The neutron has none, so a neutron beam passes straight through.
• “Isotopes have different chemical properties because they have different masses.” Repair Isotopes have the same chemical properties because they have the same electronic configuration; only mass and density differ.
• “Cl⁻ is smaller than Cl because it has gained an electron and is more tightly held.” Repair Cl⁻ is larger. The nuclear charge is the same, and the extra electron increases repulsion in the outer shell, which expands.
• Potassium written as 1s²2s²2p⁶3s²3p⁶3d¹. Repair 4s is lower in energy than 3d and fills first: [Ar]4s¹.
• Fe²⁺ written as [Ar]3d⁴4s². Repair The 4s electrons are removed first: [Ar]3d⁶.
• Cr written as [Ar]3d⁴4s² and Cu as [Ar]3d⁹4s². Repair [Ar]3d⁵4s¹ and [Ar]3d¹⁰4s¹, the two exceptions, because a half-filled or filled 3d sub-shell is particularly stable and 3d and 4s are close in energy.
• Nitrogen's 2p drawn as one box ↑↓ and one box ↑. Repair Three boxes, one electron in each, parallel spins. Pairing costs repulsion energy while an empty orbital of the same energy is available.
• “A free radical is a charged atom.” Repair A free radical is a species with one or more unpaired electrons. Cl• is neutral; Cl⁻ is an ion and not a radical.
• “The first ionisation energy is the energy to remove an electron from an atom.” Repair From each atom in one mole of gaseous atoms, forming one mole of gaseous 1+ ions. The mole and the state symbols are part of the definition.
• Al(g) → Al³⁺(g) + 3e⁻ given as “the third ionisation energy”. Repair Al²⁺(g) → Al³⁺(g) + e⁻. One electron per equation.
• “Ionisation energy increases across a period because the atom gets smaller.” Repair The atom gets smaller because the nuclear charge rises with almost constant shielding. Give the cause, not only a consequence.
• “Aluminium's first IE is lower than magnesium's because it has more shielding from inner shells.” Repair The inner-shell shielding is the same. Aluminium's outer electron is in the higher-energy 3p sub-shell, shielded by the 3s electrons.
• “Sulfur's IE dips because its outer shell is nearly full.” Repair The dip is caused by spin-pair repulsion in the first doubly occupied 3p orbital.
• Reading the group from the biggest jump in a full set of successive ionisation energies. Repair Count the electrons removed before the first big jump. The later jump into 1s can be as large or larger, and says nothing about the group.
• “An orbital is the path an electron follows around the nucleus.” Repair An orbital is a region of space where there is a high probability of finding an electron. It holds at most two electrons, of opposite spin.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Interleave with the chapters that use this one. When you reach topic 3 (bonding), re-answer: what are the shapes of s and p orbitals, and how many unpaired electrons does each Period 2 atom have? At topic 9 (periodicity), re-explain the Period 3 ionisation-energy graph. At topic 22 (analytical techniques), re-define an isotope. At topic 28 (transition elements), re-write the configurations of Fe²⁺, Fe³⁺, Cu⁺ and Cu²⁺ and say why 4s is emptied first. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 1 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• A Paper 1 item on this topic turns on one precise fact or one careful count: which ion is largest, which configuration belongs to Fe³⁺, which equation is the second ionisation energy. A Paper 2 question asks you to define first ionisation energy, determine particle numbers or configurations, and explain a trend or a dip in the language of nuclear charge, shielding, distance and spin-pair repulsion.
• The numbers here are counts (protons, neutrons, electrons) and ionisation energies. The first four ionisation energies of selected elements are printed in the Data section and supplied in the examination, so you are never expected to recall them. What you must be able to do is compute ratios, take log₁₀ values and read a group, a period and a configuration from a table.
• Topic 1 supplies no laboratory procedure for Paper 3. Its link to the practical papers is data handling: transforming a table of values, choosing axes and a scale that fill the grid, and drawing a conclusion from the pattern, which is a Paper 5 skill. The Data and practical focus section of this chapter sets that out.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.