1.4 Ionisation energy

Use ground-state electron configurations for the assessed ionisation-energy scope: atoms and ions are treated in their lowest-energy available arrangement, using the supported range from hydrogen to krypton.

Ionisation energy refers to removing one mole of electrons from one mole of gaseous species, one electron-removal step at a time, under the stated standard conditions. First ionisation uses gaseous atoms; successive ionisation uses the increasingly positive gaseous ions formed after earlier removals.

When successive-ionisation data are interpreted, use the ground-state configuration to locate the outer-shell electrons. A large jump means the next electron is being removed from an inner shell with stronger nuclear attraction; the number removed before that jump indicates the outer-shell count for the supported s- and p-block cases.

Do not use an excited-state promotion or an element outside the assessed range as the default configuration, and do not remove several electrons in one ionisation step. The scope limit defines the cases to apply; it does not claim that other states or elements cannot exist. Detailed trend explanations remain in the neighbouring ionisation-energy objectives.

First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms, forming one mole of gaseous 1+ ions. It is measured under the stated standard conditions, in kJ mol⁻¹, and is positive because electron removal is endothermic.

Write the process for one atom as X(g) → X⁺(g) + e⁻, while the definition refers to one mole of gaseous atoms and one mole of electrons removed. The electron removed is the outer electron in the ground-state atom, so its attraction to the nucleus and the shielding by inner electrons affect the energy required.

Keep every state and amount in the definition: the atom and ion are gaseous, the product is a gaseous 1+ ion, and only one electron is removed in the first step. Do not write a solid ion, remove several electrons at once, or confuse first ionisation energy with a successive ionisation energy of an already positive gaseous ion.

Use this card for the definition and equation of first ionisation energy. Periodic trends, successive-ionisation jumps and detailed electronic-configuration deductions belong to the neighbouring objectives, even though they use the same gaseous-ion convention.

Successive ionisation energies are the energies required to remove one mole of electrons, one electron-removal step at a time, from the same element as its gaseous ion becomes more positive. The second step starts from X⁺(g), the third from X²⁺(g), and so on.

Write each gaseous species explicitly: X⁺(g) → X²⁺(g) + e⁻ for the second ionisation, then X²⁺(g) → X³⁺(g) + e⁻ for the third. As electrons are removed, shielding decreases and the proton-to-electron ratio increases, so the remaining electrons are generally more strongly attracted to the nucleus and successive values increase.

Interpret the sequence by locating a large jump between successive values. The jump means the next electron is being removed from a new inner shell, closer to the nucleus and more strongly attracted; the number of electrons removed before the jump gives the outer-shell count in the supported s- and p-block inference.

Do not compare successive values as if each came from a fresh neutral atom, and do not remove several electrons in one step. Keep the gaseous state and changing positive charge in every equation; use the jump as evidence about shell structure, not as a standalone claim about an unsupported element or configuration.

Across a period, first ionisation energy generally increases. Proton number rises while added electrons enter the same principal shell, so shielding changes relatively little and the outer electron experiences a stronger overall attraction to the nucleus.

The rising nuclear charge pulls the outer electron closer, while atomic radius generally decreases across the period. The stronger attraction makes the outer electron harder to remove, so more energy is required; the trend is general rather than perfectly smooth.

Explain a small dip from the electron arrangement, not from a fall in nuclear charge. A p electron can be higher in energy than an s electron, as for Al compared with Mg, or paired p electrons can repel each other, as for S compared with P; either makes removal slightly easier.

When reading a period trend, compare the relevant outer-electron sub-shell, distance, shielding and spin-pairing evidence together. Do not claim a perfectly straight increase or explain every dip only by atomic number; detailed successive-ionisation jumps and group deductions belong to the neighbouring objectives.

Successive ionisation energies generally increase because each electron is removed from a more positively charged gaseous ion. Shielding decreases and the proton-to-electron ratio increases, so the remaining electrons are held more strongly by the nucleus.

A large jump means the next electron is being removed from a new inner shell. It is closer to the nucleus and experiences stronger attraction, so much more energy is required; smaller changes can occur within a shell or sub-shell.

To infer the outer-shell count, find the first large change in the successive-ionisation data and count the electrons removed before it. For example, if the third value is the first very large value, two outer electrons were removed before the shell change, supporting a Group 2 inference for the supported case.

Do not treat every increase as a shell jump, or read a jump without checking the gaseous-ion sequence and the stated period or element information. The method infers shell structure from a change of scale; it does not replace the separate first-ionisation trend explanation or justify unsupported configurations.

Ionisation energy reflects the attraction between the nucleus and the electron being removed. A larger ionisation energy means that, under the same definition and gaseous-state conditions, more energy is needed to overcome that attraction.

Compare four linked factors: greater nuclear charge strengthens attraction; greater distance from the nucleus weakens it; inner-shell electrons shield the outer electron; and spin-pair repulsion can make a paired electron easier to remove. The observed value is the combined result, not a proton count alone.

Use the electron’s actual shell and sub-shell before predicting a value. A 3p electron is generally easier to remove than a 2p electron because it is farther from the nucleus and more shielded; within a paired orbital, electron–electron repulsion also lowers the energy needed for removal.

Do not say that more protons always give a higher ionisation energy without checking distance, shielding, sub-shell energy and pairing. Keep these attraction factors distinct from the separate definition of first ionisation energy and from the successive-ionisation jump method.

For a first-ionisation-energy comparison, identify the electron removed in each species and check four linked factors: nuclear charge, distance from the nucleus, shielding by inner shells, and spin-pair repulsion.

State how each factor changes the attraction: greater nuclear charge tends to increase ionisation energy, whereas greater distance or shielding tends to reduce it. Pairing can lower the value because two electrons repel within one orbital. Then decide which effect dominates for the actual comparison.

Use the sub-shell and pairing evidence to explain exceptions to a general period trend. A 3p electron can be easier to remove than a 3s electron because it is higher in energy, while a paired 3p electron can be easier to remove than an unpaired counterpart; these explain the Mg/Al and P/S-type dips supported by the SME.

Do not list four factors without linking them to the electron being removed, and do not claim that proton number alone decides the result. Keep this comparison method separate from the definition of first ionisation energy, the across-period overview and the successive-ionisation jump method.

Use successive-ionisation data to infer an atom’s outer-shell configuration: locate the largest jump, then count the electrons removed before that jump. Those electrons belonged to the outer shell; the next value begins removal from a new inner shell.

Combine the outer-electron count with the stated period or other supplied information. The period identifies the occupied principal shell, so the count can then be used to infer the relevant outer sub-shells and write the supported ground-state configuration.

For example, a first large jump after seven removals supports seven outer electrons; with Period 3 information, the outer arrangement is 3s²3p⁵. Treat this as evidence-led deduction from the jump and the supplied period, not as a guess from the jump alone.

A jump gives the number of outer electrons, not automatically the element. Check the period, the gaseous-ion sequence and the supported configuration scope before naming an element; do not confuse a shell jump with the smaller within-shell or sub-shell changes.

Read successive-ionisation data in sequence: compare adjacent values, locate the largest change of scale, and count the electrons removed before that jump. The count gives the number of outer-shell electrons before the next electron is taken from an inner shell.

Use the outer-electron count as a group pattern for the supported s- and p-block cases, then combine it with the stated period or configuration to identify the outer sub-shells and possible element. A large jump between the seventh and eighth values therefore indicates seven outer electrons; Period 3 information supports the chlorine configuration.

Treat the jump as evidence of a shell-depth change: the next electron is closer to the nucleus and more strongly attracted. Preserve the order of the successive values and compare their scale, rather than calling any larger next value a shell jump.

The jump alone does not name an element. Check the period, supplied information and ground-state scope before writing a configuration, and keep this data-reading method distinct from explaining the four attraction factors or the general across-period trend.