SC180's Unit 3 atomic structure explanation, as shown here, traces two dips in period 3 ionization energies, magnesium to aluminum and phosphorus to sulfur, to electron configurations. Searches like "sc 180 unit 3 assignment example", "sc180 unit 3 sample" and "sc180 unit 3 example" land here.
What a finished SC180 Unit 3 atomic structure explanation looks like
Three pages with one table and one plot. The table lists sodium through argon with each atom's noble-gas configuration, [Ne]3s1 for sodium through [Ne]3s2 3p6 for argon, beside its first ionization energy in kJ/mol. The plot shows the general rise from 495.8 at sodium to 1520.6 at argon. The explanation argues that rise from effective nuclear charge: ten core electrons shield about equally across the row while the nuclear charge climbs, so a simple core-count estimate grows from roughly +1 at sodium to +7 at chlorine. Two paragraphs take the exceptions one at a time. Aluminum's 577.5, below magnesium's 737.7, comes from a lone 3p electron sitting higher in energy than the 3s pair. Sulfur's 999.6, just under phosphorus, comes from the first paired 3p electron and the repulsion it feels.
How a SC180 Unit 3 example is structured
Most finished explanations move from model to data to exceptions. An opening paragraph sets out the rules that build a configuration: the aufbau order, the Pauli limit of two electrons per orbital and Hund's rule for filling a sublevel singly before pairing, each stated in one sentence with a period 3 example. Orbital box diagrams for phosphorus and sulfur appear beside the text, since the sulfur argument depends on seeing the paired box. The trend section defines first ionization energy as the energy to remove one electron from a gaseous atom, then explains the general increase. The exceptions section is the graded core and treats each dip as a prediction the simple model gets wrong and the configuration repairs. A short closing paragraph links the same reasoning to atomic radius, which shrinks across the row for the same reason.
Configurations in noble-gas shorthand
Sodium through argon are written as [Ne] plus the 3s and 3p electrons, which keeps attention on the valence shell, where every argument in the explanation takes place.
Effective nuclear charge as the engine
A rough estimate, nuclear charge minus the ten core electrons, rises from +1 at sodium to +7 at chlorine. That growing pull on the valence shell is offered as the reason the trend climbs at all.
The aluminum dip, traced to a sublevel
Aluminum's outermost electron opens the 3p sublevel, slightly higher in energy and partly screened by the 3s pair, so removing it takes about 160 kJ/mol less than magnesium's 3s electron.
The sulfur dip, traced to pairing
Sulfur's fourth 3p electron must share an orbital, and repulsion within the pair makes one of them easier to remove. The drop from phosphorus is only about 12 kJ/mol, and the explanation says why it is smaller.
Radius as a cross-check
The closing paragraph notes atomic radius shrinking across the same row, the mirror image of the ionization trend, which shows the effective nuclear charge argument doing double duty.
Where marks go in SC180 Unit 3
Explanations lose the most when they describe the trend without a cause, stating that ionization energy increases across a period and stopping there. Configuration errors are the next largest cost: 3p filled before 3s, or argon given a 3d electron that does not belong. The two exceptions are where most sections place the heaviest weight, and answers that call them anomalies without naming the electron involved earn little of it. Mixing up shielding and nuclear charge, as in a claim that shielding rises steeply across a row, contradicts the explanation's own trend. Hund's rule misapplied, with sulfur's 3p electrons paired too early in the diagram, undermines the pairing argument before it starts. Units missing from the energy values and data quoted without a source draw the smaller deductions in most rubrics.
Get a SC180 Unit 3 example written to your instructions
Forward SC180's Unit 3 question and its rubric, noting the row, group or property it asks about and whether a plot is required, since some prompts use radius or electronegativity instead. The explanation follows that trend through configurations, orbital diagrams and cited values. It reaches you in 24-48h at no charge the first time.
SC180 Unit 3 questions, answered
Why does aluminum have a lower ionization energy than magnesium?
Magnesium's outermost electrons fill the 3s orbital, while aluminum's highest-energy electron is the first one in 3p. A 3p electron sits slightly higher in energy and is partly screened by the filled 3s pair, so less energy removes it. The drop, about 160 kJ/mol, is large enough that answers are expected to name that specific electron.
Do I need orbital diagrams or just configurations?
Configurations usually cover the general trend, but the sulfur exception is much clearer with a diagram, because the argument rests on one orbital holding two electrons. Many SC180 prompts ask for at least one orbital diagram, and when they do not, a small one beside the phosphorus and sulfur paragraph tends to strengthen the explanation.
Is effective nuclear charge calculated or just described?
At this level it is usually estimated rather than calculated precisely. Nuclear charge minus the number of core electrons gives a rough figure that rises across a row, which is enough to explain the trend. Some sections introduce Slater's rules for a closer estimate, so the unit reading decides which method your explanation should use.