NLN NEX Science common mistakes
NLN NEX Science · Understand the traps and how to avoid them.
When asked for the number of neutrons in an atom, the candidate subtracts nothing and simply repeats the mass number, or subtracts the mass number from the atomic number in the wrong order.
Atomic number and mass number are both written next to the element symbol and are memorized as a pair of numbers without a fixed meaning, so under time pressure the candidate grabs whichever number looks familiar.
What to do instead
Rehearse the definitions aloud in a fixed sentence each time: atomic number = protons (defines the element); mass number = protons + neutrons; so neutrons = mass number minus atomic number. Practice on 10 elements including one isotope before moving on.
Plugging values straight into a formula such as speed = distance/time or density = mass/volume while the numbers are still in centimeters or grams, and then choosing an answer choice in meters or kilograms.
The arithmetic feels correct and the answer matches a choice, so the mismatch is invisible; candidates skip conversion because it feels like extra work rather than part of the problem.
What to do instead
Before computing, write the units of every given quantity next to its number and convert to SI (m, kg, s) first. Then check that the units of the computed answer match the units requested in the question stem.
Stating that mitosis produces four genetically different haploid cells, or that meiosis produces two identical diploid cells.
The two processes are studied back-to-back and both involve prophase/metaphase/anaphase, so the chromosome counts and daughter-cell numbers get swapped.
What to do instead
Build a two-column comparison card with four fixed rows: number of divisions, daughter cells produced, chromosome number (haploid/diploid), and genetic variation. Recite the meiosis row (two divisions, four haploid, varied) before answering any cell-division item.
Labeling the pulmonary artery as the vessel carrying oxygen-rich blood and the pulmonary vein as carrying oxygen-poor blood, reasoning from 'artery = oxygenated.'
The systemic rule 'arteries carry oxygenated blood away from the heart' is learned first and is true everywhere except the pulmonary circuit, so the exception is overridden by the rule.
What to do instead
Anchor on the destination, not the vessel name: blood going to the lungs is oxygen-poor, blood returning from the lungs is oxygen-rich. Sketch the heart-lung loop once and label the four vessel types by oxygen status rather than by artery/vein.
Listing blood flow through the heart in the order right atrium, right ventricle, left atrium, left ventricle, or placing the bicuspid/mitral valve on the right side.
The chambers are memorized as a list rather than as two parallel pumps, so the two sides get braided together and valve names get attached to the wrong side.
What to do instead
Trace one drop of blood end to end and name every structure it passes: vena cava, right atrium, tricuspid, right ventricle, pulmonary valve, pulmonary artery, lungs, pulmonary vein, left atrium, mitral, left ventricle, aortic valve, aorta. Repeat the trace from memory three times.
Saying insulin raises blood glucose or that glucagon lowers it, or describing ADH as increasing urine output.
Hormone names carry no built-in hint about direction, so the effect is guessed; the pairing of hormone with gland is often known while the direction of the effect is not.
What to do instead
For each hormone, memorize a one-line effect in plain language with the trigger: insulin = glucose into cells, glucose falls; glucagon = liver releases glucose, glucose rises; ADH = water reabsorbed, urine concentrates. Test yourself by stating the effect before the name.
Recommending an antibiotic for a sore throat or flu-like illness described as viral, or describing antibiotics as effective against viruses at low doses.
Antibiotics are strongly associated with 'getting better from an infection,' and the distinction between pathogen classes feels like a technicality rather than the deciding fact.
What to do instead
Adopt a two-question filter for every treatment item: is the pathogen bacterial, viral, or fungal, and does the drug act on that class? Memorize the one-liner: antibiotics target bacterial structures (cell wall, ribosomes) that viruses lack entirely.
Describing Gram-positive bacteria as staining pink/red with a thin peptidoglycan layer, or reversing the order of the crystal violet and safranin steps.
The stain result is memorized as a color with no structural reason attached, so the color and the wall thickness get cross-wired in recall.
What to do instead
Tie color to structure: thick peptidoglycan retains crystal violet, so Gram-positive stays purple; thin wall loses it and takes the pink counterstain. Walk through the four reagents in order once, then recall the color from the wall description.
Identifying the variable plotted on the x-axis as the dependent variable, or naming the dependent variable as the one the experimenter deliberately changes.
In everyday language 'depends on' and 'changes' are used loosely, and graphs are read left-to-right so the first axis encountered is assumed to be the outcome.
What to do instead
Use the sentence frame: the experimenter changes the ___ (independent), and measures the ___ (dependent) in response. Apply the frame to the axis labels before reading any values off the graph.
Concluding that one factor causes another because the two rise together on a graph, or extending a trend line far past the last plotted data point and reporting the extrapolated value as a finding.
The graph visually invites a straight-line continuation and a causal story is more satisfying than a correlational one, so the limits of the data are not treated as part of the evidence.
What to do instead
For every trend item, state aloud what was measured and over what range, then ask whether the study manipulated the variable and whether the prediction falls inside the plotted range. Mark any answer that goes beyond the data or asserts cause as incorrect.