Q.24Mg and 27Al both undergo (α,n) reactions and the products are radioactive. These emit β particles having positive charge (called positrons). Write balanced nuclear reactions in both.

Step 1. An (α,n) reaction means an alpha particle (4-2-He) is absorbed by the target nucleus and a neutron (1-0-n) is ejected, so the product's mass number rises by 3 (add 4 for the alpha, subtract 1 for the ejected neutron) and its atomic number rises by 2 (the alpha's 2 protons stay behind).

Step 2. For 24-12-Mg: mass number 24+4-1 = 27; atomic number 12+2 = 14. So 24-12-Mg + 4-2-He → 27-14-Si + 1-0-n.

Step 3. The product, silicon-27, is stated to be radioactive and to emit a positron (a positively-charged beta particle, 0,+1,e). Positron emission lowers the atomic number by 1 while leaving the mass number unchanged (the mirror image of ordinary beta-minus decay): 27-14-Si → 27-13-Al + 0-1-e (writing the positron as 0-1-e per this chapter's plain-text convention for its charge/mass superscript-subscript pair). …

For each (α,n) reaction, add the alpha particle's mass (4) and atomic (2) numbers to the target and subtract the ejected neutron's mass number (1); then apply positron-emission's own r …

• Forgetting to subtract the ejected neutron's mass number, over-counting the product's mass by 1.
• Applying the ordinary beta-minus rule (atomic number UP by 1) instead of the positron (beta-plus) rule (atomic number DOWN by 1) to the rad …