Final answer:
The head of the humerus articulates with the scapula at the glenoid cavity to form the glenohumeral joint, which is the shoulder joint responsible for a wide range of arm motion.
Step-by-step explanation:
The part of the humerus that articulates with the scapula to form the shoulder joint is the head of the humerus. This large, rounded region is located at the proximal end of the humerus, facing medially towards the body's center. The head of the humerus fits into the glenoid cavity of the scapula, making up the glenohumeral joint, also referred to simply as the shoulder joint. This ball-and-socket joint is responsible for the wide range of motion allowed in the human arm, although this mobility comes with a reduced level of stability.