BIO 140 Human Biology I - Questions and Solutions

1. Together, just four elements make up more than 95 percent of the body’s mass. These include ________.

1. calcium, magnesium, iron, and carbon
2. oxygen, calcium, iron, and nitrogen
3. sodium, chlorine, carbon, and hydrogen
4. oxygen, carbon, hydrogen, and nitrogen

1. The most abundant elements in the foods and beverages you consume are oxygen, carbon, hydrogen, and nitrogen. Why might having these elements in consumables be useful?

Solution: These four elements—oxygen, carbon, hydrogen, and nitrogen—together make up more than 95 percent of the mass of the human body, and the body cannot make elements, so it is helpful to have them in consumables.

2. Oxygen, whose atomic number is eight, has three stable isotopes: ¹⁶O, ¹⁷O, and ¹⁸O. Explain what this means in terms of the number of protons and neutrons.

Solution: Oxygen has eight protons. In its most abundant stable form, it has eight neutrons, too, for a mass number of 16. In contrast, ¹⁷O has nine neutrons, and ¹⁸O has 10 neutrons.

3. Magnesium is an important element in the human body, especially in bones. Magnesium’s atomic number is 12. Is it stable or reactive? Why? If it were to react with another atom, would it be more likely to accept or to donate one or more electrons?

Solution: Magnesium’s 12 electrons are distributed as follows: two in the first shell, eight in the second shell, and two in its valence shell. According to the octet rule, magnesium is unstable (reactive) because its valence shell has just two electrons. It is therefore likely to participate in chemical reactions in which it donates two electrons.