Figure 2-15. Per each 99 blue beads weighing 12 g each there is one red bead that weighs 13 g.
There are two more contributors to the atomic weights in the periodic table being fractional.
- The masses of protons and neutrons are not exactly 1 a.m.u. (Table 1).
- Mass defect
is the small yet detectable difference between the mass of an atomic nucleus and the sum of masses of the protons and neutrons comprising that nucleus. This difference is due to the release of energy upon formation of an atomic nucleus from isolated protons and neutrons, according to Einstein's formula E = mc2
For the vast majority of areas of chemistry, however, the contribution from these two factors are not significant enough to be taken into account.
In section 2.1 above, we mentioned the three "discrepancies" where an element with a lower atomic number has a higher atomic weight than its neighbor with a higher atomic number (Ar vs. K; Co vs. Ni; and Te vs. I). Now that we are aware of the existence of isotopes, we should be able to see that there is no discrepancy there. It is all about the number of neutrons in the nuclei of naturally occurring isotopes within each of the Ar-K, Co-Ni, and Te-I pairs. The most abundant isotope of argon (40
Ar, >99%) is heavier than the most abundant isotope of potassium (39
K, >93%). Although a potassium atom is one proton heavier than an argon atom, the natural isotopes of argon beat those of potassium in terms of the number of neutrons. This is also the case with the Co-Ni and Te-I pairs.
Guessing how many isotopes an element has on the basis of its atomic weight specified in the periodic table is useless. For instance, one might have thought that bromine (Br) is a monoisotopic element, 80
Br, because its atomic weight (79.9) is just 0.1 a.m.u. short of 80. This is not the case however, as 80
Br is actually not a natural stable isotope. The naturally occurring isotopes of bromine are 79
Br and 81
Br, existing in a roughly 1:1 ratio, which makes the average approximately 80. Likewise, the atomic weight of tin (118.71) says nothing about its isotopic composition. In fact, tin is the element with the greatest number of stable isotopes (ten!), their abundance ranging from ~0.3% for 115
Sn to ~32.6% for 120
Perhaps you do not like my use of the terms "atomic mass" and "atomic weight" interchangeably. In physics, mass and weight mean two different things: mass is the quantity
of matter, whereas weight is the gravity force
acting on an object. In chemistry however, both mass
mean the same thing, the quantity of matter. It is noteworthy that while "mass" is more frequently applied to a single isotope of an atom, "weight" is conventionally used for the average mass of all of the naturally-occurring isotopes of a given element. 2.2.7. Exercises.
1. An atom is (a) much bigger in size than its nucleus; (b) comparable in size with its nucleus; (c) slightly bigger in size than its nucleus. Answer
2. It is sometimes said that matter is largely made of emptiness. Why? Answer
3. An atomic nucleus consists of (a) positively charged protons and negatively charged electrons; (b) positively charged protons, electroneutral neutrons, and negatively charged electrons; (c) positively charged protons and electroneutral neutrons; (d) uncharged neutrons and negatively charged electrons. Answer
4. The atomic number of an element is (a) the number of electrons an atom of that element has; (b) the number of protons an atom of that element has; (c) the number of protons and neutrons an atom of that element has; (d) the charge of the atomic nucleus; (e) the number of protons minus the number of neutrons in the nucleus of an atom of that element. Answer
5. The nucleus of an element consists of 15 protons and 16 neutrons. What element is that? What is the isotope? Is this element monoisotopic? Answer
6. A neutron is (a) slightly heavier than a proton and much heavier than an electron; (b) heavier than electron but much lighter than a proton; (c) much heavier than a proton but lighter than an electron; (d) lighter than a proton and an electron. Answer
7. Different isotopes of the same element have identical chemical poroperties. True or false? Answer
8. Find fluorine in the periodic table. Fluorine is a monoisotopic element. How many protons, neutrons, and electrons does a naturally occurring fluorine atom have? Answer
9. Why is the atomic weight of argon (atomic number: 18) higher than that of potassium (atomic number:19)? [Answer: See section 2.2.6]
10. Magnesium (Mg, atomic #12) has three naturally occurring stable isotopes: 24
Mg (79%), 25
Mg (10%), and 26
Mg (11%). How many protons and neutrons does each of these isotopes have? Using the isotopic distribution percentages, calculate the atomic weight of naturally occurring magnesium. Answer