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Chapter 2: Problem 21

(a) Define atomic number and mass number. (b) Which of these can vary without changing the identity of the element?

Short Answer

Expert verified
(a) The atomic number (Z) is the number of protons in an atom's nucleus and also equals the number of electrons in a neutral atom. The mass number (A) is the total number of protons and neutrons in the nucleus. (b) The mass number can vary without changing the element's identity, as changing the number of neutrons results in different isotopes of the same element, while the atomic number remains the same.

Step by step solution

01

Define Atomic Number

The atomic number, denoted by Z, is the number of protons in an atom's nucleus. It is also equal to the number of electrons orbiting the nucleus in a neutral atom. Atomic numbers are whole numbers and are used to identify elements in the periodic table.
02

Define Mass Number

The mass number, denoted by A, is the total number of protons and neutrons in an atom's nucleus. Unlike the atomic number, the mass number does not uniquely identify an element, as atoms of the same element can have different numbers of neutrons. Atoms with the same atomic number but different mass numbers are called isotopes.
03

Determine which of the two can vary without changing the element's identity

The atomic number determines which element an atom belongs to, as elements are defined by the number of protons in their nuclei. Changing the atomic number would result in a different element. On the other hand, mass number can vary without changing the identity of the element as it only changes the number of neutrons in the nucleus. When mass number changes while the atomic number remains the same, the atom is still the same element but exists as a different isotope. Thus, mass number can vary without changing the identity of the element.

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Most popular questions from this chapter

Give the chemical names of each of the following familiar compounds: (a) \(\mathrm{NaCl}\) (table salt), (b) \(\mathrm{NaHCO}_{3}\) (baking soda), (c) \(\mathrm{NaOCl}\) (in many bleaches), (d) \(\mathrm{NaOH}\) (caustic soda), (e) (NH \(_{4}\) ) \(\mathrm{CO}_{3}\) (smelling salts), (f) \(\mathrm{CaSO}_{4}\) (plaster of Paris).

Provide the name or chemical formula, as appropriate, for each of the following acids: (a) hydrobromic acid, (b) hydrosulfuric acid, (c) nitrous acid, (d) \(\mathrm{H}_{2} \mathrm{CO}_{3}\) (e) \(\mathrm{HClO}_{3}\), (f) \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\)

Write the correct symbol, with both superscript and subscript, for each of the following. Use the list of elements inside the front cover as needed: (a) the isotope of platinum that contains 118 neutrons, (b) the isotope of krypton with mass number 84, (c) the isotope of arsenic with mass number \(75,(\mathrm{~d})\) the isotope of magnesium that has an equal number of protons and neutrons.

Elements in the same group of the periodic table often form oxyanions with the same general formula. The anions are also named in a similar fashion. Based on these observations, suggest a chemical formula or name, as appropriate, for each of the following ions: (a) \(\mathrm{BrO}_{4}\) -, (b) \(\mathrm{SeO}_{3}{ }^{2-},(\mathrm{c})\) arsenate ion, (d) hydrogen tellurate ion.

The natural abundance of \({ }^{3} \mathrm{He}\) is \(0.000137 \%\). (a) How many protons, neutrons, and electrons are in an atom of \({ }^{3} \mathrm{He}\) ? (b) Based on the sum of the masses of their subatomic particles, which is expected to be more massive, an atom of \({ }^{3}\) He or an atom of \({ }^{3} \mathrm{H}\) (which is also called tritium)? (c) Based on your answer for part (b), what would need to be the precision of a mass spectrometer that is able to differentiate between peaks that are due to \({ }^{3} \mathrm{He}^{+}\) and \({ }^{3} \mathrm{H}^{+} ?\)
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