Ch.2 - Atoms, Molecules & Ions

McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook

McMurry 8th Edition

Ch.2 - Atoms, Molecules & Ions

Problem 96

Chapter 2, Problem 96

What is the difference between an atom's atomic number and its mass number?

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Understand that the atomic number of an atom is the number of protons in the nucleus of the atom. It is denoted by the symbol Z.

Recognize that the mass number of an atom is the total number of protons and neutrons in the nucleus. It is denoted by the symbol A.

Note that the atomic number (Z) is unique to each element and determines the element's identity on the periodic table.

Realize that the mass number (A) is not unique to each element because isotopes of an element have the same atomic number but different mass numbers due to varying numbers of neutrons.

Remember that the difference between the mass number and the atomic number gives the number of neutrons in the atom: Neutrons = Mass Number (A) - Atomic Number (Z).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Atomic Number

The atomic number of an element is the number of protons found in the nucleus of an atom. It uniquely identifies an element and determines its position on the periodic table. For example, hydrogen has an atomic number of 1, indicating it has one proton. The atomic number also defines the element's chemical properties and its identity.

Mass Number

The mass number is the total number of protons and neutrons in an atom's nucleus. It reflects the mass of the atom and is not unique to an element, as different isotopes of the same element can have different mass numbers. For instance, carbon-12 has a mass number of 12, while carbon-14 has a mass number of 14 due to the presence of additional neutrons.

Isotopes

Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons, resulting in different mass numbers. For example, both carbon-12 and carbon-14 are isotopes of carbon, with carbon-12 having 6 neutrons and carbon-14 having 8 neutrons. Isotopes can exhibit different physical properties and stability, influencing their applications in fields like medicine and archaeology.

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Related Practice

Textbook Question

A period at the end of sentence written with a graphite pen-cil has a diameter of 1 mm. How many carbon atoms would it take to line up across the period if a single carbon atom has a diameter of 150 pm?

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Textbook Question

Prior to Rutherford's gold foil experi-ment, the 'plum pudding' model of the atom represented atomic structure. In this model, the atom is composed of elec-trons interspersed within a positive cloud of charge. If this were the correct model of the atom, predict how the results of Rutherford's experiment would have been different. (a) The alpha particles would pass right through the gold foil with little to no deflection.(b) Most of the alpha particles would be deflected back toward the source.(c) Most of the alpha particles would be absorbed by the atom and neither pass through nor be deflected from the gold foil.

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Textbook Question

The subscript giving the atomic number of an atom is often left off when writing an isotope symbol. For example, 6 often written simply as 13C. Why is this allowed?

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Textbook Question

A period at the end of sentence written with a graphite pencil has a diameter of 1 mm. If the period represented the nucleus, approximately how large is the diameter of the entire atom in units of m?

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Textbook Question

What is the difference between an element's atomic number and its atomic weight?

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