Atomic Mass Unit, Isotopes, and Relative Atomic Mass

A set of practice flashcards covering atomic mass units, isotopes, atomic numbers, average atomic mass, and relative atomic mass.

What is the unified atomic mass unit (AU or u) and its SI definition?

It is defined as 1.66054 × 10^-27 kilograms.

Why was the unified atomic mass unit defined as 1.66054 × 10^-27 kg?

To make the masses of protons and neutrons roughly equal to 1 amu, while electrons are much lighter (about 1/2000 of a proton).

What are the approximate masses of a proton and a neutron in unified atomic mass units?

Proton ≈ 1.007 amu; neutron ≈ 1.008 amu.

How does the mass of an electron compare to protons/neutrons?

An electron is about 1/2000 the mass of a proton, so it contributes little to the overall atomic mass.

What primarily determines an element’s atomic mass?

The weighted average of the masses of its isotopes, dominated by the protons and neutrons in the nucleus.

What does the atomic number Z represent?

The number of protons in the nucleus; defines the element (e.g., Z=1 for hydrogen, Z=20 for calcium, Z=36 for krypton).

What is the most common form of hydrogen in the universe in terms of protons and neutrons?

Protium (1H): 1 proton, 0 neutrons; about 99.98% of hydrogen.

What is an isotope?

Atoms of the same element with the same number of protons but different numbers of neutrons.

What is the approximate mass of a hydrogen atom after including its electron?

Approximately 1.008 amu (mass ≈ proton + electron).

What is the term for the weighted average mass of an element’s isotopes?

Average atomic mass (also called atomic weight in older texts).

What does relative atomic mass mean on a periodic table?

A unitless value representing the atom’s mass relative to others (e.g., carbon is about 12 times the mass of a hydrogen atom on average).

Which part of the atom contributes most to its mass?

The nucleus (protons and neutrons); electrons contribute far less to the mass.