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Define atomic number (Z).

Atomic number (Z) is the number of protons in the nucleus.

Note that there is no special envelope or barrier that defines the nucleus (like in a biological cell). The protons (and neutrons) ARE the nucleus. The atomic number is unique to each element. It therefore is ultimately responsible for the trait of an element and the identity of an atom.

An atom by definition is a neutral particle - which means the number of protons = number of electrons (equal number of postive and negative charges). Therefore, for an atom, the atomic number also defines the number of electrons.


Define mass number (A).

Mass number (A) represents the total number of protons and neutrons.

The mass number is always an integer and is NOT a mass. Rather, the mass number is associated with each unique isotope of an element.


Define average atomic mass.

Average atomic mass is the mass of an atom considering the relative abundance of the naturally occuring isotopes.

In the context of mass spectrometry measurements (and isotope-type problems), this mass is reported in unified atomic mass units (u). In the context of mole mass relationships, this mass can be understood to have units of grams/mole (g/mol).


Define unified atomic mass unit (u)

One (u) is exactly 1/12th the mass of one carbon-12 atom.

This unit is therefore defined in terms of the most abundant isotope of carbon. This has been the approach since 1961. This unit was originally defined in terms of hydrogen, the smallest element, and then later in terms of oxygen.


Which atomic particles are responsible for the bulk of an atom's mass?

The proton and neutron make up most of the mass of an atom. In relative terms, the mass of an electron is considered insignificant.


State the relative masses of the proton, neutron and electron.

The relative masses of the proton, neutron and electron are about 1 vs 1 vs 0. The actual masses are: proton = 1.0073 u; neutron = 1.0078 u; electron = 0.00055 u. Therefore the neutron is slightly more massive than the proton.


How does adding an electron to an atom affect its mass?

An atom is neutral - equal number of protons and electrons. One additional electron will result in a negatively charged particle (anion) - but given that the mass of an electron is negligible compared to the proton and neutron, there is essentially no change to the mass of the atom.


An element has a unique number of neutrons. T or F?

This best answer for this is false. One element may have several naturally occuring isotopes and therefore is associated with several different numbers of neutrons.

There are a few elements that only have one naturally occuring isotope and this isotope has a unique number of neutrons - but this situation is less common.  Also it wouid be possible to alter this element to have a different number of neutrons, but this would not alter the element's identity. 


The number of neutrons can be determined by subtracting the number of protons from the average atomic mass. T or F?

False - the average atomic mass should not be used to determine the number of neutrons for an isotope. To determine the number of neutrons one should subtract the number of protons from the the mass number (A).


A cation is produced through a gain of protons. T or F?

False - while it is true that cations have more protons than electrons, this is the result of an atom losing an electron to become postively charged. Electons are chemistry's currency.


An anion always has more electrons than protons. T or F?

True - an anion is negatively charged. This can only be the result of the number of electrons exceeding the number of protons. This occurs when an atoms gains electrons. Electrons are chemistry's currency.


Isotopes of an element usually have the same abundance.

False, in the universe we know, no element has two or more istopes with an identical abundance.


Lithium has an approximate average atomic mass of 6.94 u and consists of two naturally occurring isotopes: 6Li and 7Li. Which isotope is more abundant?

This information tells us that isotope 7Li is more abundant given that the average atomic mass is closest to the mass of isotope 7Li.


What equation can be used to calculate the average atomic mass for an element with three isotopes?

AAM = fa1m1 + fa2m2 + fa3m3 (where fa = fractional abundance for each isotopes and m = mass for each isotope.


The number of protons in the nucleus.


Atomic number (Z)


The total number of protons and neutrons.


Mass number (A)


The mass of an atom considering the relative abundance of the naturally occuring isotopes.


Average atomic mass


Exactly 1/12th the mass of one carbon-12 atom.


Define unified atomic mass unit (u)