 # what is the difference between mass number and atomic number

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#### Studying Goal

• Decide the connection between the mass variety of an atom, its atomic quantity, its atomic mass, and its variety of subatomic particles

#### Key Factors

• Impartial atoms of every ingredient comprise an equal variety of protons and electrons.
• The variety of protons determines a component’s atomic quantity and is used to tell apart one ingredient from one other.
• The variety of neutrons is variable, leading to isotopes, that are totally different types of the identical atom that adjust solely within the variety of neutrons they possess.
• Collectively, the variety of protons and the variety of neutrons decide a component’s mass quantity.
• Since a component’s isotopes have barely totally different mass numbers, the atomic mass is calculated by acquiring the imply of the mass numbers for its isotopes.

#### Phrases

• atomic massThe common mass of an atom, bearing in mind all its naturally occurring isotopes.
• mass numberThe sum of the variety of protons and the variety of neutrons in an atom.
• atomic numberThe variety of protons in an atom.

## Atomic Quantity

Impartial atoms of a component comprise an equal variety of protons and electrons. The variety of protons determines a component’s atomic quantity (Z) and distinguishes one ingredient from one other. For instance, carbon’s atomic quantity (Z) is 6 as a result of it has 6 protons. The variety of neutrons can fluctuate to supply isotopes, that are atoms of the identical ingredient which have totally different numbers of neutrons. The variety of electrons will also be totally different in atoms of the identical ingredient, thus producing ions (charged atoms). As an example, iron, Fe, can exist in its impartial state, or within the +2 and +3 ionic states.

## Mass Quantity

A component’s mass quantity (A) is the sum of the variety of protons and the variety of neutrons. The small contribution of mass from electrons is disregarded in calculating the mass quantity. This approximation of mass can be utilized to simply calculate what number of neutrons a component has by merely subtracting the variety of protons from the mass quantity. Protons and neutrons each weigh about one atomic mass unit or amu. Isotopes of the identical ingredient may have the identical atomic quantity however totally different mass numbers.

You're reading: what is the difference between mass number and atomic number Atomic quantity, chemical image, and mass quantityCarbon has an atomic variety of six, and two secure isotopes with mass numbers of twelve and 13, respectively. Its common atomic mass is 12.11.

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Scientists decide the atomic mass by calculating the imply of the mass numbers for its naturally-occurring isotopes. Typically, the ensuing quantity incorporates a decimal. For instance, the atomic mass of chlorine (Cl) is 35.45 amu as a result of chlorine consists of a number of isotopes, some (the bulk) with an atomic mass of 35 amu (17 protons and 18 neutrons) and a few with an atomic mass of 37 amu (17 protons and 20 neutrons).

Given an atomic quantity (Z) and mass quantity (A), you will discover the variety of protons, neutrons, and electrons in a impartial atom. For instance, a lithium atom (Z=3, A=7 amu) incorporates three protons (discovered from Z), three electrons (because the variety of protons is the same as the variety of electrons in an atom), and 4 neutrons (7 – 3 = 4).

Present Sources

Boundless vets and curates high-quality, overtly licensed content material from across the Web. This specific useful resource used the next sources:

“Boundless.” “atomic quantity.” “Boundless.” “A-level Chemistry/OCR/Atoms, Bonds and Teams/Atoms and Reactions/Atoms.” “OpenStax Faculty, Biology. October 16, 2013.”

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