Two numbers describe any nucleus completely
Proton number (atomic number), Z
The number of protons in the nucleus of an atom.
Nucleon number (mass number), A
The total number of protons and neutrons in the nucleus of an atom.
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Once you know ZZZ and AAA for a nucleus, you know everything about what it is made of, because the neutrons are whatever is left over.
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The number of neutrons, sometimes written NNN, is found by subtraction.
N=A−Z N = A - Z N=A−Z -
This works because AAA counts all the nucleons while ZZZ counts only the protons, so taking one from the other leaves the neutrons.
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The proton number also tells you the relative charge on the nucleus, because each proton contributes +1+1+1 and each neutron contributes nothing, so a nucleus with Z=92Z = 92Z=92 has a relative charge of +92+92+92.
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The nucleon number tells you the relative mass of the nucleus, because protons and neutrons have almost equal mass, so a nucleus with A=235A = 235A=235 is about 235235235 times the mass of a single nucleon.
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In a neutral atom the number of electrons also equals ZZZ, since the charges must cancel.
Nuclide notation packs both numbers around the chemical symbol
Nuclide
A particular type of nucleus, defined by a specific number of protons and a specific number of neutrons.
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A nuclide is written with the nucleon number on top and the proton number underneath, both to the left of the chemical symbol.
ZAX ^{A}_{Z}\text{X} ZAX -
The bigger number always goes on top, because a nucleus can never contain fewer nucleons than it contains protons.
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For uranium-235, written 92235U^{235}_{92}\text{U}92235U, the nucleus has 929292 protons, 235235235 nucleons in total and therefore 235−92=143235 - 92 = 143235−92=143 neutrons.
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The symbol and the proton number carry the same information, so 614C^{14}_{6}\text{C}614C and "carbon-14" describe exactly the same nucleus.
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Isotopes of one element share the bottom number and differ in the top one, as in 612C^{12}_{6}\text{C}612C and 614C^{14}_{6}\text{C}614C.
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The same notation is used for the emitted particles, which is what makes decay equations possible to balance.
- An alpha particle is 24α^{4}_{2}\alpha24α, with two protons and two neutrons.
- A beta particle is −10β^{0}_{-1}\beta−10β, an electron, which is why its top number is 000 and its bottom number is negative.
- A neutron is 01n^{1}_{0}\text{n}01n, with one nucleon and no charge.
- Do not read the top number as the neutron count; AAA counts protons and neutrons together.
- Do not swap the positions: the larger number sits above the smaller one.
- Do not assume the electron count equals ZZZ for an ion; that only holds for a neutral atom.
- Do not describe the nucleon number as a mass in kilograms; it is a count that happens to be proportional to the nucleus's mass.
- Neutron-number questions are one mark and are lost by subtracting the wrong way round, so write A−ZA - ZA−Z out in full before putting numbers in.
- When a definition is asked for, say number of protons and number of protons plus neutrons; "the number at the bottom" earns nothing.
- Copy the symbols carefully when you complete a nuclide in a table, because a missing subscript is treated as a missing answer.
- Learning 24α^{4}_{2}\alpha24α and −10β^{0}_{-1}\beta−10β now saves time later, since every decay equation is balanced using these two.
- Define proton number and nucleon number.
- Write down the equation for finding the number of neutrons in a nucleus.
- How many protons, neutrons and electrons are there in a neutral atom of 2656Fe^{56}_{26}\text{Fe}2656Fe?
- Which of the two numbers gives the relative charge on the nucleus, and which gives its relative mass?
- Give the nuclide notation for an alpha particle and for a beta particle.
