Nuclide Symbols: Atomic Number, Mass Number, Ions, and Isotopes

hey it's professor dave, let's talk about nuclide symbols

we know that atoms have a tiny nucleus with positively charged

protons and neutrons which have no charge

as well as negatively charged electrons that are some distance away from the


every element is a combination of some number of these three particles

the one that determines which element an atom belongs to

is the proton, one proton means hydrogen two is helium

three is lithium and so forth. so an element has an atomic number

which refers to the number of protons in the nucleus, each atom also has a mass


while only protons count for atomic number both protons

and neutrons count for mass number because protons and neutrons

are each about one atomic mass unit. electrons are so much less massive than

protons and neutrons

that we ignore them when we look at mass, so mass number is really just the number

of particles

in the nucleus. if an atom has six protons and six neutrons

it is an atom of carbon 12, here

the number is referring to the mass. carbon atoms always have six protons

if they didn't they wouldn't be carbon. when we represent an atom we use a

nuclide symbol

this will consist of one or two letters that abbreviate the element, if one

letter it's capitalized

if two the first is capitalized and the second is not. to the bottom left we

sometimes put the atomic number

which as we said is the number of protons in the nucleus. this is kinda

redundant because the type of element implies the number of protons

but sometimes we do it anyway. to the upper left is the mass number

this is not redundant because atoms of a given element can have different masses

due to differing numbers of neutrons. these are called

isotopes of a given element. remember that the mass number is the number of

protons plus the number of neutrons

which means that the number of neutrons in an atom is the mass number

minus the atomic number. and to the upper right if the atom is an ion meaning a

particle with electrical charge

the charge will be listed here. if the number of protons and electrons in an

atom are the same

the positive and negative charges cancel out and it will be a neutral atom

but if it gains an electron

there will be one more negatively charged electrons than there are protons

so the atom will have an overall minus one charge. if instead it loses an electron

it will lose a negatively charged particle and there will now be one more

positively charged proton

than there are electrons so we will have an overall plus one charge

two particles with different numbers of protons are different elements

particles of a given element with differing numbers neutrons

are different isotopes and particles of a given element

with differing numbers of electrons are different ions

if we look at the periodic table all the elements are arranged by atomic number

one proton in the nucleus all the way to a hundred and more

then we can look at the atomic masses. but if the atomic mass

is the sum of the particles in the nucleus shouldn't they all be whole numbers?

there aren't any fractions of a neutron after all

the reason they have decimals is because they're average atomic masses

for all the isotopes of that element. remember that isotopes have differing

numbers of neutrons

the average has to reflect the relative abundance

of each isotope. for example chlorine can be chlorine 35

or chlorine 37. all chlorine atoms have 17 protons

but they can have either 18 or 20 neutrons

however about 75 percent of chlorine atoms are chlorine 35

and only 25 percent are chlorine 37 so the average is not 36

because there is much more chlorine 35 than 37

so we have to do some math. multiplying each mass number by a fraction of one

that represents

its abundance and then adding these up gives us a more accurate number for

the average mass

of all chlorine atoms. so on the periodic table

we have average atomic masses but any individual atom

will have its own mass number which must be a whole number

let's check comprehension

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