Atomic number, mass number and isotopes

Almost every carbon atom in your body is carbon-twelve. About one in a trillion is carbon-fourteen - and that tiny trace is how a museum can tell how old a bone is. The two kinds sit in the same box of the periodic table. They burn the same, react the same and make the same compounds. And yet one is heavier than the other.

This is video four of seven in Atomic structure. It builds on Protons, neutrons and electrons, so start there if the three particles are hazy.

The atomic number is the number of protons in an atom. It decides which element you have. Change the number of protons, and you have a different element. The mass number is the number of protons plus the number of neutrons. Those are the two heavy particles, so the mass number counts everything in the nucleus. Both numbers are written with the symbol. The mass number goes on top, and the atomic number underneath, both to the left of the symbol. The bottom number is never bigger than the top one. To find the neutrons, take the atomic number away from the mass number. Write it as a subtraction every time: mass number, take away atomic number, equals neutrons. And in a neutral atom, the electrons equal the protons, because their charges have to cancel. Here is magnesium, mass number twenty-five, atomic number twelve. The bottom number gives twelve protons. The neutrons are twenty-five take away twelve, which is thirteen. The atom is neutral, so it has twelve electrons to match its twelve protons. The periodic table you are given shows two numbers in each box. The smaller whole number is the atomic number. The other is often not a whole number - and the video on relative atomic mass explains why. Now one for you. Lithium has atomic number three and mass number seven. Which is true: seven protons, three neutrons, four neutrons, or seven electrons? Four neutrons: seven take away three. Seven protons and seven electrons both read the top figure as if it were the bottom one. Three neutrons copies the bottom figure instead of subtracting. An ion is an atom that has lost or gained electrons, so its charges no longer cancel. The nucleus stays exactly as it was. Now, take the magnesium ion, written M g two plus. Its nucleus is the same one as before. How many electrons does the magnesium two plus ion have? Ten. Two plus means two electrons have gone, and twelve take away two is ten. The twelve protons and thirteen neutrons do not change.

Now, back to the two kinds of carbon. Isotopes are atoms of the same element that have equal numbers of protons but different numbers of neutrons. Isotopes of one element have the same electrons, arranged the same way, so they react in the same way. Only the mass, then, is different. The definition has two boxes to tick. The protons are the same. The neutrons are different. And they are always atoms of one element, never elements. Electrons stay out of the definition on purpose. Change the electrons, and you have made an ion, which is a different idea altogether. Here are two particles to test against the checklist. Seventeen protons with eighteen neutrons, and with twenty: isotopes? Write why. Yes. They are atoms of the same element, because both have seventeen protons, with different numbers of neutrons. Here are three answers a student might write instead. Isotopes are elements with the same number of protons. An isotope is an atom with more neutrons than protons. Isotopes have the same protons but different electrons. Each of those three breaks the checklist in its own way. Which part of the checklist does each answer fail? The first says elements where it means atoms. The second compares the neutrons and protons inside one atom, when isotopes are about two atoms. The third changes the electrons, and that describes an ion. One examiner's report puts it plainly. The usual definition, "atoms with the same number of protons but a different number of neutrons", "was not well known". It also says "Electrons are not part of the definition of an isotope; their inclusion can lead to this type of confusion and ambiguous answers." Learn the sentence word for word, and write it as it stands: atoms of the same element, same number of protons, different number of neutrons. Then keep it as a pair with the ion. An isotope changes the neutrons and the mass. An ion changes the electrons and the charge. Both ideas come back to the two numbers by the symbol, which gives this video's handle: the bottom number names it, the top number weighs it.

Right, time to test it - and the last question goes back to the carbon in your body. Oxygen-eighteen has atomic number eight. How many neutrons does it have? Eighteen take away eight leaves ten neutrons. Another sort: sulfur two minus has sixteen protons. How many electrons? Eighteen. Two minus means two extra electrons, so sixteen and two more makes eighteen. Now carbon-fourteen, atomic number six: how many neutrons, and why is it still carbon? Eight neutrons, fourteen take away six. It is still carbon because it has six protons. Only the neutrons, and so the mass, set it apart from the carbon-twelve in the rest of your body.

When you can read any symbol, with or without a charge, and give all three counts straight off, you own this one, so give it the thumb. If the isotope sentence still slips, keep the video; every video left in this chapter leans on these two numbers, so they will come round again.

Next in the chapter: Relative atomic mass from isotopic abundances.

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