Atomic number, mass number, and isotopes (video) | Khan Academy
As you go from left to right in the same period (row) each new element adds one proton (its atomic number) and one electron. Because the protons are densely. Atoms are made up of particles called protons, neutrons, and electrons, which are Determine the relationship between the mass number of an atom, its atomic. For example, the atomic number of sodium is Every sodium atom has 11 protons and 11 electrons. It has 11 positive charges and 11 negative charges.
So let's talk about the next isotope of hydrogen. So this is called deuteriums. Let me go ahead and write deuterium here. Deuterium is hydrogen, so it must have one proton in the nucleus and it must have one electron outside the nucleus, but if you look at the definition for isotopes, atoms of a single element that differ in the number of neutrons, protium has zero neutrons in the nucleus.
So let me go ahead and draw in deuterium's one neutron. I'll use blue here, so neutrons are going to be blue.
So deuterium has one neutron and since neutrons have mass, deuterium has more mass than protium.
So isotopes have different masses because they differ in terms of number of neutrons. Notice though, that they have the same atomic number, they have the same number of protons in the nucleus. Right, it's one proton in the nucleus.
Relation Between Atomic Number and Atomic Mass
And that's important because if you change the number of protons, you're changing the element, and that's not what we're doing here. We're talking about atoms of a single element. Deuterium is still hydrogen, it's an isotope.
Finally, our last isotope, which is tritium. So tritium has one proton in the nucleus, one electron outside the nucleus, and we draw that in here, and it must differ in terms of number of neutrons, so tritium has two neutrons.
Let me go ahead and draw the two neutrons here in the nucleus. And so those are the isotopes of hydrogen. How do we distinguish between the different isotopes? Well we're going to write little symbols to represent these isotopes. And so the symbol that we'll draw here for protium is going to have the element symbol, which is, of course, hydrogen, and then down here we're going to write the atomic number. So the subscript is the atomic number which is one, because there's one proton in the nucleus, and then for the superscript, we're going to write in the mass number.
So let me move down here so we can look at the definition for the mass number.
The mass number is the combined number of protons and neutrons in a nucleus, so it's protons and neutrons, and it's symbolized by A. So A is the mass number, which is equal to the number of protons, that's the atomic number which we symbolized by Z, plus the number of neutrons. So A is equal to Z plus N.How To Calculate The Number of Protons, Neutrons, and Electrons - Chemistry
And for protium, let's look at protium here. So in the nucleus there's only one proton and zero neutrons, so one plus zero gives us a mass number of one. And I'll use red here for mass number so we can distinguish. Alright, so mass number is red and let me use a different color here for the atomic number. Let me use magenta here. So the subscript is the atomic number and that's Z, and the superscript is the mass number and that's A.
So this symbol represents the protium isotope. Let's draw one for deuterium. So it's hydrogen so we put an H here. There is still one proton in the nucleus, right one proton in the nucleus, so we put an atomic number of one.
The mass number is the superscript, it's the combined number of protons and neutrons. So we look in the nucleus here. There's one proton and one neutron. So one plus one is equal to two. So we put a two here for the superscript. And finally for tritium, it's still hydrogen.
So we put hydrogen here. There's one proton in the nucleus, atomic number of one, so we put a one here. And then the combined numbers of protons and neutrons, that would be three. So one proton plus two neutrons gives us three.
So there's the symbol for tritium. So here are the isotopes of hydrogen and using these symbols allows us to differentiate between them. So let's take what we've learned and do a few more practice problems here. So let's look at a symbol for carbon. So here we have carbon with subscript six, superscript And if we want to know how many protons, electrons and neutrons there are. So let's first think about protons.
Well we know that the subscript is the atomic number and the atomic number is equal to the number of protons. So there are six protons in this atom of carbon.
Atomic number - Wikipedia
And if it's a neutral atom of carbon, the number of electrons must be equal to the number of protons. So if there are six protons, there must also be six electrons. And finally, how do we figure out the number of neutrons? Loosely speaking, the existence or construction of a periodic table of elements creates an ordering of the elements, and so they can be numbered in order.
Dmitri Mendeleev claimed that he arranged his first periodic tables first published on March 6th, in order of atomic weight "Atomgewicht". A simple numbering based on periodic table position was never entirely satisfactory, however.
Besides the case of iodine and tellurium, later several other pairs of elements such as argon and potassium, cobalt and nickel were known to have nearly identical or reversed atomic weights, thus requiring their placement in the periodic table to be determined by their chemical properties. However the gradual identification of more and more chemically similar lanthanide elements, whose atomic number was not obvious, led to inconsistency and uncertainty in the periodic numbering of elements at least from lutetium element 71 onward hafnium was not known at this time.
Niels Bohrcreator of the Bohr model. The Rutherford-Bohr model and van den Broek[ edit ] InErnest Rutherford gave a model of the atom in which a central core held most of the atom's mass and a positive charge which, in units of the electron's charge, was to be approximately equal to half of the atom's atomic weight, expressed in numbers of hydrogen atoms.
This proved eventually to be the case. Moseley's experiment[ edit ] Henry Moseley in his lab. The experimental position improved dramatically after research by Henry Moseley in This led to the conclusion Moseley's law that the atomic number does closely correspond with an offset of one unit for K-lines, in Moseley's work to the calculated electric charge of the nucleus, i. Among other things, Moseley demonstrated that the lanthanide series from lanthanum to lutetium inclusive must have 15 members—no fewer and no more—which was far from obvious from the chemistry at that time.
The proton and the idea of nuclear electrons[ edit ] In the reason for nuclear charge being quantized in units of Z, which were now recognized to be the same as the element number, was not understood. An old idea called Prout's hypothesis had postulated that the elements were all made of residues or "protyles" of the lightest element hydrogen, which in the Bohr-Rutherford model had a single electron and a nuclear charge of one.
- Atomic number
- Atomic number, mass number, and isotopes
If Prout's hypothesis were true, something had to be neutralizing some of the charge of the hydrogen nuclei present in the nuclei of heavier atoms. In Rutherford succeeded in generating hydrogen nuclei from a nuclear reaction between alpha particles and nitrogen gas,  and believed he had proven Prout's law.