As you may know, atoms room composed of three different varieties of particles: protons, neutrons, which are uncovered in the nucleus, and also electrons which are discovered in the electron cloud approximately the nucleus. Protons and also neutrons room responsible for most of the atom mass.
You are watching: What is the mass number of an element that has 19 protons, 19 electrons, and 20 neutrons?
The atomic mass for protons and also neutrons is the same. A proton has an atom mass of 1 u (unified atomic mass). The fixed of one electron is really small. Protons have a positive (+) charge, neutrons have actually no charge --they room neutral. Electrons have a negative charge (-). The variety of protons and electrons is the same in a neutral (uncharged) atom.
How deserve to you find out how many protons (= electrons) an element has? girlfriend look at the periodic table. In this table the facets are noted according to their atomic number. Hydrogen is the very first element with an atomic number of 1. Now, the atom number is additionally the number of protons in an element. it is always constant. (e.g., H=1, K=19, Ur=92). So if friend look for potassium (symbol K in the periodic table), friend will discover that it has actually the atomic variety of 19. This tells you the potassium has 19 protons and also - due to the fact that the number of protons is the same as the number of electrons- likewise 19 electrons.
To find out the number of neutrons you need to look in ~ the atomic mass or load of the element. This can additionally be uncovered in the regular table. It is the number under the element symbol. For potassium the is about 39. This method that the atomic load is 39 because that both protons and also neutrons. Because we know that the number of protons is 19 we have the right to calculate the number of neutron (39 19) as 20.
Things deserve to be a little bit more complicated however. The same facet may contain varying numbers of neutrons; facets that have actually the same variety of protons yet different number of neurons are dubbed isotopes. Oxygen, v atomic number of 8, because that example, can have 8, 9, or 10 neutrons. (The atomic number for oxygen is 8, and also the atom mass is 15.9994).
If you review the atomic weight for potassium it reads 39.098. This tells you the there must be some various other isotopes for potassium. In fact, natural potassium has actually three isotopes through 20, 21 and 22 neutrons v the diversity of 93.26 %, 0.01 % and 6.73 % respectively.
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