Answer to Question #254501 in General Chemistry for shamsa

Question

1. After the discovery of protons and electrons, it was noticed that there was trend in the periodic table. Helium weighs ~4 and is atomic number 2, Carbon weights ~12 and has atomic number 6. Heavier elements can weigh more than double the atomic number (i.e., gold: atomic mass = 197, atomic number = 79, while for hydrogen atomic mass = 1, atomic number = 1.) How can we best explain this deviation in the trend?

Answer 1

So, when the atomic number increases, the mass number of the isotopes is higher (though they are different isotopes), and so the relative atomic mass will increase. Since this occurs as you go down a group, the atomic mass increases as you go down a group.