Download Atomic Structure Protons, neutrons and electrons

Atomic Structure Protons, neutrons and electrons Many millions of tiny little atoms make up all elements and therefore compounds. They are so small it’s very hard to imagine. Even smaller again, the atom is made from the sub atomic particles: protons, neutrons and electrons. Different elements simply have different numbers of protons, neutrons or electrons. The table below shows a comparison between these particles. The word “relative” is there as we are directly comparing the charge and mass of just these three particles, so the values are relative to each other. Particle Relative Charge Relative Mass Proton +1 1 Neutron Neutral (or zero) 1 Electron -­‐1 1/1836 Without looking too much into the exact weights, you need to know that electrons are very small in comparison to protons and neutrons. Just for fun, an electron roughly weighs 6.63 x 10-­‐34 kg! What the atom looks like The atom is very simple, it consists of a positively charged nucleus with negatively charged electrons whizzing all around it. The atom is held together by the electrostatic attraction between these positive and negative charges. The nucleus is made up from the protons and neutrons. Most of the mass is in the nucleus as the electrons are so light in comparison. Electrons don’t just fly around any old style, they do have a bit of structure, with some electrons closer to the nucleus than others. We will look at this in more detail in the electronic structure tutorial. Symbols The periodic table is your friend. There is a lot of very useful information you can get from using it properly. In terms of this atomic stuff, we are using the periodic table to look for the numbers of protons, neutrons and electrons in different elements. Generally, if you pick an element, for example, sodium, it will say: Mass Number Na Atomic Number 23 Na 11 From this information you can work out the number of protons, neutrons and electrons. The mass number = the number of protons plus the number of neutrons. The atomic number = the number of protons (or the number of electrons) Immediately we can say that: Number of protons = 11 Number of electrons = 11 If we subtract the atomic number from the mass number we get the number of neutrons, therefore: Number of neutrons = 23-­‐11 = 12 Note: Remember that for an element or a compound, the number of protons and the number of electrons must equal each other as the overall charge is 0 or neutral. The neutrons are not charged so you can have a different number of those. Ions The only time the number of protons and electrons will differ is when we are looking at ions i.e. an ion has a negative or positive charge. A positive ion will have more protons than electrons and a negative ion will have more electrons than protons. For example, the magnesium ion, Mg2+, to work out the number of protons, neutrons and electrons for the ion, we firstly start at the periodic table again. 24 Mg 12 We can instantly see that Mg has 12 protons (and therefore 12 electrons). If we do 24-­‐12 then we get the number of neutrons, which is also 12. So for Mg2+, we need to lose 2 electrons to get the 2+ charge, so we can say that the ion has 12 protons and neutrons but only 10 electrons. Tip: some students don’t understand what a ‘+’ charge means on an ion. A positive charge tells you that you have lost electrons and how many. The misconception is that a ‘+’ means additional electrons. You have to look at it as an increase in positive charge or a loss of negative charge. Isotopes Another variation on all this number of protons, neutrons and electrons stuff is isotopes. An isotope is atoms of the same element that has a different number of neutrons (the number of protons and electrons don’t change). Below are the two isotopes of chlorine. We can see that there are 17 protons in each and therefore 17 electrons. We can also see that the mass number in the first example is 35 and 37 in the second example. This means that the number of protons must be different. We have 35-­‐17 = 18 neutrons and 37-­‐17 = 20 neutrons In the periodic table you will find that chlorine has a mass number of 35.5, which is an average of its two isotopes. Chemical reactivity is dependent on the electrons, so if isotopes have the same number of electrons, then they will have the same reactivity and properties.