atomic radius

追加: 2026-05-12

[00:00:02]hey guys so i do want to take a couple minutes um and talk about our first periodic trend that we're going to focus on this unit um there are other trends but we're going to wait until next unit to tackle those because this trend strictly deals with where the electrons are so trends are if something is going to increase or decrease and things like that are overall trends our trend we're going to focus on today is looking at atomic radius and size and then we'll also look at ionic radius and see what that does um so atomic radius is equal to one half the distance between the nuclei of two atoms of the same element when atoms are joined so think of like radius and math if you have a circle it's half the distance of the circle this is the exact same as with atomic radius the only difference is atoms because they the electrons can move around that electron cloud does not have a nice defined edge like a circle does so that's why we say it's halfway the distance between two nuclei because that's where the edge would be if there's a nice defined edge the general trend for atomic size is the atomic size increases from top to bottom in a group so going down a group it gets larger so imagine it almost like a wedge where it's larger at the bottom and smaller at the top so if we were comparing um like hydrogen and lithium number one and number three lithium is lower than hydrogen in the same column in the same group so lithium would be larger lithium would be bigger the other part of our trend is that the atomic size is going to decrease from left to right across the period so the left side is larger and then it gets smaller as it goes towards the right so like helium is actually going to be smaller than hydrogen because it is to the right of hydrogen what causes this trend so going down a group so the increasing going down the number of energy levels increases so we went from like going down the first column we went from like 1s to 2s to 3s to 4s so we added an energy level and each energy level does get larger so it starts moving away from the nucleus the electrons in the outermost energy level are shielded by electrons closer to the nucleus from the pull of the protons so because those outer electrons are truly outside all those inner electrons the inner electrons are blocking the positive charge from the protons from reaching those outer electrons and as they get further and further away there are more electrons in front of them blocking them so it's harder for the protons to pull on those electrons and try to get them closer to the nucleus whereas electrons in those lower energy levels that are close to the nucleus the nucleus is going to be able to pull on them easier um so think of it like if you had two oppositely charged magnets when they're close together you can feel that pull between the two magnets trying to connect them and pull them together but if you hold the two magnets further apart there may still be an attraction but you're not feeling that pull from one magnet to another when they're further apart where is going across a period so whether you're going across period the number of energy levels does not change so say the second period of the periodic table that second row where you're going from 2s to 2p you're still in the second energy level that has not changed so since you're still in the same energy level there's no increase of shielding so there's no additional inner electrons to block the pull of the protons from the nucleus and as we go across from left to right there are actually more protons in the nucleus as you go across so the nucleus is stronger therefore it actually pulls those electrons in tighter because the nucleus has a higher charge with no increase of shielding so this would be kind of like our magnets that we had with the first example where we're holding them close and we can feel the pull between them well imagine if the magnet representing our nucleus was larger and stronger we would feel that pull even more from one magnet to another and how questions like this may come up is it may simply give you some atoms and want to know which one is larger which one's smaller or ask you to rank them and put them in order it's like if we wanted to order um let's do calcium silicon and fluorine or let's change one let's do calcium silicon and let's do cobalt co and sorry i'm having to write with my track pad so if we wanted to put these by increasing size and rank them so these are the ones that tend to trick people so remember if you want to rank them by increasing size you want to start with the smallest so the smallest elements are going to be towards the top and right of our periodic table so out of these three the one that is closest to the top right is silicon that is our smallest the one out of the following two that are closest to the top right is cobalt and then our biggest is towards that bottom left-hand corner which out of the three is calcium so calcium will be the biggest of these three in comparison we're also going to look at ionic radius so we said ions are charged particles that are formed whenever a neutral atom either gains or loses electrons so we end up with cations or anions are two types of ions we come across cations are going to be our positively charged items or ions excuse me these ions lose electrons to have that positive charge because that means there's more protons than electrons um our cations are going to be our metals tend to form cations and since electrons are lost that means that there's more protons than electrons so therefore there's more positive charges pulling on the negative charges and it's able to pull them closer to the nucleus another way to think of this is with like weight if you lose weight you get smaller therefore cations lose electrons and they get smaller as ions than their original element so we might have like lithium and the lithium ion so lithium by itself has three protons and three electrons well whenever it becomes an ion and it will have a positive one charge that means it loses one electron therefore there are three protons pulling on two electrons so they can pull on them tighter making the cation smaller so the lithium atom is larger than the lithium ion the cation gets smaller whereas anions these are negatively charged ions these are negative because they gain electrons and these are going to be our non-metals they're in that top right hand corner and since electrons are gained we'll have more electrons than protons so the protons are not able to hold on to more electrons as tightly as it does its normal number of electrons um so think of if you gain weight you get bigger or if you've ever babysat the more kids you have the harder they are to control because you're outnumbered it gets harder than if it was a one-on-one situation works the exact same way so an anion gains electrons and ends up getting bigger because that outer layer of electrons can spread out more because it does not feel the pro pull of the protons as much so we might have something like let's do fluorine and our fluoride ion which is just going to have a negative 1 charge so fluorine has 9 protons 9 electrons whereas the fluoride ion has 10 electrons and 9 protons so the protons cannot hold as tightly so that fluoride ion actually gets bigger that outer layer is going to expand we're going to deal with this even more next unit but this does deal with filling our layers of electrons things become ions to become stable but it does change their overall size