Measure and to classify electrons and atom as 
either being balance or quarrel act rounds that 
before electrons are the ones closest to the 
nucleus the valence electrons are the ones that 
are on the outside
that are going to be especially important when we 
talk about bonding
so the rule is we're gonna look adjust elements in 
the S and the P blocks of the periodic table
if they you have an omen as block the valence 
electrons of the outermost acts and in the P block 
its the outermost pass in the outermost P
so for example with carbon
and we have two electrons in the one as we own 
count those the we do count the two electrons in 
the two acts and we count the two electrons in the 
two piece of carbon has a total of
four valence electrons
a shortcut instead of writing of the orbital diagram
is just look of the periodic table and find the group 
number
carbon using group number 14 the last digit gives 
you the number of valence electrons
so similarly florine is found in group 17 florine 
atoms will have seven valence electrons
and potassium and group number one has one 
valence electrons
and so on a compare the sizes of the atoms
a magnesium atom has a radius of about 160 
people meters and a barium atom is much larger 
222 people meters
magnesium and barium are both found in group 
number two the periodic table
the outermost electrons for magnesium are no 
three S orbital
and for barium the electrons are found in a 
success orbital
both orbital source you recall
that the success is much larger than the three S
as the quantum number and increases the size
of the orbital increases
this is a comparison between two elements in the 
same column but if you look at two elements in the 
same row
for example so what can and chlorine
silicon 117 people meters is larger than chlorine 99 
people meters
and at first glance this trend looks the opposite of 
the previous trend that we're looking at
silicon and chlorine are both found
their last electrons are in the
three P sub shell of the periodic table solich and 
has
two electrons in the three P
and chlorine has five electrons
in that three P sub show
the difference between silicon and chlorine
both of these atoms have their left ron's there are 
a must electrons in the three P orbital
so we can't say that 13 P
is larger than the other three P just on the basis of 
what orbital it is instead will we have to do is look at 
the effective nuclear charge
a major difference between silicon and chlorine is 
how many protons they have the atomic number of 
solich and is 14
where the atomic number of chlorine is 17
so if you picture a nucleus in the middle of this 
three P orbital
there are 14 positive charges for silicon and it has 
14 electrons if are looking at the last electron
all the other electronics are blocking some of the 
positive charge of that
nucleus
compare that to chlorine the amount of blocking is 
about the same the blocking is called shielding
so the other electrons for example the three S 
electrons the two us electronegativity this electrons
all of those electrons are blocking some of the 
positive charge is
so then make the reason that chlorine a smaller is 
it has greater number of positive charges which is 
going to increase the effective
nuclear charge what the last electron with the 
outermost electrons feel when they are signs that 
looking at the nucleus 17 positive charges are 
more attractive and they can pole on those 
electrons more closely
than 14 positive charge its
if you put both of these together
you can have a general trend of atomic sides so 
this rectangle is showing the generic periodic table
size of the atoms in general increases as you 
move down a column and the size increases as 
you move from right to left
if you left to right that affected nuclear charge the 
effective increases and that's why atoms on the 
right side in a column are smaller than the atoms 
on the left side
