The 17 so when subtracting these numbers you

The Periodic
table helps us to figure out a lot of things about a certain element that may
be whether how reactive it is or ionising and how many electrons it can hold
and so on. When starting with the period table we see the elements in symbols
such as Na for sodium this makes it very easier for scientists and other people
write the elements without writing the full name of it. However the first thing
we do when we look at the periodic table is the mass and atomic number of that
element, the mass number is the protons and neutrons added together and the
atomic number is just the atoms inside the nucleus. When wanting to find out
the amount of neutrons all you do is take away the mass number from the atomic
number for example oxygen has a mass of 16 (the mass number will always be at
the top and always will be the highest number of the element) and it also has
an atomic number of 8 (this will always be the lowest number and will always be
at the bottom of the element) so all you would do is 16-8=8 to find out the
number of neutrons an element has. However some elements may have a different
number of neutrons these are called isotopes the main example is Chlorine it
has a mass number of 35 and an atomic number of 17 so when subtracting these
numbers you get 18 neutrons this element would be classed as an isotope. The
next thing you learn about the periodic table is the group number of the
elements there are only 1 to 7 groups including group 0 which are known as the
noble gases. Firstly, elements are arranged by their electronic structure a
shell can only hold 2, 8, 8 electrons. 
Elements such as Lithium and sodium are placed in group 1 because they
both occupy 1 electron in their outer shell and you can also predict how many
electrons they occupy in their highest energy level by looking at the group
number for example nitrogen is in group 5 so we can tell by looking at the
periodic table that is occupies 5 electrons in its shell. Group 1 also has an
effect on it called shielding this is when the electrons are further away from
the nucleus making them less attracted to them so the positive charge cannot
hold them in place, that’s why group 1 metals get more reactive as you go down
the group. Group 1 metals are also known as alkali metals so as you go down the
group they become more reactive because its outer electron is lost and also its
further away from the nucleus, they have low melting and boiling points and
also they have low density. These metals also form ionic compounds (when an
atom loses or gains an electron) with non-metals. They ‘re keen to lose their 1
electron in their outer shell so they lose it to become an ion and they produce
white compounds that dissolve in water to form colourless solutions. Group 7
elements are known as halogens they also have an effect of shielding they have
a much high energy level because all their outer shell is complete so this also
makes it difficult for the group 7 elements to be attracted to the nucleus so
that’s why they get less reactive when you go down the group. They have a high
melting and boiling point and they are less reactive as well they can also form
ionic bonds with metals for example they form 1- ions called
halides. The period is the horizontal group is the elements who have the same
amount of electronic shells but not electrons so in group 1 if we look
horizontally lithium only has 2 shells exactly like boron they don’t share any
similar properties except for the period group they are in.