Why does atom's attraction to electron increase in periods ?

[KingCrimson]

Why does the nucleus' attraction to the outermost electron increase across a period?
I have been told that this is because each time you go across a period one step to the right, you add one proton and so there is more positive charge, but isn't it true that you also add one electron ? and so the positive charge even though it has increased is divided between more electrons and so the attraction of electrons by the nucleus stays the same ?
This could only mean that the positive charge's attraction to each electron is not divided, but this defies what my a level book says.
thanks in advance

 

[Bandersnatch]

You're correct, it is not divided.

The electrostatic force a single charge like an electron "feels" from the nucleus increases with the number of protons, and it doesn't decrease as you add more electrons in the same way as the force of Earth's gravity on a single person doesn't decrease as you add more people.

but this defies what my a level book says.

There's probably some misunderstanding there. Can you quote what your book says?

 

[KingCrimson]

Page 75 Edexcel AS chemistry bob fullick and ann mcduel
"A positive ion always has a smaller ionic radius than the original atom, This is because the loss of electron/s means that the remaining electrons each have a greater (share) of the positive charge of the nucleus so are more tightly bound"
they say Share as if the force is divided between electrons.

 

[Bandersnatch]

This looks like a blunder to me, but you better wait for other members to voice their opinions.

 

[Bystander]

Are you discussing atoms, ions, ionization potential, or electron affinity?

 

[Nugatory]

Page 75 Edexcel AS chemistry bob fullick and ann mcduel
"A positive ion always has a smaller ionic radius than the original atom, This is because the loss of electron/s means that the remaining electrons each have a greater (share) of the positive charge of the nucleus so are more tightly bound"
they say Share as if the force is divided between electrons.

That's a completely awful wrong terrible somewhat misleading explanation, but it is true that (all else being the same) a positive ion has a smaller diameter than the corresponding neutral atom. The force between the electrons is repulsive, so tends to force the electrons away from one another and outwards. When we remove one electron this force becomes smaller while the attraction to the nucleus remains the same, so the remaining electrons can settle in a bit closer.

 

[sophiecentaur]

That's a completely awful wrong terrible somewhat misleading explanation, but it is true that (all else being the same) a positive ion has a smaller diameter than the corresponding neutral atom. The force between the electrons is repulsive, so tends to force the electrons away from one another and outwards. When we remove one electron this force becomes smaller while the attraction to the nucleus remains the same, so the remaining electrons can settle in a bit closer.

That's a much better way of putting it. It's due to the net charge (i.e. a difference) and not the "sharing" which is a ratio effect.