ATOMIC STRUCTURE AND THE PERIODIC TABLE (The Electrons (They move around…
ATOMIC STRUCTURE AND THE PERIODIC TABLE
It's in the middle of the nucleus
It contains protons and neutrons
The nucleus has an overall positive charge because protons are positively charged and neutrons have no charge
Almost the whole mass of the atom is concentrated in the nucleus
But size-wise it's tiny compared to the atom as a whole.
They move around the nucleus
They have a negative charge
They're tiny compared to the nucleus
The size of their orbits determines how big the atom is
They virtually have no mass
They occupy shells around the nucleus
Neutral atoms have no charge overall
The charge on the electrons is the same size as the charge on the protons, but opposite
This means the number of protons always equals the number of electrons in an atom
The number of neutrons isn't fixed but is usually about the same as the number of protons
It's the number of protons in an atom that decides what element it is.
Isotopes are different atomic forms of the same element, which have the same number of protons but different number of neutrons.
Isotopes must have the same number if protons but different mass numbers.
If they had different number of protons, they'd be different elements together
In 1869, a Russian scientist called Dmitri Mendeleev arranged 50 or so known elements according to their properties and the properties of their compounds to make a Table of Elements.
Mendeleev's table placed elements with similar chemical properties in the same vertical groups - but he found that he had to leave gaps in his table to make this work.
Mendeleev's Table Predicted New Elements
The gaps in Mendeleev's table of elements were really clever because they predicted the properties of undiscovered elements.
Since then new elements have been found which into the gaps left in Mendeleev's table
In fact, a whole new group of elements have been discovered and added to the table. This group includes helium, neon and argon. These are colourless gases that react with much at all.
Over the last hundred years or so the table has been refined to produce the periodic table.