History of the Atom

what did James Chadwick discover?

he discovered the existence of neutrons

what was James Chadwick’s model of an atom look like?

• it had a nucleus containing protons and neutrons

• the nucleus was surrounded by electrons —> which occupied shells

• (similar to modern day model of atom)

explain how Chadwick’s experiments on the atom led to a better understanding of isotopes?

• he discovered neutrons

• which meant that it explained why atoms of the same element had the same atomic number (same no.protons) —> but had different mass numbers (different no.neutrons)

• therefore created a better understanding of isotopes

what did John Dalton believe?

• he believed that atoms were solid spheres

• and that different spheres made up different elements

what was John Dalton’s model? and describe it?

the ‘Billiard Ball’ model

• solid ball (sphere)

what did JJ Thompson discover?

• he concluded that atoms weren’t solid spheres (disproving John Dalton’s model)

• he discovered the existence of electrons

what was JJ Thompson’s model? and describe it?

the ‘plum pudding’ model

• positively charged ball with negative charges placed in it

what did Ernest Rutherford discover?

he discovered that most of the atom is empty space and positive charges in the atom must be concentrated in the centre

what experiment did Ernest Rutherford carry out to discover what he discovered?

he fired positively charged alpha particles at a thin sheet of gold —> it was called the Gold Foil experiment

what was observed by the Gold Foil experiment?

• most alpha particles passed straight through the foil

• some alpha particles were deflected

• this meant that the ‘plum pudding’ model couldn’t have been right

what was Ernest Rutherford’s model? and describe it?

the ‘nuclear’ model

• tiny positively charged nucleus in the centre (where most of the mass is concentrated)

• and ‘cloud’ of negative electrons surrounding the nucleus

what did Niels Bohr discover?

• he discovered that a ‘cloud’ of electrons around the nucleus would be attracted to the nucleus causing the atom to collapse

  • and so he suggested that all the electrons were contained in shells

(he took Rutherford’s model and improved it bascially)

what was Niels Bohr’s model? and describe it?

Bohr’s nuclear model

• electrons orbit nucleus in shells

• electrons are at specific distances from the nucleus

what are the similarities between the ‘plum pudding’ model and the ‘nuclear’ model?

• both have positive charges

• both have negative charges

• both have no neutrons (not yet discovered)

what are the differences between the ‘plum pudding’ model and the ‘nuclear’ model?