1b.1 History of the atom

Question 1

Name four models of the atom, on chronological order of description.

Answer 1

1. John Dalton’s billiard ball model (early 1800s)
2. J J Thompson’s plum pudding model (1897)
3. Ernest Rutherford’s nuclear model (1909)
4. Niels Bohr’snuclear model (1913)

Question 2

Describe John Dalton’s model of the atom

Answer 2

• billiard ball model (early 1800s)
• atoms are solid spheres
• different spheres made up different elements

Question 3

Describe J J Thompson’s model of the atom

Answer 3

• plum pudding model (1897)
• identified electrons as smaller negatively charged particles within the atom
• atom is a ball of positive charge with electrons stuck in it

Question 4

Describe Rutherford’s model of the atom

Answer 4

• nuclear model (1909)
• discovered when firing positively-charged alpha particles (two protons + two neutrons) at a thin sheet of gold, and observing that some particles were deflected (this would not occur in a plum pudding model)
• hypothesised that the atom has a positively charged nucleus where most mass is concentrated, with a cloud of negative electrons, and most atom is empty space

Question 5

Describe Bohr’s model of the atom

Answer 5

• improved nuclear model (1913)
• suggested that a cloud of electrons would be attracted to the nucleus, so would collapse, so the previous model could not be correct
• electrons orbit nucleus in fixed shells with fixed energy (at fixed distances from the nucleus), and could not be found in between

Question 6

Who discovered neutrons?

Answer 6

James Chadwick (English physicist), discovered them in 1932