4.1 - Development of the Model of the Atom Flashcards
(19 cards)
When Democritus first conceived of atomic theory, around 500 BC, how did he describe atoms?
The smallest possible unit of matter
Small spheres
Separated from each by empty space
In the 1800’s John Dalton described atoms as solid
___________ , and suggested that different types of spheres make up the different
__________ .
spheres
elements
In 1897 J J Thomson theorised that an atom consists of _____________________
What do we call this model?
a ball of positive charge, with negative electrons mixed throughout it.
Plum pudding model
How Rutherford developed the nuclear model ( 4 steps)
In Rutherford’s experiments, alpha particles were fired at a thin sheet of gold foil.
Most particles passed through, but some were deflected off course.
This caused him to hypothesise that there was a dense region of positive charge at the centre of the atom that repelled the alpha particles.
As a result he developed the nuclear model of the atom, in which there was a central positive nucleus, surrounded by negative electrons.
One issue with Rutherford’s nuclear model was that _________________
In response to this, in 1913 Bohr suggested that electrons ___________________, which kept the atom from collapsing.
atom should collapse as the negative electrons would be attracted to the positive nucleus, causing them to rush inwards.
orbit the nucleus in shells
In the 20th century, Chadwick discovered neutral particles in the atomic nucleus. What are these particles called?
Neutrons
What was Dalton’s atomic model called?
The Solid Sphere Model
What did John Dalton believe about atoms?
That atoms were tiny, solid, indivisible spheres, and each element had its own type
Who discovered the electron?
J.J. Thomson
What was J.J. Thomson’s model called?
The Plum Pudding Model
Describe the Plum Pudding Model.
A positively charged sphere with negatively charged electrons scattered through it
What experiment did Rutherford do?
The Gold Foil Experiment
What did Rutherford discover from his experiment ?
Alpha particles went off course because they were pushed away by the positively charged nucleus.
proving that the atom has a small, dense, positively charged nucleus; most of the atom is empty space
What was the rutherfords experiment
He shot alpha particles (positively charged) at a thin sheet of gold foil.
Expected: All particles should go straight through (based on the Plum Pudding Model).
Actual result:
Most went through ➡️ the atom is mostly empty space
Some were deflected ➡️ there’s a small, dense, positive center
A few bounced back ➡️ the center must be massive and positively charged
Q: What are the similarities between J.J. Thomson’s plum pudding model and Rutherford’s gold foil experiment?
Both models recognized that atoms contain electrons (negative particles).
Both tried to explain how positive and negative charges are arranged inside atoms.
Q: What are the differences between J.J. Thomson’s plum pudding model and Rutherford’s gold foil experiment?
Thomson’s model couldn’t explain alpha particle deflections; Rutherford’s gold foil experiment explained it by the nucleus causing repulsion.
Nuclear model was developed in the 20th century other was 19th
N —> considers protons and neutrons as separate entities within the nucleus
P –> Does not differentiate between protons and neutrons, considers them as part of the same structure
Who was Niels Bohr and what did he propose
A scientist who proposed that electrons orbit the nucleus in fixed shells or energy levels
What did Bohr say about electron energy levels?
Electrons can only exist in certain allowed shells and have specific energy
What does bohrs model say
Bohr’s model says that electrons orbit the nucleus in fixed energy levels called shells.
Electrons can only exist in these shells and not between them.
When an electron absorbs energy, it jumps to a higher shell and becomes excited.
When it falls back to a lower shell, it gives off energy as light. This explains why atoms emit light at specific colors.
