A deep dive into the building blocks of matter and the history of atomic discovery.
Key Concepts:
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Which of the following statements from John Dalton’s original atomic theory was later proven incorrect by the discovery of isotopes?
All atoms of a given element are identical in mass and properties.
Dalton postulated that all atoms of a particular element are identical in mass. The existence of isotopes—atoms of the same element with different masses (different numbers of neutrons)—proved this specific postulate wrong.
In Rutherford's gold foil experiment, most alpha particles passed directly through the foil with no deflection. What did this observation suggest about the structure of the atom?
The atom consists mostly of empty space.
Since the vast majority of alpha particles passed through undeflected, Rutherford concluded that the atom is composed mostly of empty space, with the mass concentrated in a tiny nucleus.
Which scientist proposed the "plum pudding" model, suggesting that the atom was a positively charged sphere with negatively charged electrons embedded within it?
J.J. Thomson
J.J. Thomson proposed the plum pudding model after discovering the electron, conceptualizing the atom as a positive mass with electrons embedded like plums in a pudding.
A chemist analyzes two compounds composed of carbon and oxygen. Compound A contains 1.33 g of oxygen per gram of carbon. Compound B contains 2.66 g of oxygen per gram of carbon. This observation best illustrates which law?
Law of Multiple Proportions
The Law of Multiple Proportions states that when two elements form more than one compound, the ratios of the masses of the second element that combine with a fixed mass of the first element can be reduced to small whole numbers (in this case, 2.66:1.33 is a 2:1 ratio).
Which subatomic particle has a charge of zero and a mass of approximately 1 amu?
Neutron
Neutrons are neutral particles (charge of 0) located in the nucleus and have a mass of approximately 1 atomic mass unit (amu).
Where are electrons located in the modern model of the atom?
In a cloud or region of space surrounding the nucleus
Electrons orbit the nucleus in a diffuse cloud or specific orbitals, occupying the vast majority of the atom's volume but very little of its mass.
Which statement correctly compares the relative masses of protons and electrons?
A proton is approximately 1800 times more massive than an electron.
Protons and neutrons have a mass of ~1 amu, while electrons have a mass of ~1/1836 amu. Therefore, a proton is roughly 1800 times heavier than an electron.
In the Rutherford gold foil experiment, what caused the deflection of the small percentage of alpha particles?
Repulsion by the dense, positively charged nucleus
Alpha particles are positively charged. When they approached the small, dense, positively charged nucleus of the gold atoms, the strong electrostatic repulsion caused them to deflect.
Which subatomic particle identifies an atom as a specific element?
Proton
The number of protons (atomic number) defines the identity of an element. Changing the number of protons changes the element.
An atom of Carbon-14 (14C) contains how many neutrons? (Atomic number of Carbon is 6).
8
The number of neutrons is calculated by A−Z, where (A) is the mass number (14) and (Z) is the atomic number (6). 14−6=8 neutrons.
