2A1 Atomic Structure Flashcards
Explain the current atomic model, the quantum mechanical model, and the experimental basis for both models.
List the three subatomic particles and their charges.
- Protons: positive charge
- Neutrons: no charge
- Electrons: negative charge
Protons and neutrons reside in the nucleus while electrons orbit around it.
Define:
atomic number
The number of protons in an atom’s nucleus.
It determines the element’s identity.
All atoms of a given element have the same atomic number.
Atomic number also equals the number of electrons in a neutral atom.
How is the atomic weight of an element calculated?
By considering the weighted average of all naturally occurring isotopes of the element.
This involves multiplying each isotope’s percent abundance by its mass number. The atomic weight of carbon is 12.001, which is the weighted average of isotopes carbon-12 and carbon-13.
What does the mass number of an element represent?
The total number of protons and neutrons in an atom’s nucleus.
What does a neutral atom imply about protons and electrons?
It has the same number of protons and electrons.
This balance results in no overall charge.
Define:
isotopes
Atoms of the same element with the same number of protons but different number of neutrons.
Isotopes have the same atomic number but different mass numbers.
Isotopes of an element have similar chemical properties but different physical properties. For example, uranium has three naturally occurring isotopes: Uranium-238, Uranium-235, and Uranium-234. Each varies in terms of the number of electrons in their nuclei.
What notation is used to represent isotopes?
A hyphen and a number following the elemental name.
It includes the element name followed by a hyphen and the mass number.
Example: Carbon-14 indicates carbon with a mass number of 14 (6 protons and 8 neutrons).
List three isotopes of hydrogen.
- Protium
- Deuterium
- Tritium
Protium has no neutrons, deuterium has one, and tritium has two.
How can you determine the number of neutrons from mass number?
Neutrons = Mass Number (A) - Atomic Number (Z)
This formula helps in identifying the number of neutrons in the nucleus.
How is the atomic number represented on the periodic table?
As a whole number next to the elemental symbol.
What is the mass number of an oxygen atom with 8 protons and 8 neutrons?
16
This can be represented as oxygen-16.
Define:
isobars
Atoms of different elements with the same atomic mass.
They differ in the number of protons and neutrons.
What is the purpose of atomic theory?
To describe the structure of matter as being composed of tiny particles called atoms.
List the main scientists involved in atomic theory.
- Democritus
- John Dalton
- J.J. Thomson
- Ernest Rutherford
- Niels Bohr
- Robert Millikan
- Erwin Schrodinger
Each of these scientists contributed significant elements of our modern atomic theory. With each discovery, the theory has evolved and grown into what it is today.
Define:
matter
Anything that has mass and takes up space.
All matter is made up of atoms of different elements.
Who proposed the idea that matter could be divided infinitely?
Aristotle
Turns out, he was wrong when it comes to matter! Eventually, the atomic theory would show that there are indivisible particles.
Who proposed the concept of atoms as indivisible particles?
Democritus
Democritus used the term “atomos,” meaning indivisible, to describe the smallest piece of matter. He was also incorrect, since atoms are divisible into smaller particles like neutrons and protons.
What was the name of John Dalton’s atomic model?
The solid sphere model
Dalton described atoms as indivisible, solid spheres that form elements and compounds.
List five postulates of Dalton’s atomic theory.
- Matter is composed of exceedingly small particles called atoms.
- An element consists of only one type of atom, which has a mass characteristic of the element and is the same for all atoms of that element.
- Atoms of one element differ in properties from atoms of all other elements.
- A compound consists of atoms of two or more elements combined in a small, whole-number ratio.
- Atoms are neither created nor destroyed during a chemical change.
These postulates were first published in 1807.
Dalton’s atomic theory provides a microscopic explanation of the many macroscopic properties of matter.
What discovery did J.J. Thomson make about the atom in 1897?
The existence of electrons.
Thomson’s experiments with a cathode ray tube led to the discovery of negatively charged electrons.
What experiment led to the discovery of the nucleus?
The gold foil experiment.
Rutherford observed that alpha particles were deflected, indicating a dense positively charged central nucleus.
Define:
Thomson’s plum pudding model.
It viewed the atom as a positively charged sphere with negatively charged electrons embedded throughout.
The model resembled a dessert with dispersed raisins, which helped to conceptually visualize the idea.
How did the gold foil experiment disprove the plum pudding model?
It showed that atoms have a concentrated nucleus rather than a uniform distribution of charge.
The deflection of alpha particles indicated a central dense mass.
Describe the Bohr model of the atom.
It shows an atom as a central nucleus containing protons and neutrons with the electrons in circular orbits around the nucleus at fixed energy levels.
It explains atomic spectra but is limited for complex atoms.
What is a key difference between the Bohr model and Rutherford’s model?
The Bohr model illustrated specific orbits for electrons, while Rutherford’s model suggested random positions.
This distinction is critical in understanding atomic structure.
What are energy levels in Bohr’s model?
Fixed paths where electrons orbit the nucleus.
Each energy level corresponds to a specific amount of energy and are often called shells.
Electrons in higher energy levels are farther from the nucleus.
What key concept did Bohr introduce to explain atomic spectra?
Electron transitions between fixed energy levels.
Bohr linked light emission to electron jumps between orbits.
Electrons absorb energy to move to higher levels and emit light when returning to lower levels producing distinct light spectra.
What is the significance of atomic spectra?
They reveal the energy levels of electrons in atoms.
The emission of light corresponds to electron transitions between levels.
How did the concept of atomic orbitals evolve from Bohr to Schrodinger?
Bohr proposed fixed orbits, while Erwin Schrodinger described orbitals as regions of probability.
Orbitals reflect quantum mechanical behavior of electrons.
Define:
atomic orbitals
Regions around the nucleus where electrons are likely to be found.
Orbitals are located at discrete distances from the nucleus.
Electrons closer to the nucleus have lower energy.
True or False:
Electrons can be precisely located in an atom.
False
The Heisenberg Uncertainty Principle states that exact positions and velocities cannot be simultaneously known. If we can estimate the position of the electron, we cannot estimate the velocity, and vice versa.
What is the significance of the Bohr model for hydrogen?
It explained the discrete spectral lines of hydrogen.
The model was a stepping stone to the quantum mechanical model.
The quantum mechanical model describes electrons as wave-like entities in probabilistic orbitals around the nucleus.
Describe Millikan’s contribution to atomic theory.
He measured the charge of the electron using his oil drop experiment.
Millikan balanced oil drops between electrical forces to calculate the electron’s charge.
The data from this experiment was later used to determine the mass of the electron.
How does the quantum mechanical model explain atomic behavior?
It uses mathematical equations to predict electron locations and behaviors.
The model accounts for wave-particle duality of electrons.
What is the difference between a ground state and an excited state?
- Ground state: electrons occupy the lowest energy levels.
- Excited state: one or more electrons occupy higher energy levels.
Excitation occurs when energy is absorbed.
True or False:
An electron in a higher energy level is more stable.
False
Electrons in lower energy levels are more stable due to proximity to the nucleus.
