Atomic Theory and Subatomic Particles: Key Concepts Explained
Key Concepts in Atomic Theory
Which of the following was one of Dalton’s improvements over Democritus’s ideas?
Atoms retain their identity in a chemical reaction.
Dalton’s Atomic Theory
Atoms of the same element are always identical.
Which discovery did J. J. Thomson make that improved upon Dalton’s atomic theory?
Atoms contain tiny, negatively charged electrons
Which of the following was originally a tenet of Dalton’s atomic theory, but had to be revised about a century ago?
Atoms are tiny indivisible particles.
The range in size of most atomic radii
5 
10
m to 2
10
m
Why did J. J. Thomson reason that electrons must be a part of the atoms of all elements?
Charge-to-mass ratio of electrons was the same, regardless of the gas used.
Which of the following is true about subatomic particles?
The mass of a neutron nearly equals the mass of a proton.
Which of the following is correct concerning subatomic particles?
The neutron was discovered by Chadwick in 1932.
Explain how Dalton improved upon atomic theory more than 2000 years after Democritus’s hypotheses about atoms.
John Dalton used experimental methods to hypothesize that atoms of the same element are identical, and that they differ from atoms of other elements. Dalton also noted that atoms of different elements can form compounds in which the elements combine in whole-number ratios. He noted that individual atoms are not transformed into different atoms as a result of a chemical reaction. What Dalton did not contribute was the fact that individual atoms are divisible into subatomic particles, which came from the discovery of the electron and the atomic nucleus by Thomson and Rutherford, respectively.
Melting point of water = 0
water in 4 degrees c hs ability to decompose to hydrogen and oxygen
Explain how electron microscopy and scanning tunneling microscopes have contributed to the understanding of atoms.
Using an electron beam as the source radiation to image objects greatly enhances the magnifications that can be achieved. Electron microscope magnifications are typically 100,000
. Such magnifications allow scientists to routinely image groupings of atoms. Scanning tunneling microscopy (STM) also enables researchers to generate images of groups of atoms.
AMU= PERCENTAGE X MASS AND THEN ADD ALL RESULTS
What observations by Rutherford led to the hypothesis that atoms are mostly empty space, and that almost all of the mass of the atom is contained in an atomic nucleus?
His gold-foil experiment led to this hypothesis. Alpha particles were observed to mostly pass through a gold foil, which suggests that the volume of individual gold atoms consists mainly of empty space. The observation that some alpha particles were scattered at large angles led to the suggestion that the gold atom has a central core, or nucleus, composed of a concentrated mass capable of deflecting the alpha particles.
Explain how the atoms of one element differ from those of another element.
Different elements have different numbers of protons in their nuclei. All atoms of the same element have the same number of protons. Because atoms are neutral, the number of electrons in an atom equals the number of protons.
In what way are two isotopes of the same element different? Explain why isotopes of the same element have the same chemical behavior.
Isotopes of the same element have different numbers of neutrons, and therefore, different mass numbers and different atomic masses. Isotopes of the same element have the same number of protons and electrons. The electrons, not the neutrons, are responsible for an element’s chemical behavior.
proton+
electron-
mass number: total number of protons and neutrons in neucleus
atomic number: number of protons in neucleus
atomic mass: weiht average of masses of the isotopes of element.
1/12 mass of carbon atom with 6prot and 6 neut
Chlorine has two naturally occurring isotopes, Cl-35 and Cl-37. The atomic mass of chlorine is 35.45. Which of these two isotopes of chlorine is more abundant?
Cl-35
Explain on what basis the elements in the periodic table are arranged and how that contributes to the usefulness of the periodic table.
The elements in the periodic table are separated into groups according to a set of repeating properties. In each vertical column of the periodic table, called a group, the elements have similar chemical and physical properties. This allows you to easily compare the properties of one element (or a group of elements) to another element (or group of elements).
What is the difference between a solution and a heterogeneous mixture? Give an example of each.
A solution is a mixture that has a uniform composition. The composition of a heterogeneous mixture is not uniform throughout. All solutions are mixtures, but not all mixtures are solutions. Salt water is a solution; sand in water is a heterogeneous mixture.
Define element and compound. Explain the difference between an element and a compound.
An element is the simplest form of matter that has a unique set of properties. A compound is a substance that contains two or more elements chemically combined in a fixed proportion. An element cannot be broken down into simpler components through chemical reactions. Compounds are substances that can be broken down into simpler substances through chemical reactions.
What does the chemical formula of a compound indicate? Use an example to illustrate your points.
The chemical formula shows the elements that are in the compound and gives the proportions in which elements are found in the compound. The formula for sodium chloride (NaCl), for instance, indicates that the elements in the compound are sodium (Na) and chlorine (Cl). The formula also indicates that the elements are present in a proportion of 1:1 in the compound.