Week 2 - Day 2

Download Word (docx):

Navigate using audio

Audio 0:03:42.054463
Using data from
- Millikan’s experiment (–1.60 × 10–19 C/electron);
- Thomson’s mass-to-charge ratio for electrons, it can be deducted that the mass of an electron is as follows:

Audio 0:07:20.979240
Beta particles: negatively charged = electrons
Alpha particles: positively charged Ernest Rutherford showed that these are chemically part of helium atoms
Audio 0:07:58.216800
Gamma Rays: like light and X-rays, only more energy
So where are the Electrons and the Positive Charge in an atom?
Audio 0:08:27.289910

Audio 0:09:44.109094
(as carried out by Ernest Marsden under the direction of Johannes Gieger and Ernest Rutherford) Rutherford’s Experimental Design
Audio 0:10:06.370006
Audio 0:11:02.745604

Multiple collisions, small deflections
Rutherford on the large deflections:
’.. About as credible as if you had fired a 15- inch [artillery] shell at a piece of paper and it came back and hit you.’
Audio 0:12:11.955108
Audio 0:12:34.844121
Audio 0:12:53.542528
Audio 0:13:40.032752
Audio 0:14:53.573853

Audio 0:16:11.946000
The nuclear theory of the atom has three basic parts
1. Most of the atom’s mass and all of its positive charge are contained in a small core called a nucleus.
2. Most of the volume of the atom is empty space, throughout which tiny, negatively charged electrons are dispersed.
3. There are many negatively charged electrons outside the nucleus as there are positively charged particles (named protons) within the nucleus, so that the atom is electrically neutral.

Audio 0:20:04.055851
All atoms are composed of the same subatomic particles:
- Protons
- Neutrons
- Electrons
Protons and neutrons have nearly identical masses
- The mass of the proton is 1.67262 * 10^-27 kg
- The mass of the neutron is 1.67493 × 10^–27 kg.
- The mass of the electron is 9.1 × 10^31 kg.
The charge of the proton and the charge of the electron are equal in magnitude but opposite in sign. The neutron has no charge.

The most important number to the identity of an atom is the number of protons in its nucleus.
The number of protons defines the element.
The number of protons in an atom’s nucleus is its atomic number and is given the symbol Z.

Audio 0:24:33.380011
All atoms of a given element have the same number of protons; however, they do not necessarily have the same number of neutrons.
- Example:
  - All neon atoms contain 10 protons, but they may contain 10, 11, or 12 neutrons.
  - All three types of neon atoms exist, and each has a slightly different mass.
Atoms with the same number of protons but a different number of neutrons are called isotopes.

Audio 0:26:45.772174
The sum of the number of neutrons and protons in an atom is its mass number and is represented by the symbol A.
X is the chemical symbol, A is the mass number, and Z is the atomic number.

Audio 0:28:13.708943
A second common notation for isotopes is the chemical symbol (or chemical name) followed by a dash and the mass number of the isotope.

Audio 0:28:56.545513
The relative amount of each different isotope in a naturally occurring sample of a given element is roughly constant.
The percentages are called the natural abundance of the isotopes.

Audio 0:34:19.624557
The number of electrons in a neutral atom is equal to the number of protons in its nucleus (designated by its atomic number Z).
In chemical changes, however, atoms can lose or gain electrons and become charged particles called ions.
- Positively charged ions are called cations.
  - Metal elements, such as Na+, form cations.
- Negatively charged ions are called anions.
  - Nonmetal elements, such as F–, form anions.

Audio 0:36:03.449187
Atomic mass is sometimes called the atomic weight or standard atomic weight.
The atomic mass of each element is directly beneath the element’s symbol in the periodic table.
The atomic mass of an element represents the average mass of the isotopes that compose that element
- It is a weighted value based on the element’s natural abundance of each isotope.
Audio 0:36:29.805279

The masses of atoms and the percent abundances of isotopes of elements are measured using mass spectrometry—a technique that separates particles according to their mass.

Naturally occurring chlorine consists of 75.77% chlorine-35 atoms (mass 34.97 amu) and 24.23% chlorine-37 atoms (mass 36.97 amu).
Calculate chlorine’s atomic mass.
Audio 0:39:29.943281
75.77 % / 100 % * 34.97 amu + 24.23%/100% * 36.97 amu

term	definition
atomic number	number of protons in an atom’s nucleus (Z)
isotopes	atoms with the same number of protons but a different number of neutrons
mass number	the sum of the number of neutrons and protons in an atom
natural abundance	the relative amount of each different isotope in a naturally occurring sample of a given element (it is roughly constant)
atomic structure	highly condensed mass in nucleus with mostly empty space in electron cloud
nucleus	the small core of an atom (contains most of it’s mass and the positive charge)
protons	positively charged particles (in the nucleus of an atom)
cations	positively charged ions
anions	negatively charged ions
mass spectrometry	technique that separates particles according to their mass