Atomic models & the periodic table Flashcards
Chapter 1
What is an atom?
An atom is the smallest unit of ordinary matter that forms a chemical element.
What is an element?
An element is a pure substance (only one type) that cannot be broken down into simpler/smaller units using chemical reactions.
Element vs atom example
For example, water (H2O) is composed of 2 hydrogen atoms and one oxygen atom.
An element is a collection of atoms of the same type
Dalton (DESCRIBE)
Solid sphere model, 1803
Thomson (DESCRIBE)
Plum pudding model, 1897
Rutherford (DESCRIBE)
Nuclear model, 1911
Planetary model (DESCRIBE)
Bohr 1913
Simplified atomic model (DESCRIBE)
Chadwick, 1932
Solid sphere model
John Dalton described atoms as tiny particles that could not be divided.
Dalton had no idea of the size of atoms or how they were constructed. His model was called the Solid Sphere Model.
From who did Dalton take his idea?
He took this basic idea from Democritus, who used the word “atomos” to describe the smallest possible piece of matter.
Describe J.J Thomson’s experiment
Thomson experimented with cathode ray tubes, cutting edge technology of his time.
Strong potential differences (voltage) between two metal plates cause electrons to be ejected from the cathode plate.Thomson was able to deflect the electrons using electric fields.
What did J.J. Thomson conclude from his experiment?
Thomson was able to deflect the electrons using electric fields, demonstrating that the particles had negative charge.
How did Thomson modify Dalton’s model?
Thomson modified Dalton’s model to better explain his findings.
Atoms must contain smaller negative particles, electrons.
Electrons can be removed from atoms.
What was the problem with Thomson’s model?
His problem was that he could not find the positive charge in the atom that would balance the charge of the negative electrons.
Describe the developments in technology that were rapid in 1908 (Ernest Rutherford).
Following cathode rays, scientists quickly discovered how to produce and use many kinds of radiation: x-rays, rays made of alpha, beta, gamma particles.
Scientists experimented with alpha particles, which are much bigger than electrons and have a positive charge.
Describe the gold foil experiment
Rutherford expected most of the alpha particles to pass through the gold foil. It was only 160 atoms thin (0.00004 cm)!
(average human hair = 0.007 cm)
This was mostly true, but every now and then an alpha particle would bounce back, like it hit something much bigger with a strong positive charge.
(remember: like charges repel, similarly to magnetic repulsion)
What is Rutherford’s theory?
Rutherford’s theory is that the nucleus is very dense, and has a positive charge. The electrons revolve around of the nucleus.
His theory was that the atom looked like the solar system with the sun in the middle and the planets revolving around it. His model is called the Planetary Model.
What was the problem with Rutherford’s model?
But many scientists had trouble understanding how the electrons could remain circling around the nucleus.
Since positive and negative charges attract, shouldn’t the electrons eventually crash into the nucleus?
The Collapse of the Atom!
By 1913, scientists knew that …
light was related to the movement of electrons.
They were able to make light by exciting electrons in gas molecules with electric potential (voltage).
Using a prism, light of different wavelength (color) is refracted and viewed separately.
What was Bohr able to understand?
Bohr was able to understand why only certain frequencies of light were being emitted.
Remember, light is emitted from a moving electron.
Since only very specific types (energies) of light were being emitted, the electrons had to be moving in very specific ways too.
What is wrong with Bohr’s model?
Bohr’s is still incomplete. Remember how charges react: unlike charges attract (+ / -); like charges repel (+/+) (-/-).
What did James Chadwick discover?
Building on the work of colleagues across the world, Chadwick discovered the neutron in 1932.
Scientists had discovered a new type of radiation particle that was very energetic, like alpha particles, but it did not carry a charge.
What did the neutron explain?
The neutron eventually helped us understand why the nucleus doesn’t repel itself apart.
Neutrons allow the Strong Nuclear Force to hold the protons in the nucleus together.
Quickly describe Chadwick’s experiment
Performing this experiment on paraffin and gases, Chadwick was able to determine that the new radiation was a neutral (no charge) particle about the same mass as a proton.
Describe Neil’s Bohr experiment
Cr
Chromium
Fe
Iron
Ni
Nickel
Cu
Copper
Zn
Zinc
Br
Bromine
I
Iodine
Ag
Silver
Hg
Mercury
Pb
Lead
Valence electron
Valence means the outermost layer/shell of an atom.
Valence electrons are the electrons in the outermost layer/shell of an atom.
Columns (Different groups)
There are 2 types of groups on the periodic table: A and B
Each element of the same group in section A has the same number of valence electrons
The elements in section B are transition metals. These elements can have different numbers of valence electrons.
Families (how many + similiarities)
Like human members of a family, atoms of the same family have similar appearance and behaviour.
The reason for this is because atoms of the same group (column) have the same number of Valence Electrons (group A only).
Valence electrons are largely responsible for how an atom behaves and reacts. (remember the clothing analogy)
There are 4 special groups in the periodic table that make up these families.
Group 1A
: Alkali Metals
Group 2A
Alkaline Earth Metals
Group 7A:
Halogens (gases)
Group 8A:
Noble Gases
Alkali metals characteristics
Soft
HIGHLY reactive - for this reason they are not found in their pure state naturally.
In their pure state, they must be stored in oil
Alkaline Earth Metals properties
Highly malleable (can be bent into shapes)
Very reactive - but will not react on contact with the air (like the alkali metals)
They can be found in their pure state in nature
Transition metals characteristics
Harder metals - all solid at room temperature EXCEPT Hg (mercury)
Halogens characteristics
Very reactive with metals to form salts
Powerful disinfectants (think of chlorine in pools)
Noble gases characteristics
Very stable - non reactive
They are found in their pure state in nature
Periods: rows
Each element of the same period will have the same number of electron shells
Octect rule
Since we are only investigating the structure of the first 20 atoms, we simplify by using the octet rule.
Octet Rule:
The first electron shell (1st row) may contain a maximum of 2 electrons
The second, third and fourth shells (2nd, 3rd and 4th rows) may contain a maximum of 8 electrons.
Metals
Metals are located to the left of the staircase.
good conductors of electricity and heat, they are ductile, malleable and often shiny, they are usually solid at room temperature and react with acid.
Metals tend to give away its valence electrons in presence of nonmetals
Non-metals:
Non-metals are located to the right of the staircase
Poor conductors of electricity and heat, many are gases at room temperature. when solid, they can easily be reduced to powder.
Nonmetals tend to accept valence electrons in presence of metals to reach a full octet.
Metalloids:
Located on the staircase
The properties of these elements vary depending on conditions
Atomic number
The whole number found next to each element symbol. These numbers are in order: 1 is Hydrogen (top left) and increase from left to right.
This number represents the number or protons in the nucleus of that element.
Chemical Symbol
Symbol used to represent each element. The first letter will always be a capital letter. If there is a second letter (i.e. Mg) it will be lowercase.
(Relative) Atomic mass:
the mass of an atom (determined by comparing it to carbon). We will revisit this in our chemistry unit!
Mass number
Round the relative atomic mass to a whole number. Indicates the sum of protons and neutrons in an atom.
How to calculate the number of neutrons
The atomic number (6) tells us the # of protons in the nucleus.
The mass number (atomic mass(12.01), rounded (12)) tells us the total # of particles in the nucleus (protons + neutrons).
In order to know the # of neutrons:
subtract the atomic number from the mass number.
neutrons = (Mass Number) - (Atomic Number)
A Z E Notation
An effective way to keep track of mass, protons (and neutrons).
The upper number (A) is the mass number of the atom (always larger)
The lower number (Z) is the atomic number of the atom (always smaller)
A = Z + N, N = A - Z
Rutherford-Bohr Atomic Model
characteristics
Nucleus in the center (composed of protons)
A neutral atom will have the same number of protons as electrons.
Electrons travel around the nucleus on specific paths. These paths are called energy levels
Not every path can hold the same amount of electrons.
For now the rule is:
1st shell: 2 electron,
2nd shell: 8 electrons,
3rd shell: 8 electrons
Simplified Atomic Model
characteristics
This new model adds information about the nucleus.
Like the Rutherford-Bohr model, we state the number of positive protons in the nucleus.
We also state the number of neutral neutrons in the nucleus.
Electron shell orbits are represented by semi-circles, and the number of electrons in the shell written below.
Valence Electrons/Valence Shell
When understanding atoms and the electrons of an atom, we often need to think of Valence.
Valence means the outermost layer/shell of an atom.
Valence electrons are the electrons in the outermost layer/shell of an atom.
Why are valence electrons important?
When one atom encounters another atom, they may interact.
The interaction usually involved Valence Electrons, because they are the outermost electrons.
The inner electrons are essentially “hidden” by the valence shell.
Lewis notation
Lewis notation is a simple representation of an element’s outer electron shell (valence shell).
Key components:
The element’s symbol (i.e. lithium → Li, fluorine → F)
Dots representing the number of valence electrons
Always fill single dots on each side before doubling
Isotopes
Isotope: atoms of the same element with different numbers of neutrons.
Because they contain different numbers of neutrons, isotopes have different atomic masses.
How is the average atomic mass calculated
The average atomic mass of an element is calculated by taking the weighted average masses of the element’s naturally occurring isotopes.
