Unit 2Atomic Theory and Periodicity

The Periodic Table

The Periodic Table The periodic table is a tool used for the

investigations of: average atomic mass, mass number, and

atomic number;

Mass Number Mass number is the total amount of

nuclear particles, neutrons and protons, and stated as AMU, atomic mass units. Neutron is a non-charged particle that

lends mass to the nucleus. Proton is a positively charged particle that

has the same mass as the neutron. When added together, the proton mass

and neutron mass equal the mass number of an isotope of an element.

Isotope Isotopes are atoms of the same element

with different numbers of neutrons While some isotopes are radioactive,

many are not. Below are the three isotopes of

hydrogen.(Note mass number is often called atomic mass.)

Atomic Number The atomic number of an element equals

the number of protons in a single atom of that element. This number does not change.

In a stable atom, the atomic number also equals the number of electrons surrounding the nucleus of an atom.

All atoms of a particular element have the same number of protons.

The periodic table is arranged in order of increasing atomic numbers.

Atomic Number and Mass Number In the drawing below of a carbon atom,

there are 6 protons, 6 electrons and 6 neutrons.

The atomic number is 6. The mass number is 12 amu.

Average Atomic Mass Calculating the average mass number of an

element involves the weighted average of all known isotopes and their abundance in nature.

Example: The element chlorine exists as two naturally occurring isotopes. Cl-35 which occurs 75% of the time and Cl-37 which occurs 25% of the time.

What is the average atomic mass of chlorine? 

(.75 x 35) + (.25 x 37) = 35.5 amu

Writing Atomic Number and Mass Number Atomic Number is only used as an

identifier of the number when written it is the bottom number, the mass number is the top.

The mass number is written as above or as a dashed number, such as C-12.

Determining Number of Neutrons from Atomic Number and Mass Number

To determine the number of neutrons from the mass number and atomic number, simply subtract the two numbers.

Example: Br – BromineMass number – 79.904 Atomic number – 35

Number of protons and electrons in a neutral atom = 35 eachNumber of neutrons = Mass number – atomic number = 45 (rounded)

Half-life of Radioactive Isotopes Half-life is the length of time required for one half of a given

sample of a radioactive isotope to decay. How to Calculate: An ExampleA substance has a half-life of 25 days. If 200. g of that substance is found in a storage facility, how grams of the substance will be left after 125 days. 200100 25 days100 50 25 days 50 25 25 days 25 12.5 25 days 12.5 6.25 25 days (total of 125 days) Mass after 125 days = 6.25 g

Radioactive Decay Radioactive decay – the spontaneous

disintegration of a nucleus into a slightly lighter and more stable nucleus, accompanied by the emission of particles, electromagnetic radiation, or both.

Types of Radioactive Decay - alpha particle – two protons and two neutrons bound

together and emitted from a nucleus. They are often referred to as helium nuclei particles with a +2 charge. This particle has the lowest penetration of all forms of radiation.

- beta particle – an electron emitted from the nucleus during some kinds of radioactive decay. Please note these electrons come from a radioactive nucleus and not from the electron cloud around the nucleus. This particle has medium penetration.

- gamma radiation – (ray) a high-energy electromagnetic wave emitted from a nucleus as it changes from an excited state to a ground energy state. This ray has high penetration.

Historical and Quantum Models of the Atom Discoveries and insights related to the

atom’s structure have changed the model of the atom over time

Major insights regarding the atomic model of the atom and principal scientists include: Particles/atomos – Democritus first atomic theory of matter – John Dalton Dalton also

stated that atoms are solid spheres that resemble a billard ball

discovery of the electron – J. J. Thompson discovery of the nucleus – Ernest Rutherford = gold foil

experiment – the atom is mostly empty space, there is a small, dense center that positively charged.

discovery of charge of electron – Robert Millikan planetary model of atom – Niels Bohr quantum nature of energy – Max Planck uncertainty principle – Werner Heisenberg wave theory – Louis de Broglie.

Particles/atomos – Democritus The theory of Democritus and Leucippus

held that everything is composed of "atoms", which are physically, but not geometrically, indivisible; that between atoms lies empty space; that atoms are indestructible; have always been, and always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, which differ in shape, and size.

John Dalton

John Dalton (1766 – 1844) proposed a basic model of the atom that helped establish many scientific concepts and also created the foundation for more modern models. His model suggested that atoms are the smallest particle of an element, that atoms of different elements have different masses, and that they are solid, indestructible units - much like a billiard ball.

John Dalton First Well Defined Atomic Theory Five main points of Dalton's atomic theory

1. Elements are made of extremely small particles called atoms.

2. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.

3. Atoms cannot be subdivided, created, or destroyed.

4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

5. In chemical reactions, atoms are combined, separated, or rearranged.

J. J. Thomson

Sir Joseph John "J. J." Thomson, 18 December 1856 – 30 August 1940) was a British physicist and Nobel laureate. He is credited with discovering electrons and isotopes, and inventing the mass spectrometer. Thomson was awarded the 1906 Nobel Prize in Physics for the discovery of the electron and for his work on the conduction of electricity in gases. He is best known however for his plum pudding model of the atom.

JJ Thomson Thomson fashioned his model after the

dessert known as a plum pudding, seen below.

The Plum pudding model, shows the electrons and positively charged particles randomly stuck in the nucleus.

Robert Millikan

discovery of charge of electron Millikan's experiment involved measuring the force

on oil droplets in a glass chamber sandwiched between two electrodes, one above and one below. With the electrical field calculated, he could measure the droplet's charge, the charge on a single electron being (1.592×10−19 C).

Ernest Rutherford Ernest Rutherford, (30 August 1871 – 19 October 1937) was a New Zealand born British chemist and physicist who became known as the father of nuclear physics. He is the only Nobel prize winner to have his most famous work performed after receiving the prize.

Rutherford, Gold Foil Experiment As a result of the gold foil experiment,

Rutherford surmised the atom is mostly empty space, there is a small, dense center that positively charged.

Rutherford shot alpha particles in a beam through a thin sheet ofgold foil, the beam scattered, indicating a dense positive center.

Niels Bohr

Niels Henrik David Bohr (October 7, 1885 –November 18, 1962)a Danish physicist who made foundational contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922.

Niels Bohr Created the Planetary Model of the

Atom. He stated that the electrons were in

fixed orbits around the nucleus.

Max Planck Max Karl Ernst Ludwig Planck April 23, 1858 – October 4, 1947) was a German

theoretical physicist who originated quantum theory, which won him the Nobel Prize for Physics in 1918

According to the quantum theory, energy is held to be emitted and absorbed in tiny, discrete amounts. An individual bundle or packet of energy, called a quantum, thus behaves in some situations much like particles of matter; particles are found to exhibit certain wavelike properties when in motion and are no longer viewed as localized in a given region but rather as spread out to some degree.

Werner Heisenberg Uncertainty Principle

States that there is a level of uncertainty as to the exact location of an electron in the electron cloud.

Louis de Broglie Wave–particle duality states that all

particles exhibit both wave and particle properties. This means that particles can have both mass and movement, often in the form of vibrations.

Quantum Mechanical Model The modern atomic theory is called the

Quantum Mechanical Model. This model uses different letters to denote placement of electrons and energy levels.

Electron configurations, Valence electrons, and Oxidation numbers

Electron configuration is the arrangement of electrons around the nucleus of an atom based on their energy level.

Atoms can gain, lose, or share electrons within the outer energy level.

Electrons are added one at a time to the lowest energy levels first (Aufbau Principle).

An orbital can hold a maximum of two electrons (Pauli Exclusion Principle). Electrons occupy equal-energy orbitals so that a maximum number of unpaired

electrons results (Hund’s Rule). Energy levels are designated 1–7. Orbitals are designated s, p, d, and f

according to their shapes s, p, d, f orbitals relate to the regions of the Periodic Table. Loss of electrons from neutral atoms results in the formation of an ion with a

positive charge (cation). Gain of electrons by a neutral atom results in the formation of an ion with a

negative charge (anion). Transition metals can have multiple oxidation states.

Electron Configuration

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