C-4 Atomic Structure

The nucleus and the moving electrons.

Law of Definite Proportions • Specific substances always contain

the same elements in the same ratio by mass.

• Ex. Water is made up of about 2 grams of hydrogen to 16 grams of oxygen. Therefore we come up with the formula for water to be H2O.

Law of Multiple Proportions• The ratio of masses of one

element that combine with a constant mass of another element can be expressed in small whole numbers. The ratio of hydrogen to oxygen can be different in 2 different compounds.

• Ex. Water = H2O and Hydrogen peroxide = H2O2.

Atomic Mass• The proton and neutron are

essentially equal in mass while the electron is considerable less.

• electron = 9.10953 X 10-28 g• proton = 1.67265 X 10-24 g• neutron = 1.67495 X 10-24 g• Notice the mass of an electron is

about 1800 times lighter than a proton or neutron.

• The orbiting of electrons around the nucleus is much like a satellite orbiting the earth.

• Electrons are continuously giving off energy thus should slow down and be pulled in by the nucleus and the + charge: Just as a satellite would be pulled into the earth if it lost energy and slowed down.

• Electrons never collapse into the nucleus even though they are continuously giving off energy.

Electromagnetic Radiation

• The energy given off by electrons is called electromagnetic radiation.

• This energy is not coming off continuously but in spurts.

• Once the energy comes off it travels at the speed of light. (c)

c=300,000,000 m/secWhen the energy spurts out it travels in all directions and is analogous to ripples in water

when a rock is dropped in water.

• Therefore we can say that electromagnetic radiation (energy) comes off in waves.

• The longer the time between emissions of EMR from electrons the further the waves are apart. The distance between 2 successive waves is called the energy’s wavelength.

• The more frequently an electron emits energy, the higher the number of waves that come off each second. The # of waves emitted per second is called frequency.

• The larger the frequency of emission of energy, the closer the waves are to each other or in other words the smaller the wavelength.

• The product of the wavelength and frequency is always a constant and that constant has been found to be the speed of light. “ wavelength X frequency = c”

• lambda X f = c

• The greater the frequency of energy emission, the more energy you’ll be bombarded with each second. Therefore, the frequency determines the amount of energy coming off an electron.

• Planck was the first to determine the equation for energy and frequency.

• E = h x f ( h is a constant)

Photons

• Let’s take a closer look at these waves of energy that are being emitted by electrons.

• The energy coming off is in packets or quanta so that waves would look like:

• These little packets of energy are called photons.

• These photons eventually hit some form of matter and are absorbed thus giving the matter more energy.

• This energy is absorbed on the atomic level of the matter (in the electrons) and thus these atoms have more energy.

• An atom which has absorbed energy in this way is called an excited atom.

• When excited atoms radiate energy, the radiation must be given off in photons.

Types of E.M.R.• We have already decided that EMR

can have different frequencies of emission.

• The difference in frequencies determines the type of EMR.

• Types of EMR: x-rays, ultraviolet, infrared, radio, visible, etc.

• Your eyes are sensitive to certain frequencies of EMR.---light.

Spectra of Atoms

• Different elements are excited differently when energy is added and thus emit energy differently also.

• This fact can be used to identify the elements.

• Know the spectrum p.96

Radiation• There are 3 forms of radiation with the

first 2 being comprised of particles.

– 1. alpha particle-is a helium nucleus made up of 2 protons and 2 neutrons.

– 2. beta particle-is a high speed electron from a radioactive nuclei.

– 3. gamma rays-very high energy x-rays

The Hydrogen Atom and the Quantum Theory

• The electrons travel in many different orbits around the nucleus. The smallest of these orbits is called the ground state.

• The further out an electron travels the more energy it has.

• Therefore, an electron at ground state has the lowest amount of energy.

• An electron needs to gain energy to move to an outer orbit, it gives off energy when it moves into a closer orbit.

Atomic Mass• The proton and neutron are

essentially equal in mass while the electron is considerable less.

• electron = 9.10953 X 10-28 g

• proton = 1.67265 X 10-24 g

• neutron = 1.67495 X 10-24 g

• Notice the mass of an electron is about 1800 times lighter than a proton or neutron.

Average Atomic Mass

• Not all atoms of the same element have the same mass. They have a mass that is close to an average for that element.

• The average mass is calculated by taking the masses for all the isotopes of an element and their relative amount of existence.

• Ex. Hydrogen has an atomic mass of 1.00797 amu that is based on 2 isotopes, deuterium-2amu and protium-1amu.

• Read & Work the sample on p.103