Atoms and Their Structure Unit 3: Atoms and Their Structure

The model of the atom through time

Dalton’s Atomic Theory John Dalton - proposed his atomic theory of matter atom - The smallest particle of an element that retains the properties of that element

Dalton’s Atomic Theory 1. All matter is made up of atoms. 2. All atoms of one element are exactly alike, but are different from atoms of other elements. 3. Atoms are indestructible and cannot be divided into smaller particles. (NO LONGER CONSIDERED TO BE TRUE)

Atoms CAN be divided into smaller pieces 2 regions – the nucleus and the electron cloud 3 particles – the proton (p+), the electron (e-), and the neutron (n0)

Structure of the atom In 1909, a team of scientists led by Ernest Rutherford carried out experiments that discovered the existence of a positively charged nucleus

Rutherford’s Gold Foil Experiment Today, we know that the particles of the beam consisted of clusters containing two protons and two neutrons and are called alpha particles. The sheet of gold foil was surrounded by a screen coated with zinc sulfide, which glows when struck by the positively charged particles of the beam.

The Nuclear Model of the Atom Because so few particles were deflected, they proposed that the atom is nearly all empty space. It has a small, dense, positively charged central core, called a nucleus with electrons surrounding it. To explain the results of the experiment, Rutherford’s team proposed a new model of the atom.

The Electron

Discovered by :Thomson experiments used a Cathode-Ray Tube The Electron Discovered by :Thomson experiments used a Cathode-Ray Tube Conclusion - cathode rays are made up of invisible, negatively(-) charged particles, electrons (-). A vacuum tube has had all gases pumped out of it. At each end of the tube is a metal piece called an electrode, which is connected through the glass to a metal terminal outside the tube. These electrodes become electrically charged when they are connected to a high-voltage electrical source When the electrodes are charged, rays travel in the tube from the negative electrode, which is the cathode, to the positive electrode, the anode. Because these rays originate at the cathode, they are called cathode rays. Thomson found that the rays bent toward a positively charged plate and away from a negatively charged plate.

Cathode-Ray Tube opposites attract These electrons came from the matter ( or atoms) of the negative electrode. Click here He knew that objects with like charges repel each other, and objects with unlike charges attract each other.

Charge – the electron is negatively charged electrons have very little mass; about 1/1840 the mass of a hydrogen atom Charge – the electron is negatively charged Location – electrons move about in the electron cloud surrounding the nucleus Attraction between the positive nucleus and the negative electrons hold the atom together

Role Electrons being on the outside part of the atom, are the parts that interact with other atoms the number of electrons determines the properties of an element

Why don’t atoms collapse upon themselves? Electrons have energy of motion that enables them to overcome the attraction of the positive nucleus. Niels Bohr.

The Electromagnetic Spectrum energy travels in the form of waves that have both electrical and magnetic properties. These electromagnetic waves can travel through empty space, as you know from the fact that radiant energy from the sun travels to Earth every day.

The Electromagnetic Spectrum One way to increase the energy of an electron is to supply energy in the form of high-voltage electricity. Another way is to supply electromagnetic radiation, also called radiant energy.

Electromagnetic radiation includes radio waves microwave radiation X- rays UV rays Gamma Rays visible light. Infrared All of these forms of radiant energy are parts of a whole range of electromagnetic radiation called the electromagnetic spectrum.

The Electromagnetic Spectrum

Frequency and wavelength high frequency = low wavelength and high energy

Our eyes see different frequencies of light as different colors

Electrons and Light Excited electrons release light called the emission spectrum of that element.

The emission spectrum is unique for each element carbon

This space is called the electron cloud. The Electron Cloud Model Electrons take up little space but travel rapidly through the space surrounding the nucleus. This space is called the electron cloud.

Evidence for Energy Levels Bohr theorized that electrons absorb energy and move to higher energy states. Then, these excited electrons give off that energy as light waves when they FALL BACK to a lower energy state.

Evidence for Energy Levels Electrons have certain amounts of energy, so they can move around the nucleus only at distances that correspond to those amounts of energy. Energy level - the region of space in which electrons can move about the nucleus

Each energy level can hold a limited number of electrons. Electrons in Energy Level Each energy level can hold a limited number of electrons. The lowest energy level is the smallest and the closest to the nucleus.

This first energy level holds a maximum of 2 electrons. Electrons in Energy Level This first energy level holds a maximum of 2 electrons. The second energy level holds a maximum of 8 electrons. The third energy level holds a maximum of 18 electrons.

******A full valence shell = 8 The lowest energy levels fill up first ****Electrons in the outermost energy level are called valence electrons*** ******A full valence shell = 8

Electrons in Energy Level the periodic table is a tool to predict the number of valence electrons in any atom in Groups IA – VIII A. atoms in Group IA, like hydrogen, have one valence electron. atoms in Group IIA have two valence electrons. Etc.

The Bohr Model An oxygen atom has eight electrons. Two of these fill the first energy level, and the remaining six are in the second energy level. Oxygen has 6 valence electrons

Lewis Dot Diagrams A Lewis dot diagram illustrates valence electrons as dots around the chemical symbol of an element.

Each dot represents one valence electron. Lewis Dot Diagrams Each dot represents one valence electron. In the dot diagram, the element’s symbol represents the core of the atom—the nucleus plus all the inner electrons.

The Proton Logically there must be a positively charged particle because matter is not negatively charged Years later, after the discovery of the nucleus scientists determined there were rays composed of positively(+) charged subatomic particles called protons.

Location – inside nucleus Charge – positive The atomic number of an element is the number of protons in the nucleus atomic number determines the identity of an element,.

1 amu is approximately the mass of a single proton or neutron. Atomic Mass chemists have devised a different unit of mass called an atomic mass unit, which is given the symbol amu. 1 amu is approximately the mass of a single proton or neutron. 1 amu = 1 gram

The Neutron Thomson discovered that neon consisted of atoms of two different masses isotopes - atoms of an element that are chemically alike but differ in mass

Neutrons Because of the discovery of isotopes, scientists hypothesized that atoms contained a third type of particle that explained these differences in mass. The existence of this neutral particle, called a neutron, was confirmed in the early 1930s. Calculations showed that such a particle should have a mass equal to that of a proton but no electrical charge.

The mass of a neutron is almost the same as the mass of a proton. The sum of the protons and neutrons is the atomic mass number

Masses Isotopes of an element have different mass numbers because they have different numbers of neutrons, but they all have the same atomic number.

Atomic Mass The atomic mass on the periodic table is an average of ALL the known isotopes of that element. The atomic mass depends upon the mass and relative abundance of each isotope of that element

The neutron is located inside the nucleus Charge - neutral

Information in the Periodic Table The larger number in each box is the average atomic mass of that element. The smaller number is the atomic number

# of protons = # of electrons In a neutral atom: # of protons = # of electrons atomic mass # = neutrons + protons The number of protons determines the identity of the element

Element shorthand

The number represents the mass number Chemists will write hydrogen-1 and hydrogen-2 to indicate isotopes of an element The number represents the mass number Ex - krypton-84 84 = mass number # of protons =36 # of neutrons = 48

mass number SYMBOL atomic number Element = gold # of (p+) = 79 # of (e-) = 79 # of (n0) = 197 – 79 = 118

Nuclear fission, fusion, radioactivity, and nuclear power

nuclear fusion. - combining of atomic nuclei Fusion reactions can release very large amounts of energy and require extremely high temperatures. They are also called thermonuclear reactions

For example, nuclear fusion occurs within the Sun, where hydrogen atoms fuse to form helium atoms.

Nuclear Reactions Nuclear fission - splitting of a nucleus into fragments Heavy atoms (mass number > 60) BREAK into smaller atoms when struck by neutrons Nuclear fission releases a large amount of energy.

Nuclear power plants use the process of nuclear fission to produce heat in nuclear reactors. The heat is used to generate steam, which is then used to drive turbines that produce electricity.

Pros and Cons of Nuclear Power produce no greenhouse gases nuclear disasters are rare nuclear power plants are cost effective produces radioactive waste no good storage options must remain stored for at least 10,000 years potential for nuclear disaster uranium is non-renewable resource only enough uranium for the next 30 to 60 years

Radioactivity Radioactivity is the process whereby unstable atomic nuclei breakdown and release energetic subatomic particles In small doses it is beneficial – especially in medicine In large doses - lethal