Atomic Structure

Stepwise Timeline of Atomic Theory Rutherford 1911 Dalton 1803 Modern Theory Thomson 1897 Bohr 1913 Chadwick 1932

Democritus ~ 460 B.C. to 360 B.C. Who Greek Philosopher What Atoms cannot be created, destroyed or divided. How Observing nature

Dalton’s Atomic Theory Who John Dalton (1766-1844), an English schoolteacher and chemist What proposed his atomic theory of matter in 1803. Although his theory has been modified slightly to accommodate new discoveries, Dalton’s theory was so insightful that it has remained essentially intact up to the present time.

Dalton What ( write this outside the text box) Cannot be created or destroyed. All atoms of one element are exactly alike, (same size, mass, properties) but different from atoms of other elements Combine in whole number ratios to form compounds. How- work with gases

J.J. Thomson -1903 Who a British physicist. What Plum Pudding model (or Chocolate Chip Cookie model) Discovered electrons Model Atom was a positively charged sphere with negative electrons in it like chips POSITIVE CHARGE ELECTRONS

J. J. Thomson How discovered that cathode rays are made up of invisible, negatively charged particles referred to as electrons. http://tinyurl.com/cathodert

Ernest Rutherford What Who Found nucleus (1911) British chemist Occupies a small volume of the atom Contains almost all the mass of the atom Electrons orbit around nucleus Discovered proton Who British chemist and physicist.

Ernest Rutherford How Gold Foil Experiment Alpha particles which are positively charged pass through unmolested most of the time. Occasionally they would bounce off when they hit something (proton) that was also positively charged.

Niels Bohr - 1913 Who Danish Physicist What Planetary Model Electrons (e-) have definite path around the nucleus (orbit) e- arranged around the nucleus according to energy level e- with lowest energy level are closest to nucleus How Spectral emission lines Who Danish Physicist

Chadwick What Discovered the neutron in 1932 How Used alpha particles Who British Scientist What Discovered the neutron in 1932 How Used alpha particles

Modern Atomic Theory 1. All matter is made up of very tiny particles called atoms. 2. Atoms of the same element are chemically alike. 3. Individual atoms of an element may not all have the same mass. However, the atoms of an element have a definite average mass that is characteristic of the element. 4. Atoms of different elements have different average masses. 5. Atoms are not subdivided, created, or destroyed in chemical reactions.

Atom and Elements Element - a substance that is composed of a single type of atom. Atom - the smallest particle of an element that retains the properties of that element. The diameter of an atom is measured in nanometers 1 nm = 1 x 10-9 m = 0.000000001 m Atoms are composed of sub-atomic particles.

Proton Discovered by Ernest Rutherford in early 1900’s Determines the identity of an atom Relative mass of 1 atomic mass unit Part of the nucleus of an atom Positive charge If you change only the # of protons, you change the element being described.

Neutron Discovered by James Chadwick in 1932 Determines the isotope of an atom Relative mass of 1 atomic mass unit Part of the nucleus of an atom No charge (neutral) If you change only the # of neutrons, you have a new isotope (variety) of the element .

Changing the number of neutrons Creates ISOTOPES - Atoms of the same element but with a different number of neutrons. Isotopes of an element have nearly identical chemical properties

Electron Discovered by J. J. Thomson in 1903 Determines the charge of an atom (charged atoms are called ions) Relative mass of 0 (~1/1836) atomic mass unit Make up the electron cloud of an atom Negative charge

Changing the number of electrons When an atom loses electrons, it results in a net positive charge and is called a CATION ions are I itive

Example of a cation Neutral potassium (K) has 19 protons and 19 electrons. 19 protons = +19 19 electrons = -19 0 If potassium (K) loses an electron, it only has 18 electrons. 19 protons = +19 18electrons = -18 +1 This is written as K+1 and is called a cation

Changing the number of electrons When an atom gains electrons, it results in a net negative charge and is called an ANION

Example of an anion Neutral bromine (Br) has 35 protons and 35 electrons. 35 protons = +35 35 electrons = -35 0 If bromine (Br) gains an electron, it has 36 electrons. 35 protons = +35 36 electrons = -36 -1 This is written as Br -1 and is called an anion

Gained electron Lost electron cation anion

Describing an atom Equals the number of protons in an element. ATOMIC NUMBER Equals the number of protons in an element. In a neutral atom, the atomic number also equals the number of electrons. - All atoms of the same element have the same number of protons. The smaller of the two numbers in the periodic table square, always a whole number

Describing an atom ATOMIC MASS A weighted average of the mass of all the isotopes (varieties) of an atom Each element has only one atomic mass Also called “average atomic mass” The larger of the two numbers in the periodic table square Always a decimal number

Describing an atom MASS NUMBER Equals the # protons + # neutrons in an atom Not always the same for atoms of an element  isotopes Not listed on the periodic table Always a whole number

APE MAN A = Atomic Number P = Number of Protons E = Number of Electrons M= Mass number A = Atomic Number (again) N = Number of Neutrons Always the same number in a neutral atom Mass Number minus Atomic number equals Number of neutrons

Isotope Notation mass number C 12 6 element symbol atomic number

Isotope Name name of the element dash mass number Example: Carbon -14 is the isotope name for a carbon atom with a mass number of 14

C Isotope Notation mass number 14 element symbol 6 atomic number number of neutrons = mass number – atomic number. How many protons and neutrons in this isotope?

Practice 14 18 Isotope notation Isotope name Atomic number Mass # # of p+ # of no # of e- Silicon - 14 18 Helium - 4

Determining (Average)Atomic Mass To determine the atomic mass you must know what percent of each isotope of the element is found in nature. This is called the relative abundance. Example: There are 2 common isotopes of Chlorine. 25% is chlorine - 37 75% is chlorine - 35 Calculate the average atomic mass of chlorine.

Average Atomic Mass Neon in nature is 90.5% Neon-20, 0.3% Neon-21, and 9.2% Neon-22. What is the average atomic mass of Neon?

REACTIONS CHEMICAL NUCLEAR involve the transfer or sharing of electrons involve the absorption or emission of particles by the nucleus of an atom

Nuclear Chemistry Vocabulary Nuclide- General name given to the nucleus of an atom Parent nuclide- initial nucleus Daughter nuclide- the nucleus after the decay has occurred

Nuclear Chemistry Vocabulary Radiation - energy that is emitted from a source and travels through space. Ionizing Radiation- Has enough energy to change atoms and molecules into ions; examples: X-rays and gamma rays. Nonionizing Radiation- Does not have enough energy to ionize matter; examples: radio waves, microwaves Accidentally discovered by Henri Becquerel in 1896 when he was performing a lab with fluorescent screens. Radioactivity is the spontaneous emission of radiation from the nucleus of an atom.

Types of Ionizing Radiation Symbol Charge Penetrating Ability Size Alpha He or  positive (deflected by a magnet) Limited ability to pass through matter. (can be stopped by paper) Big Beta formed when a neutron splits e or -1 negative (deflected by a magnet) Can penetrate better than alpha (can be stopped by a few mm of Al) Small Gamma  Neutral (is not deflected by a magnet) Penetrates the farthest. (several cm of lead or a larger layer of concrete will block ) nothing 4 2

penetrating ability

Why decay happens To become more stable. Large atoms are stable when the neutron: proton ratio is 1.5:1 Decay happens when the neutron: proton ratio is too high.

Alpha decay

Alpha Decay Occurs when an alpha particle leaves the nucleus - alpha particle = Helium nucleus Parent  daughter: mass decreases by 4 and atomic number decreases by 2 Example: Thorium-230 undergoes alpha decay. Write the decay reaction. Th ------> He + Ra

Alpha decay practice Write the decay reaction for alpha decay of Uranium-238.

Beta decay Ac +

Beta Decay occurs when a beta particle e is emitted from the nucleus Parent  daughter: equal mass but atomic number increases by 1. a neutron becomes a proton. Example: Carbon-14 undergoes beta decay. Write the decay reaction. C -----> e + N

Beta decay practice Write the decay reaction showing beta decay of Thorium-234.

Fission and Fusion

Fission Reaction Nuclear reaction Splitting an atom’s nucleus Releases energy Alpha, beta are examples Used in nuclear reactors Causes a chain reaction Problem: produce radioactive waste; storage of fuel is dangerous

chain reaction

Fusion Nuclear reaction Two light nuclei are combined to form one heavier more stable nuclei Energy is released this is how stars are fueled Problem with using on Earth: requires EXTREMELY high temps and high pressure

fusion in the sun

How come the protons hang out with each other? The charge of a proton is positive. It is repelled by other protons. So, how do the protons stay in the nucleus? Shouldn’t they want to avoid each other? The answer is that a Strong Nuclear Force exists, which is a very strong, but short range, force between quarks that keep the nucleus together by overcoming the repulsion between the protons.