Atoms 14.1 The Structure of the Atom 14.2 Electrons

Learning Goals Identify and describe particles which comprise atoms. Describe the effects of radioactivity. Compare and contrast forces inside atoms

Atoms – an atom is a single piece of an element that retains the element’s properties Elements – has only one kind of atom. All elements are found on the periodic table of elements Molecules – any combination of two or more atoms Compounds- an combination of two or more different atoms

Atomic Structure Investigation 14A Key Question: What is inside an atom?

14.1 Structure of the Atom In order to understand atoms, we need to understand the idea of electric charge. We know of two different kinds of electric charge and we call them positive and negative.

14.1 Electric charge in matter We say an object is electrically neutral when its total electric charge is zero.

Democritus Proposed that there had to be a smallest part of matter, he called these atomos (indivisible)

John Dalton Published the theory of the atom in 1808. All atoms of a particular element are the same. Atoms of different elements have different properties, mass, and chemical reactivity. Atoms are not changed by chemical reactions, just rearranged in order or number.

14.1 An early model In 1897 English physicist J. J. Thomson discovered that electricity passing through a gas caused the gas to give off particles that were too small to be atoms. (plum pudding model) These negative particles were eventually called “electrons.”

14.1 The nuclear model In 1911, Ernest Rutherford, Hans Geiger, and Ernest Marsden did a clever experiment to test Thomson’s model. Rutherford said the atom is mostly empty space with a solid, central nucleus.

Niels Bohr 1913, while studying light realized that electrons can only exist in certain energy levels or orbitals.

Subatomic Particles Subatomic means “smaller than the atom”

14.1 Inside an atom The mass of the nucleus determines the mass of an atom because protons and neutrons are much larger and more massive than electrons.

Nucleus Neutrons Protons

The Atom Electron Protons Neutrons Parts of the atom

14.1 Force inside atoms Electrons are bound to the nucleus by the attractive force between electrons (-) and protons (+).

Electrons Travel in a cloud and has levels (-) negative charge Tiny- so small it almost doesn’t exist Bond- determines how the atom will bond

Nucleus 2 Parts Neutron (Ø) no charge Protons (+) positive charge

Atomic Mass Unit (AMU) # neutrons + # protons = AMU

Atomic number (Atomic #) The atomic # equals # of protons Corresponds to the # for the element listed on the periodic table. Ex. He (helium) 2 protons Atomic# =2

14.1 Force inside atoms What holds the nucleus together? There is another force that is even stronger than the electric force. We call it the strong nuclear force.

How atoms of various elements are different Isotopes are atoms of the same element that have different numbers of neutrons. The mass number of an isotope tells you the number of protons plus the number of neutrons. How are these carbon isotopes different?

14.1 Radioactivity Almost all elements have one or more isotopes that are stable. “Stable” means the nucleus stays together. Carbon-14 is radioactive because it has an unstable nucleus.

Atomic # - is the # of protons Atomic Mass- is the average of all the isotopes Mass number – The #protons+#neutrons Neutrons = mass# - protons

If the # of electrons equals the # of protons the atom is neutral If the # of electrons equals the # of protons the atom is neutral. If not, it is an ion.

Solving Problems How many neutrons are present in an aluminum atom that has an atomic number of 13 and a mass number of 27?

Solving Problems Looking for: Given Relationships: Solution …number of neutrons in aluminum-27 Given … atomic no. = 13; mass no. = 27 Relationships: Periodic table says atomic no. = proton no. protons + neutrons = mass no. Solution neutrons = mass no. – protons neutrons = 27 – 13 = 14

Chapter Fourteen: Atoms 14.1 The Structure of the Atom 14.2 Electrons

Chapter 14.2 Learning Goals Compare spectra of elements. Explain the Bohr atom model. Apply principles of quantum theory to explain the behavior of electrons in atoms.

Atomic Challenge Investigation 14B Key Question: How were the elements created?

14.2 Electrons in the atom Each different element has its own characteristic pattern of colors called a spectrum. The colors of clothes, paint, and everything else around you come from this property of elements to emit or absorb light of only certain colors.

14.2 Electrons in atoms Each individual color in a spectrum is called a spectral line because each color appears as a line in a spectroscope. A spectroscope is a device that spreads light into its different colors.

14.2 Bohr model of the atom Danish physicist Neils Bohr proposed the concept of energy levels to explain the spectrum of hydrogen. When an electron moves from a higher energy level to a lower one, the atom gives up the energy difference between the two levels. The energy comes out as different colors of light.

14.2 The quantum theory Quantum theory says that when things get very small, like the size of an atom, matter and energy do not obey Newton’s laws or other laws of classical physics.

14.2 The quantum theory According to quantum theory, particles the size of electrons are fundamentally different An electron appears in a wave-like “cloud and has no definite position.

14.2 The quantum theory The work of German physicist Werner Heisenberg (1901–1976) led to Heisenberg’s uncertainty principle. The uncertainty principle explains why a particle’s position, momentum or energy can never be precisely determined. The uncertainty principle exists because measuring any variable disturbs the others in an unpredictable way.

14.2 The uncertainty principle

14.2 Electrons and energy levels In the current model of the atom, we think of the electrons as moving around the nucleus in an area called an electron cloud. The energy levels occur because electrons in the cloud are at different average distances from the nucleus.

14.2 Rules for energy levels Inside an atom, electrons always obey these rules: The energy of an electron must match one of the energy levels in the atom. Each energy level can hold only a certain number of electrons, and no more. As electrons are added to an atom, they settle into the lowest unfilled energy level.

14.2 Models of energy levels While Bohr’s model of electron energy levels explained atomic spectra and the periodic behavior of the elements, it was incomplete. Energy levels are predicted by quantum mechanics, the branch of physics that deals with the microscopic world of atoms.

14.2 Energy levels In the Bohr model of the atom, the first energy level can accept up to two electrons. The second and third energy levels hold up to eight electrons each. The fourth and fifth energy levels hold 18 electrons.

14.2 Electrons and energy levels The first energy level can accept up to two electrons. The second energy levels hold up to eight electrons.

Energy and Quantum Theory Investigation 14C Energy and Quantum Theory Key Question: How do atoms absorb and emit light energy?

Bioluminescence- Glow Live! Like a glow stick, living things produce their own light using a chemical reaction. Bioluminescence is “cold light” because it doesn’t produce a lot of heat. While it takes a lot of energy for a living thing to produce light, almost 100 percent of the energy becomes visible light.