Chapter 3: Matter and Atomic Structure

3.1 What are elements? ALL matter—everything on Earth and beyond– is made up of substances called elements. An element is a substance that cannot be broken down into simpler substances by physical or chemical means. Gold is gold whether it is melted, pulled in to a wire, hammered into a sheet, or broken into pieces There are 92 elements that occur naturally on Earth and in the stars. Other elements have been formed in lab experiments. Each element is identified by a one-, two-, or three-letter abbreviation known as a chemical symbol H for Hydrogen Au for Gold reflects it’s Latin name, aurum

Elements Are Made of Atoms Each element has distinct characteristics Each element is made up of atoms. Atom Smallest particle of an element that has all of the characteristics of that element Made up of: Nucleus that contains protons and neutrons Proton: a tiny particle that has mass and a positive electrical charge Neutron: particle with about the same mass as a proton, but is electrically neutral charged- no electrical charge All atomic nuclei have a positive charge (they are made up of protons- positive charge- and neutrons- no charge)

Elements are Made of Atoms The number of protons and neutrons in a nucleus varies widely with different atoms Hydrogen : lightest atom, with only one proton Uranium-238 : heaviest atom, with 92 protons and 146 neutrons Atomic number: number of protons Mass number: number of protons plus the number of neutrons Electrons Smaller particles that surround the nucleus Have very little mass, but have a negative charge (exactly the same magnitude of the positive charge in the proton) An atom has an equal number of electrons and protons, making the overall charge neutral.

Energy Levels Think of an atom as a jawbreaker The center is the nucleus The different colored layers represent the various energy levels The energy level represents the area in an atom where an electron is most likely to be found. The size of an atom depends on the number and arrangement of electrons

Electrons in Energy Levels Arranged in a predictable pattern, each energy level can hold a limited amount of electrons. 2 in the first level, then 8, 18, and then 32 and so on… Look at the periodic table! (next slide) Lets practice some! Na-Sodium Br-Bromine Ne-Neon Zn-Zinc

CORRECTION!!!! Max amount of electrons in energy levels. 2 in the first level, then 8, 18, and then 32 and so on… To calculate: number of possible electrons= 2(n2) Where “n” is the energy level

Electrons in Energy Levels Electrons in the outermost level determine chemical behavior of different elements The outermost electrons are called valence electrons. Elements with the same number of valance electrons have similar chemical properties. Example: Na and K, both have one valence electron. Both of these elements are highly reactive metals, meaning they easily combine with other elements. Some elements have a full outer shell. Examples: He, Ne, Argon These elements do not easily combine with other elements.

Isotopes So far, we know that all atoms of an element have the SAME number of protons. BUT, the number of neutrons can be different. Example: Chlorine atoms have 17 protons. But they could have either 18 or 20 neutrons. If it has 18 neutrons, it would have a mass number of 35 (17+18) If it has 20 neutrons, the mass number would have 37 (17+20) When the atoms of the same element have different mass numbers, they are known as isotopes. Looking at our previous example: Chlorine. Chlorine has TWO isotopes: chlorine-35 and chlorine-37. Many elements are mixtures of isotopes.

Isotopes So How do scientists know how many neutrons are in an isotope?? Scientists have measured the mass of elements and found average masses for each element. Atomic mass: is the average of the mass numbers of isotopes that make up that element. When you are given an isotope, you can figure out the number of neutrons using the mass number and atomic number (number of protons). Example: Uranium-235. (U) Atomic number: 92 Number of neutrons= 235-92=143 neutrons

Radioactivity Radioactivity: the spontaneous process through which unstable nuclei emit radiation. Because some isotope’s nuclei are unstable, they emit this radiation. During radioactive decay, a nucleus can: 1. lose protons or neutrons 2. change a proton to a neutron 3. change a neutron to a proton Remember, the number of protons in an element identifies it. So if an element loses a proton, the identity of that element is actually changed.

Try the “Using Math” question On page 58

What elements are most abundant? Astronomers have identified two of the most abundant elements in the universe as: hydrogen and helium. All the other elements account for only one percent of all atoms in the universe. Earth’s crust is different! After analysis of the composition of rocks and minerals, scientists have found that oxygen and silicon make up most of earth’s crust. 98.5% of earth’s crust is made up of only 8 elements Note: The elements that make up the universe and earth’s crust can be found alone or in chemical combinations with other elements.

Recap How many protons, neutrons, and electrons are in sodium-23? Draw the Electron configuration for sodium-23.