1. Chapter 3: Matter and Atomic Structure

2. 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

3. 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)

4. 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.

5. 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

6. 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

7. 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

9. 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.

10. 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.

11. 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= =143 neutrons

12. 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.

13. Try the “Using Math” question
On page 58

14. 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.

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