Introduction to Chemistry

What is “matter?” Matter is anything that has mass and takes up space EVERYTHING around you is matter (including you!)

What is an “atom?” Atoms are the smallest form of matter Different types of atoms exist and combine to form everything around us.

Atomic Structure Nucleus Electron Shells

Protons Positive (+) charge Found in the nucleus

Electrons Negative (-) Charge Found in electron shells Much smaller than protons

Neutrons Neutrons have no charge, so we call them “neutral” Located in the nucleus

How do different charges behave? Protons have a positive charge. Electrons have a negative charge. What happens when those charges come in contact with one another? Subatomic Particle Behavior

Describing Atoms Atomic Number = Number of protons in an atom In a neutral atom, the number of protons = the number of electrons.

Let’s Practice… An atom has 9 positive protons. Where are the protons located in an atom? The Nucleus How many negative electrons does it have? 9 electrons What is the atom’s atomic number? Atomic Number = 9 (same as protons)

Let’s Practice… A neutral atom has 13 negative electrons. Where are electrons located in an atom? Electron Shells How many positive protons will this atom have? 13 positive protons What is this atom’s atomic number? Atomic Number = 13 (same as protons)

In summary.. 1. Atoms make up EVERYTHING! 2. Atoms are made of 3 smaller subatomic particles: Protons (+) Electrons (-) Neutrons (0) 3. The number of protons is the same as electrons. 4. We identify atoms by their atomic number, which is the same as their number of protons.

But what about neutrons? The number of neutrons in an atom can change! Remember the charge of a neutron? Would adding or removing neutrons do anything to the charge of the atom? No! The charge of a neutron is zero. We could add as many neutrons as we wanted, but the atom wouldn’t become more positive or more negative.

How to find the # of Neutrons In addition to Atomic Numbers, atoms also have one more number called the Atomic Mass. The Atomic Mass is how heavy the nucleus of an atom is – so how heavy the protons and neutrons are together. Example: An atom with 6 protons and 7 neutrons in its nucleus has an atomic mass of… 13 amu.

Let’s Practice… An atom has 8 protons and an Atomic Mass of 19 amu. How many electrons will this atom have? 8 electrons What is the Atomic Number of this atom? Atomic Number = 8 (same as the protons) How many neutrons will this atom have? (Hint: Atomic Mass - # protons = # neutrons) 19 – 8 = 11 neutrons