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The building blocks of all matter Today We Will… Explain the Atomic Theory Construct the Bohr Model Identify the parts of an Atom Identify atomic number, symbol and mass number. Atomic - Molecular Theory of Matter The Atomic - Molecular Theory of Matter states that all matter is composed of small, fast moving particles called atoms. These atoms can join together to form molecules. This theory is really thousands of individual theories that provide evidence for the whole theory. Matter Since the atom is too small to be seen even with the most powerful microscopes, scientists rely upon models to help us to understand the atom. Believe it or not this is a microscope. Even with the world’s best microscopes we cannot clearly see the structure or behavior of the atom. Scientific Models Scientists create models to help them to visualize complex properties, structures or behaviors. Since the atom is so small, scientists must gather Indirect Evidence to develop their models. This is a model of a very complex molecule made of many different kinds of atoms. Each colored ball represents an atom of a different element. What should a Model look like? This is a painting of a young woman by Pablo Picasso. Does it actually look like a young woman? Scientific models may not always look like the actual object. A model is an attempt to use familiar ideas to describe unfamiliar things in a visual way. Can a Model be Changed? A model can be changed as new information is collected. From the early Greek concept to the modern atomic theory, scientists have built upon and modified existing models of the atom. The Bohr Model Invented by Niels Bohr Depicts protons and neutrons in a nucleus. Electrons around the nucleus in what is called an electron cloud Atomic Structure Atoms are composed of 2 regions: – Nucleus: the center of the atom that contains the mass of the atom – Electron Cloud : the region that surrounds the nucleus that contains most of the space in the atom 1. ___________________ 2._________________ What’s in the nucleus? The nucleus contains 2 of the 3 subatomic particles. Proton: positively charged subatomic particles Neutron: neutrally charged subatomic particles What’s in the electron cloud? Electron: the third subatomic particle with a negative charge and relatively no mass. Why are all Atoms are Electrically Neutral? Normally in an atom the number of electrons within the electron cloud is equal to the number of protons in the nucleus. The positive and negative charges cancel each other out. Therefore, the atom is said to be electrically neutral. If an atom gains or loses electrons the atom is no longer neutral . This can happen if the atom absorbs or releases energy The atom is then called an ION. IMPORTANT In all ATOMS the number of positively charged protons is always equal to the number of negatively charged electrons. Atomic Number _ Element Symbol Element Name Atomic Mass 1 H Hydrogen 1.008 How do the subatomic particles balance each other? In an atom: protons = electrons If 20 protons are present in an atom then 20 electrons are there to balance the overall charge of the atom. The neutron have neutral charge therefore they do not have to equal the number of protons or electrons. How do we know the number of subatomic particles in an atom? Atomic number: indicates the number of protons in an atom. – Ex: Hydrogen’s atomic number is 1, so hydrogen has _______ proton. – Ex: Carbon’s atomic number is 6 , so carbon has _____ protons. **The number of protons identifies the atom – 2 p = _He_______ 29 p = __Cu_____ How do we know the number of subatomic particles in an atom? Mass number: the number of protons and neutrons in the nucleus. Ex:Hydrogen can have a mass of 3. Since it has 1 proton, it must have 2 neutrons. **number of neutrons = Mass # - Atomic # Determining the number of protons and neutrons: Li has a mass # of 7 and an atomic number of 3 – Protons = 3 ( same as the atomic #) – Neutrons = 7 – 3 = 4 ( Mass # - Atomic #) Ne has a mass number of 20 and an atomic number of 10. – Protons = 20 – Neutrons = 10 What about the electrons? The electrons are equal to the number of protons. So electrons = protons = atomic number – Ex: He has a mass number of 4 and an atomic number of 2 p = ___2___ n = ___2____ e-= __2____ Determine the number of subatomic particles in the following: Cl has a mass number of 35 and an atomic number of 17 p = 17 n = 18 e- = 17 K has a mass number of 39 and an atomic number of 19. – p = 19 n = 20 e- = 19 How exactly are the particles arranged? Bohr Model of the atom: The 3rd ring can hold up to 18 eThe 4th ring and any after can hold up to 32 e- All of the protons and the neutrons The 1st ring can hold up to 2 eThe 2nd ring can hold up to 8 e- What does carbon look like? Mass # = 12 atomic # = 6 6 p and 6 n live in the nucleus p+ = ____ ____ no = ____ e- = Changing Electrons An atom that changes the number of electrons is called an ion. The number of protons and neutrons stays the same. No change in atomic number or mass number. What makes an ion? If the atom loses electrons, the atom becomes positively charged. If an atom gains electrons, the atom becomes negatively charged. Positive ion (4 +) + (3-) = 1+ Negative ion (4 +) + (5-) = 1- Changing Neutrons Changing the number of neutrons in an atom makes an isotope. Have the same atomic number Have different mass numbers (number of protons + neutrons) Sample Problem 1: For an atom with 15 protons, 16 neutrons, and 18 electrons… What is the atom’s net charge? What is the atomic number of the atom? What is the mass number? This is an atom of what element?