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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 behaviour of the atom. Scientific Models Scientists create models to help them to visualize complex properties, structures or behaviours. 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 coloured 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. Is this really an Atom? Many of the models that you have seen may look like the one below. It shows the parts and structure of the atom. Even though we do not know what an atom looks like, scientific models must be based on evidence. The model above represents the most modern version of the atom. (Artist drawing) Subatomic Particles Particle Symbol Charge Relative Mass Electron e- 1- 0 Proton p+ 1+ 1 Neutron n 0 1 Why are all Atoms 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. Location of Subatomic Particles 10-13 cm electrons protons neutrons nucleus 10-8 cm Atomic Number Counts the number of protons in an atom Atomic Number on the Periodic Table Atomic Number 11 Symbol Na All atoms of an element have the same number of protons 11 protons 11 Sodium Na Mass Number Counts the number of protons and neutrons in an atom Number of Electrons An atom is neutral The net charge is zero Number of protons = Number of electrons Atomic number = Number of electrons Subatomic Particles in Some Atoms 16 O 8 8 p+ 8n 8 e- 31 P 65 Zn 15 30 15 p+ 16 n 15 e- 30 p+ 35 n 30 e- Learning Check Write the atomic symbols for atoms with the following: A. 8 p+, 8 n, 8 e- ___________ B. 17p+, 20n, 17e- ___________ C. 47p+, 60 n, 47 e- ___________ Solution A. 8 p+, 8 n, 8 e- 16O 8 B. 17p+, 20n, 17e- 37Cl 17 C. 47p+, 60 n, 47 e- 107Ag 47 Make a diagram of the Helium atom in your notes.