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What is an atom? What is an Atom? • An atom is the smallest unit of ordinary matter. What particles are in an atom? Particles in Atoms • Protons • Neutrons • Electrons What are the charges of each particle? • Protons are Positive • Neutrons are Neutral • Electrons are Negative Nucleus • The nucleus of the atom is made up of protons and neutrons. Cloud of Electrons • Electrons move within a sphere-shaped region surrounding the nucleus • Most of an atom’s volume is the space where electrons move • Let’s say you are holding a pencil while standing at the pitcher’s mound in a baseball stadium. The nucleus would be the size of the pencil’s eraser. The electrons could be as away as the top row seats! Particle Masses • Mass of 2,000 electrons = Mass of 1 proton • Mass of 1 proton = Mass of 1 neutron Particle Masses • The protons and neutrons make up nearly all the mass of an atom Particle Masses • Particles too small to be measured in grams/kilograms • Scientists use atomic mass units (amu) Particle Masses • A proton or neutron has a mass equal to about one amu • An electron has a mass of about 1/2000 amu. What is an Element? What is an Element? • An atom with a specific number of protons. Oxygen Atom Atomic Number • The number of protons in an element’s nucleus. Atomic Mass • The sum of the protons and neutrons in the nucleus of an atom Isotopes • All atoms of the same element have the same number of protons • However, the number of neutrons can vary. Isotopes • Atoms with the same number of protons and a different number of neutrons are called isotopes Isotopes • An isotope is identified by its mass number. • (Mass number = protons + neutrons) Atomic Model • Atoms are so small, scientists create models to describe them