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Introduction to Atoms Chapter 4 Democritus • Greek Philosopher • “uncutable particle” • Named the atom from the Greek word Atomos • Atoms are small, hard, different shapes and sizes, always moving and join together Aristotle • Greek philosopher • Disagreed with Democritus • Strong influence Dalton • British Chemist 1766 • Elements combine in specific proportions because they are made of individual atoms • He observed that no matter how large or small the sample, the ratio of the masses of the elements in the compound is always the same. • Compounds have a fixed position Dalton’s Theory • Dalton proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided. Main Points of Dalton’s Theory • All elements are composed of atoms • All atoms of the same element have the same mass, and atoms of different elements have a different masses. • Compounds contain atoms of more than one element. • In a particular compound, atoms of different elements always combine in the same way What did Dalton notice that all compounds have in common? • Dalton noticed that the ratio of masses of elements in a compound is always the same! Objects with like charges repel, or push apart. • Objects with opposite charges attract, or pull together. Thomson’s Experiments • He used a sealed tube of gas in his experiments. – When the current was on, the disks became charged and glowing beam appeared in the tube. – The beam bent toward a positively charged plate placed outside the tube. • He concluded that the particles in the beam had a negative charge because they were attracted to the positive plate. • Thomson’s experiments provided the first evidence that atoms are made of even smaller particles Thomson’s Model • Thomson found a negative charge, but knew that atoms were neutral. • Atoms must contain a positive charge as well. • The negative charges were evenly scattered throughout an atom filled with a positively charged mass of matter. • Plum Pudding Model Rutherford’s Atomic Theory (1871-1937) • He hypothesized that the mass and charge at any location in the gold would be too small to change the path of an alpha particle (positive charge). • When the gold foil experiment was done more particles were deflected and bounced straight back, as though they had struck an object. • The alpha particles must have come close to another charged particle • Rutherford concluded that the positive charge of an atom is not evenly spread through the atom. • It is concentrated in a very small, central area called the nucleus. • According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus. Bohr • Danish scientist • Discovered electrons travel around nucleus in definite paths • Located in levels around nucleus • Electrons can jump from one level to another Schrodinger and Heisenberg • Modern theory • Electron clouds surround nucleus • Electrons do not travel in definite paths • Movement cannot be predicted Chapter 4 Section 2 Atom Structure Nucleus • A dense, positively charged mass located at the center of the atom • New Model: All of an atom’s positive charge is concentrated in its nucleus. Properties of subatomic particles • 1. Protons electron • 2. Electrons • 3. Neutrons neutron proton Protons • Positively charged subatomic particle that is found in the nucleus of an atom • Charge of 1+ Electron • A negatively charged subatomic particle that is found in the space outside the nucleus • Charge of 1- Neutrons • Neutral subatomic particle that is found in the nucleus of an atom • Mass almost exactly equal to that of a proton Subatomic particles • Distinguished by their mass, charge and location in the atom Atomic Number • Atoms of any element have the same number of protons • Atomic number = number of protons in an atom of that element • Ex: Hydrogen has 1 proton, atomic number is 1 Mass Number • Sum of protons and neutrons in the nucleus of an atom • Number of neutrons = Mass number – Atomic Number Isotopes • Every atom of a given element has the same number of protons and electrons, but every element does not have the same number of neutrons. • Isotopes – have same atomic number, but different mass numbers because they have different number of neutrons Isotopes • Atoms that have the same number of protons but have a different number of neutrons • Atoms that are isotopes of each other are always the same element because the number of protons in each atom is the same Bohr’s Model of the Atom • Focused on Electrons that move with constant speed in fixed orbits around the nucleus • Electrons can change energy levels when the atom gains or loses energy Electron Cloud Model • Visual model used to find the locations for electrons Energy levels, orbitals and Electrons Energy Level # of Orbitals 1 1 Maximum # of Electrons 2 2 4 8 3 9 18 4 16 32 Forces that Work in Atoms •* •* •* •* Gravity Electromagnetic Force Strong Force Weak Force