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Atoms • Once believed to be the smallest particles that made up matter • Nucleus: Protons and Neutrons. ~same mass • Protons and neutrons are made of quarks. • Electron Cloud: Electrons. Much smaller mass than protons and neutrons. • Protons are positively charged • Neutrons have no charge • Electrons are negatively charged • Atoms are neutral since they have the same number of protons and electrons. Atomic Structure • The nucleus is located in the center of the atom. • The electron cloud which surrounds the nucleus contains electrons spinning around the nucleus. Current Atomic Model • The smallest particles of matter are now believed to be the quark and the electron. • Protons and Neutrons are made from quarks. Electron Arrangements • The electrons are arranged in energy levels within the electron cloud. • Each energy level or shell is labeled with a number or letter. For example K-Shell or energy level 1 • Each energy level or shell can hold a maximum number of electrons: #e- = 2n2 where n = energy level. Level 1 holds 2 electrons. • Electrons closer to the nucleus have the lowest kinetic energy because of attractive forces between the electrons and protons. Atomic Structure Diagrams • Elements are made up of atoms with a specific atomic number (number of protons) • For example, all Helium elements contain 2 protons (atomic number 2). • Atomic structure diagrams show how the electrons are arranged in the different energy levels or shells within the electron cloud. Atomic Structure Diagram E E E E E P N E E E E E E P= protons, N= neutrons found in nucleus. How many protons should this atom have? If its mass number is 23, how many neutrons should it have? Properties of Atoms • Atomic radius – the distance from the nucleus to the outer edge of the electron cloud. • Ionization energy - The energy required to remove the outermost (highest energy) electron from a neutral atom in its ground state in order to form a cation. • Electron affinity - The energy given off when a neutral atom in the gas phase gains an extra electron to form a negatively charged ion. • Electronegativity - a measure of the attraction of an atom for the electrons in a chemical bond. Ions • If an atom gains or loses electrons, it forms an ion. • Cations – positively charged due to loss of electrons. • Anions – negatively charged due to gain of electrons. Isotopes • An atom’s mass number is the total number of protons and neutrons. • When like atoms of the same element have different number of neutrons, they are called isotopes. • Isotopes of one element all have the same number of protons (atomic number, Z) but different numbers of neutrons (thus different atomic weights, A). • Every element has several isotopes Science Changes Over Time • How did the following theories change – Democritus thought that atoms were the smallest particles that make up matter. – Dalton thought that atoms could not be created, divided, or destroyed. – Dalton thought that all atoms of the same element were exactly alike. – Scientists accepted that protons, neutrons, and electrons were the smallest subatomic particles.