The Chemical Basis of Life Chapter 4 Required Elements Structure of Atoms Chemical Bonds Properties of Water 4.1 - Life Requires About 25 Chemical Elements • Matter – Elements • 25 Essential • Trace Elements – Compounds What is Matter? Matter Not Matter How is Matter Organized? •Divided into pure substances or mixtures based on composition. –Pure substance •Uniform composition with same properties throughout •Can be classified as either an element or a compound Element •Cannot be broken down into simpler substances •Made of only one type of atom •Represented by 1-2 letters 25 Essential Elements • • 25 of 92 natural elements are essential to life CHNO – 96% of living organisms Trace Elements • Elements that make up less than 0.01 percent of you body mass, but are critical to your health. Compound •Made from two or more simpler substances •Can be broken down into those simpler parts •Always contains 2 or more elements 4.2 – Chemical Properties are Based on the Structure of Atoms • Atoms • Subatomic Particles – Electrons • Reactivity – Nucleus • Protons – Atomic Number • Neutrons – Isotopes – Radioactive Isotopes • Electrons and Reactivity The Atom • The atom is the smallest part of an element that retains all the characteristics of that element. • A Greek scientist named Democritus was one of the first to propose that all matter was composed of tiny particles called atoms Sub Atomic Particles • Electrons orbit the nucleus (a little) like planets round the sun. Electrons reside in specific shells or orbitals. • The three parts of the atom are called sub atomic particles. Sub Atomic Particles • The nucleus of an atom contains protons and neutrons. • They are held together by a strong force which keeps the positively charged protons from repelling one another.