Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 4 “Atomic Structure” Section 4.1 Defining the Atom ■ OBJECTIVES: ■ Describe Democritus’s ideas about atoms. Section 4.1 Defining the Atom ■ OBJECTIVES: ■ Explain theory. Dalton’s atomic Section 4.1 Defining the Atom ■ OBJECTIVES: ■ Identify what instrument is used to observe individual atoms. Section 4.1 Defining the Atom Have you ever been asked to believe in something that you can’t see? Yes, frequently! • The unaided eye cannot see the tiny fundamental particles that make-up all matter. • Yet all matter is composed of such particles… they’re called atoms. • An atom is the smallest particle of an element that retains its identity in a chemical reaction. The concept of the atom intrigued a number of early scholars… Although these philosophers and scientists could not see and observe individual atoms, they were still able to propose ideas (hypotheses) about the structure of atoms. Section 4.1 Defining the Atom The Atomic Model of Democritus • The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms. He said matter consisted of tiny, indivisible, unchangeable particles called atoms. • Democritus believed that atoms were indivisible and indestructible • Although his ideas did agree with later scientific theory, they did not explain chemical behavior • They also lacked supporting evidence because his approach was not based on the scientific method, just philosophy • His ideas were later challenged by the famed Greek philosophers, Plato and Aristotle Section 4.1 Defining the Atom The Atomic Model of John Dalton Remember: A model is used to study and explain things that are too large, too small, or too complex to easily see (common models include globes, subway maps, and weather maps) John Dalton (1766 – 1844) • The real nature of atoms and the connection between observable changes and events at the atomic level were not established for more than 2000 years after Democritus. • John Dalton, an English Chemist and school teacher, used experimental methods to transform the ideas of Democritus into a scientific theory. Section 4.1 Defining the Atom The Atomic Model of John Dalton • Dalton studied the ratios in which elements combined during chemical reactions (his experiments showed that the ratios in which elements combined were whole numbers) • Based on the data from his experiments, he then formulated additional hypotheses and theories to explain his observations and tested these • The result was Dalton’s Atomic Theory – based on experimental results and able to explain chemical behavior • Dalton’s Atomic Theory consists of four parts: Section 4.1 Defining the Atom The Atomic Model of John Dalton 1.All elements are composed of tiny indivisible particles called atoms. 2.Atoms of the same element are identical. The atoms of any one element are different from those of any other element. 3.Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. 4.Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction. Section 4.1 Defining the Atom The Atomic Model of John Dalton Section 4.1 Defining the Atom Sizing up the Atom Let’s use something we’re all familiar with, a US penny, to further understand Dalton’s Atomic Theory… • Pre-1857 pennies were made of pure copper (Cu) • Imagine grinding the copper penny into dust • Each speck of shiny red dust still has the properties of copper • If we had a way to make each speck even smaller, we would eventually come upon a particle of copper that could no longer be divided and still have the properties of copper • This indivisible speck of copper is an atom Section 4.1 Defining the Atom Sizing up the Atom • Copper atoms are very small • A pure copper penny contains about 2.4 x 1022 atoms or 24,000,000,000,000,000,000,000 atoms • For the sake of comparison, the Earth’s population is only about 7.4 x 109 humans (7,400,000,000 humans) • In other words, there are many more atoms in the coin than there are humans on the Earth! • If you could line up 100 million copper atoms side-by-side in a line, it would only be 1 cm long! Section 4.1 Defining the Atom Sizing up the Atom • Though atoms of different elements do vary in size, the radius of a single atom usually falls within the range of: 5 x 10-11 m to 2 x 10-10 m • It may seem impossible, but despite their small size, individual atoms are observable with scientific instruments such as a scanning tunneling microscope (STM) • The designers of the STM won the 1986 Nobel Prize in Physics for the design of this amazing tool Section 4.1 Defining the Atom Sizing up the Atom An STM works by holding a one-atom sized tip over a sample of matter that conducts electricity. Electric current flows between the tip and the sample. The current is stronger when the tip and the sample are close and weaker when they are farther apart. This change in voltage is interpreted to show surface topography. Section 4.1 Defining the Atom Sizing up the Atom Section 4.1 Defining the Atom Actual STM images of atoms – note the arrangements Above: Iron (Fe) atoms on a Copper (Cu) surface Above: Nickel (Ni) atoms Left: Silicon (Si) atoms Section 4.1 Defining the Atom Sizing up the Atom • What are we actually seeing in the STM images? Essentially, the images show the distribution range for the electrons of each atom -more on that in the future, but pretty neat, huh? • Individual atoms can not only be seen, but actually moved around and arranged in patterns. • The ability to move individual atoms holds future promise for the creation of atom-sized electronic devices such as circuits and computer chips! • This atomic-scale, “nanoscale,” technology has many potential uses and could one day become essential in medicine, communications, solar energy, and space exploration. Section 4.1 Defining the Atom Your assignments for today (due tomorrow) 1. Complete the Reading Guide/Review WS for 4.1 2. Complete the 4.1 Section Assessment # 1-7 on p. 103 (write the questions and the answers – type or write, your choice)