Atoms Unit 3 Introduction to the Atom + -

Some of the First Discussions about the Atom…. Democritus (400 BC) named the atom, which means indivisible, and said it was the smallest particle and could not be broken down. He was pretty much correct in that if you did break the atom down, it wouldn’t be that “element” any more. But he wasn’t aware that the atom was made up of particles.

Greeks Philosophers Continue the Discussion and Disagree Aristotle (350 BC) believed matter could always be broken down into smaller and smaller parts. And this is true to a certain extent… but eventually you come to the small basic particles that can no longer be broken apart.

His ideas are now called the Atomic Theory of Matter Dalton's Atomic Theory In 1808, John Dalton proposed that elements were composed of atoms & that only whole numbers of atoms can combine to form compounds His ideas are now called the Atomic Theory of Matter Dalton’s Atomic Theory (1766 - 1844 AD) was widely accepted but not totally correct

Dalton’s Atomic Theory All matter is composed of extremely small particles called atoms Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, & other properties ELEMENT 1 ELEMENT 2 ELEMENT 3 ELEMENT 4 Dalton’s Atomic Theory

Atoms cannot be subdivided, created, or destroyed atoms of different elements combine in simple whole # ratios to form chemical compounds in chemical reactions, atoms are combined, separated, or rearranged + +

Not all of Dalton’s claims held up to the scrutiny of experimentation Atoms CAN be divided into even smaller particles Not every atom of an element has an identical mass (isotopes are atoms of the same element, but with different mass numbers) Atomic Theory

Atomic Theory Dalton’s Atomic Theory of Matter has been modified. What remains is… All matter is composed of atoms Atoms of any one element differ in properties from atoms of another element One of the disputed statements of Dalton was that atoms are indivisible (we now know about protons, electrons, and neutrons) Atomic Theory

In the 1800’s it was determined that atoms are actually composed of several basic types of smaller particles It’s the number and arrangement of these particles that determine the atom’s chemical properties. A new definition of an atom is the one use today: the smallest particle of an element that retains the chemical properties of that original element. Atomic Theory

Plum Pudding Model JJ Thomson’s cathode ray tube experiment in the late 1800s showed that atoms had smaller parts, called negative corpuscles; he developed the “plum pudding model.” The plum pudding model showed electrons (the plums) mixed in together with protons (the cake batter)

Mg - - + + + - - + - - + + + - - - - + + + + - + - Negative particles embedded in a sphere of positive plasma-like matter. THINK… Chocolate Chip Cookie Mg - - + + + - - + - - + + + - - - - + + + + - + -

Scientists still didn’t really understand how the particles were put together in an atom. This was a difficult question to resolve, given how tiny atoms are. Most thought it likely that the atom resembled Thomson’s model Atomic Structure

Rutherford’s gold foil experiment In 1911, Ernest Rutherford provided a more detailed picture of the internal structure of the atom He showed that atoms had a nucleus where most of the mass resided. His model had a nucleus with a hard dense positively charged mass, negatively charged electrons outside, and that the atom was actually mostly empty space.

Rutherford Model

Bohr Model of the Atom Neils Bohr put electrons into different energy levels or shells. (This model is not correct either… electrons do not travel in orbits or paths like the model suggests)

Modern Day Theory (Electron Cloud Theory) The Modern Theory suggests that electrons are located somewhere in a cloud.

Basic and important fact to remember: All atoms contain the same basic parts, but atoms of different elements have different numbers of protons. The Periodic Table lists atoms in consecutive order by their Atomic Number The atomic number is directly related to the number of protons in the nucleus of each atom of that element

Atoms have: A nucleus small, dense part of the atom consists of protons and neutrons An electron cloud large part of the atom that is empty space except for the electrons that are moving very fast and very randomly around the nucleus Nucleus Electron Cloud

The total number of protons & neutrons determines the mass of the atom Called the “Mass Number” (atomic mass is the averaged mass of the isotopes and is given on the periodic table. Simply round the atomic mass to get the mass number) A Carbon atom, has 6 protons and 6 neutrons, so its mass number is 12 If you know the atomic number & mass number of an atom of any element, you can determine the atom’s composition and the number of neutrons.

The Proton The protons are what give the atom its charge (+) They add mass to the atom as well. Each proton is equal to one AMU (atomic mass unit) The number of protons in an atom determines what element it is. The atomic number signifies the number of protons Protons are held together in the nucleus by the “Strong Force” otherwise they would repel each other.

The Neutron The neutron adds mass (1 amu) to an atom but has NO charge Atoms of the same elements are identical due to the number of protons, but there can be different numbers of neutrons (we call those isotopes) To find the number of neutrons, subtract the atomic number from the mass number.

Electrons Electrons have almost no mass, and we DO NOT count their mass. Located outside the nucleus in the electron cloud (aka: shells, orbitals, energy levels) moving at incredibly high speeds. Electrons have a negative (-) charge. Electrons found in the outermost shells of the atom are responsible for chemical reactions. Electrons have different amounts of energy depending what energy level they are at. Electrons can be removed and added to atoms quite easily, unlike protons.

Subatomic Particles Particle Symbol Charge Relative Mass Electron e- 1- 0 Proton p+ + 1 Neutron n 0 1

Location of Subatomic Particles 10-13 cm electrons protons neutrons 10-8 cm nucleus

in an atom and determines what element it is Atomic Number Counts the number of protons in an atom and determines what element it is

Atomic Number on the Periodic Table 11 Na Atomic Number Symbol The symbol represents the element. RULE: The first letter is always capitalized, and IF there is a second letter, it is lower case.

All atoms of an element have the same number of protons 11 Na 11 protons Sodium

Atomic Mass on the Periodic Table 11 Na 22.99 Atomic Number Symbol Atomic Mass Atomic mass is the weighted average mass of all the atomic masses of the isotopes of that atom. That is why there is a decimal.

Mass Number Counts the number of protons and neutrons in an atom (note: Atomic Mass is different from Mass Number. On your periodic table of elements, the atomic mass is usually given and you need to round it to the nearest whole number to use to figure Neutrons)

Atomic Notation atomic number 11 Show the mass number and atomic number Give the symbol of the element mass number 23 Na sodium-23 atomic number 11

Number of Electrons An atom is neutral when no charge is indicated. The net charge is zero Remember: Atomic number = Number of protons and therefore…. Number of protons = Number of electrons when the atom is neutral.

Subatomic Particles Showing the P E N 16 31 65 O P Zn 8 15 30 8 p+ 15 p+ 30 p+ 8 e- 15 e- 30 e- 8 n 16 n 35 n

Isotopes Atoms with the same number of protons, but different numbers of neutrons. Atoms of the same element (same atomic number) with different mass numbers Isotopes of chlorine 35Cl 37Cl 17 17 chlorine - 35 chlorine - 37

Learning Check Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12C 13C 14C 6 6 6 #p _______ _______ _______ #e _______ _______ _______ #n _______ _______ _______

Solution 12C 13C 14C 6 6 6 #p 6 6 6 #e 6 6 6 #n 6 7 8