Atomic Structure Chapter 4

What is an atom? Draw a circle map for atoms Atom: the smallest particle of an element that retains its identity in a chemical reaction

The Atomists: The first atomic theory 460 BCE: Greek Democritus suggested that matter is “ composed of minute, invisible, indestructible particles of pure matter which move about eternally in infinite empty”

Dalton’s Atomic Theory John Dalton ( ), an English schoolteacher and chemist, Studied the theories and the results of experiments by other scientists. He formed a hypothesis, experimented, and came up with a theory. Dalton proposed his atomic theory of matter in 1803.

Dalton’s Atomic Theory (The main points) 1. All matter is made up of indivisible particles called atoms. 2. All atoms of one element are exactly alike, but are different from atoms of other elements. 3.Atoms mix in simple whole number ratios to form compounds. 4.Chemical reactions happen when atoms are separated, joined, or rearranged. * Atoms of one element can NEVER be turned into atoms of a different element!

Modern Change to Dalton’s Theory… Atoms are NOT indivisible! Atoms can be broken into three subatomic particle: 1.Electrons 2.Protons 3.Neutrons

Review Dalton’s Atomic Theory (shortened) 1.All matter is made of indivisible atoms 2.All Na atoms are the same but are different from He atoms 3.Atoms mix in whole number ratios 4.Chemical reactions happen when atoms are separated, joined, or rearranged

Modern Change to Dalton’s Theory… Atoms are NOT indivisible! Atoms can be broken into three subatomic particle: 1.Electrons 2.Protons 3.Neutrons

Discovery of the Electron Because of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts. In 1897, a British physicist, J.J. Thomson, discovered that this solid-ball model was not accurate.

Cathode-Ray Tube Cathode Ray: a stream of electrons produced at the negative electrode of a tube containing a gas at low pressure Thomson knew that objects with like charges repel each other, and objects with unlike charges attract each other

Cathode-Ray Tube

The Electron Thomson named the negative particles found in his experiment electrons. Electrons (e - ): –Are the smallest subatomic particles –Have a charge of -1

You don’t get shocked when you touch EVERYTHING So … there must be something positively charged in the atom to balance the electrons.

Rules about Electrically Charged Atoms 1.Atoms have no overall charge (neutral) 2.Electrical charges are carried by matter 3.Electrical charges always exist in whole numbers 4.# of negatively charged particles = # of positively charged particles = neutral atom

Protons Protons (p + ): –Positively charged subatomic particles (+1) –Mass 1 proton = mass 1840 electrons –Very, very heavy when compared to electrons

At this point, it seemed that atoms were made up of equal numbers of electrons and protons However, in 1910, Thomson discovered that neon consisted of atoms of two different masses

Isotopes Atoms of an element that are chemically alike but differ in mass are called isotopes of the element. So why the difference in mass?

Neutrons Neutrons (n 0 ): Neutral charge Same mass as a proton Because of the discovery of isotopes, scientists hypothesized that atoms contained still a third type of particle that explained these differences in mass.

Cookie Dough Model

Rutherford’s Gold Foil Experiment In 1909, a team of scientists led by Ernest Rutherford in England carried out the first of several important experiments that revealed an arrangement far different from the cookie-dough model of the atom.

The Gold Foil Experiment

The Nuclear Model of the Atom Since most of the particles passed through the foil, they concluded that the atom is nearly all empty space.

The Nuclear Model of the Atom Nucleus: Contains most of the mass of the atom (protons and neutrons) Is very small compared to the rest of the atom the atom has a small, dense, positively charged central core, called a nucleus.

Draw an atom

Bracing Map of an atom

Review

Elements are different because they have different numbers of protons How do you figure out the # of protons?

Atomic Number : the number of protons in the nucleus of an atom of that element Atomic number identifies the element

Where to find the atomic number on the periodic table… H 1 Atomic Number # of protons in the nucleus of the atom

Remember elements are electrically neutral…and protons are positively charged So if the atomic number is the number of protons ….. Then it must also be the number of electrons in order to balance the atom

Practice What is the atomic number of carbon? –How many protons? –How many electrons? What is the atomic number of aluminum? –How many protons? –How many electrons?

From looking at the periodic table you can tell any element’s number of electrons and protons

You know you want to know how many of these particles are in an atom, right? Atomic Number : # of protons in the atom Mass number: # of protons + # neutrons

How to get the info from the periodic table H Atomic Number: Number of protons in the nucleus of the atom. Atomic Mass: Average mass of all isotopes of that element. Hey, why isn’t this a whole number?

The atomic mass unit (amu) Atoms don’t have much mass! ( g) Use carbon-12 isotope as a ref. and make 12 carbon atom= 1 amu On the periodic table the value is a weighted average of all isotopes of the element – that is why it isn’t a whole number.

What is a weighted average? Let’s calculate your grade….

In this chapter you will fill out lots of tables… NameProtonsElectronsNeutrons Helium224-2 = 2 Carbon = 6 Carbon =7 Carbon-12 and Carbon-13 are isotopes

How many subatomic particles? # protons = atomic number (always) For a NEUTRAL atom # electrons=# protons (this will be a bit different in upcoming chapters…) # neutrons = mass number - # protons (always)

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