Atoms

Tiny Particles of Matter

Start of the atom• Dalton 1808 atomic theory

• Moseley 1st used atomic number

• 1913 Bohr used planetary model of atom

Atoms- Early Models• Democritus- first suggested that

there was tiny particles- atoms

• Dalton- provide scientific basis for atoms, and their chemical reactivity (1808

Atomic Theory

• Play atomic theory video clip

Dalton’s Atomic Theory• 1) All matter is made of atoms. Atoms are

indivisible and indestructible.• 2) All atoms of a given element are identical in mass and properties• 3) Compounds are formed by a combination of

two or more different kinds of atoms.• 4) Chemical reaction occurs when atoms separate,

or rearrange. Atoms never change into atoms of another element as a result of a chemical reaction

Ernest Rutherford’sGold Foil Experiment - 1911

Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil Particles that hit on the detecting screen (film) are recorded

Rutherford’s problem:In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

Target #1

Target #2

The Answers:

Target #1 Target #2

Rutherford’s Findings

a) The nucleus is smallb) The nucleus is densec) The nucleus is positively

charged

Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected

“Like howitzer shells bouncing off of tissue paper!”

Conclusions:

The Rutherford Atomic Model• Based on his experimental evidence:

• The atom is mostly empty space• All the positive charge, and almost all

the mass is concentrated in a small area in the center. He called this a “nucleus”

• The nucleus is composed of protons and neutrons (they make the nucleus!)

• The electrons distributed around the nucleus, and occupy most of the volume

• His model was called a “nuclear model”

Bohr Model• Niels Bohr in 1913 came up with the Bohr

model to explain how electrons are arranged around the nucleus of an atom.

• He showed that electrons move around the nucleus of an atom in an orbit• Like planets around the sun

Bohr Model

Timeline• In 1700 there were 13 elements

• In 1869 there were 26 elements

• In 1908 there were 81 elements

• Now there are 118

Mass of Atom

• The mass of 1 amu is about 1.67 x 10-24 grams.

• The proton is 1.0073 amu and the neutron is 1.0087 amu, which is essentially equal in mass.

• The mass of the electron is 0.000549u, or about 1/2000 the mass of a proton.

Size of Atoms

• If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long

• A scanning tunneling microscope allows scientist to see atoms

• These are nickel atoms from STM

Subatomic particles• Electrons, protons and neutrons

• The nucleus of the atom contains protons and neutrons

• Electrons revolve around the nucleus

Protons and Neutrons• 1886 Protons found

• Mass = 1 amu

• Positive charge

• Neutrons found in 1932• Neutral charge cathode ray video

Discovery of the ElectronIn 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively

charged particle: the electron

Modern Cathode Ray Tubes

Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Television Computer Monitor

Mass of the Electron

1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge

The oil drop apparatus

Mass of the electron is 9.11 x 10-28 g

Atomic Number• Atoms are composed of identical

protons, neutrons, and electrons• How then are atoms of one element

different from another element?

• Elements are different because they contain different numbers of PROTONS

• The “atomic number” of an element is the number of protons in the nucleus

• # protons in an atom = # electrons

Mass Number

Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p+ + n0

Nuclide p+ n0 e- Mass #

Oxygen - 10

- 33 42

- 31 15

8 8 1818

Arsenic 75 33 75

Phosphorus 15 3116

Complete Symbols• Contain the symbol of the element,

the mass number and the atomic number.

X Massnumber

Atomicnumber

Subscript →

Superscript →

SymbolsSymbols Find each of these: Find each of these:

a)a) number of protonsnumber of protons

b)b) number of number of neutronsneutrons

c)c) number of number of electronselectrons

d)d) Atomic numberAtomic number

e)e) Mass NumberMass Number

Br80 35

Isotopes• Elements have different versions

• Each version has a different number of neutrons so different mass

• Same element different # neutrons

• Keeps same chemical properties

• Play elements and Isotope video

Isotopes• Dalton was wrong about all

elements of the same type being identical

• Atoms of the same element can have different numbers of neutrons.

• Thus, different mass numbers.• These are called isotopes.

Naming Isotopes• We can also put the mass

number after the name of the element:•carbon-12

•carbon-14

•uranium-235

Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.

Isotope Protons Electrons Neutrons Nucleus

Hydrogen–1

(protium) 1 1 0

Hydrogen-2

(deuterium) 1 1 1

Hydrogen-3

(tritium)

1 1 2

Atomic Mass How heavy is an atom of oxygen?

It depends, because there are different kinds of oxygen atoms.

We are more concerned with the average atomic mass.

This is based on the abundance (percentage) of each variety of that element in nature. We don’t use grams for this mass because

the numbers would be too small.

Measuring Atomic Mass• Instead of grams, the unit we use

is the Atomic Mass Unit (amu)• It is defined as one-twelfth the

mass of a carbon-12 atom.• Carbon-12 chosen because of its isotope purity.

• Each isotope has its own atomic mass, thus we determine the average from percent abundance.

To calculate the average:• Multiply the atomic mass of

each isotope by it’s abundance (expressed as a decimal), then add the results.

• If not told otherwise, the mass of the isotope is expressed in atomic mass units (amu)

Atomic Masses

Isotope Symbol Composition of the nucleus

% in nature

Carbon-12 12C 6 protons

6 neutrons

98.89%

Carbon-13 13C 6 protons

7 neutrons

1.11%

Carbon-14 14C 6 protons

8 neutrons

<0.01%

Atomic mass is the average of all the naturally occurring isotopes of that element.

Carbon = 12.011

Electrons• Electrons determine the physical and chemical

properties of an element• P.T. classifies elements by electron configuration

(4 groupings)1. Noble gases2. Representative elements3. Transition elements4. Inner transition elements• Look at bottom of pg. 395

Electrons

• Play electron video clip

Energy Levels• Electrons in a particular path have a

fixed energy• Electrons don’t lose energy so they don’t

fall into the nucleus• The energy level is the region around the

nucleus where electrons are likely to be found

Energy Levels• Like the rungs of a ladder

• Electrons close to the nucleus have less energy

• For electrons to move from energy level to the next it must gain or lose the right amount of energy

• The farther away the less force the nucleus has on the electron, so it is easier for the electron to leave the atom

Energy Levels• The P.T. can help determine

electron levels

• Valence electrons are outer electrons- affect reactions

• Oxidation numbers follow Group numbers

Energy Levels• Like the rungs of a ladder

• Electrons close to the nucleus have less energy

• For electrons to move from energy level to the next it must gain or lose the right amount of energy

• The farther away the less force the nucleus has on the electron, so it is easier for the electron to leave the atom

Quantum Mechanical Model

• Just like the Bohr model but• Electrons don’t have set orbits

• It uses probability to show where an electron could be located.

• Math based

• Electron cloud• Areas of high probability

Electron Cloud

Energy Levels• Principal energy level= major levels• Sublevels, each principal level has a set

# of sublevels that coincides with the number of principal level• 1 = 1 sublevel• 2 = 2 sublevels

Energy Levels• Orbitals are areas where electrons are

likely to be found• Letters denote the orbitals

• S= spherical • p= dumbbell shape• d and f

Energy levels• The number of electrons in a principal

energy level is based on 2n2

• 1 2 3 4 p.level

• 2 8 18 32 sub

Orbitals• s= 1 orbitals

• P= 3 orbitals

• D= 5 orbitals

• F= 7 orbitals

Electron Configuration• 3 rules pg.367-3681. Electrons enter orbitals of lowest

energy 1st

2. An atomic orbital describes at most two electrons

3. When electrons occupy orbitals of equal energy, one electron enters each orbital until they are all full.