Atomic Structure

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Expectations

Students know how to relate the position of an element in the periodic table to its atomic number and atomic mass.

Students know the nucleus of the atom is much smaller than the atom yet contains most of its mass.

Modern Atomic Theory

All matter is composed of atoms Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!

Atoms of an element have a characteristic average mass which is unique to that element.

Atoms of any one element differ in properties from atoms of another element

The Atom

The atom consists of two parts:

1. The nucleus which contains:

2. Orbiting electrons.

protonsneutrons

Protons

• Protons are positively charged, and are located in the nucleus– p– Same number as electrons

Electrons

• Negatively charged. They orbit around the nucleus.– e-– Same number as protons

Neutrons

• No charge (neutral). Found in the nucleus. Can be a different number of these than either protons or electrons

– Note: This is important

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

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

Conclusions from the Study of the Electron

Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.

Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons

Electrons have so little mass that atoms must contain other particles that account for most of the mass

Thomson’s Atomic Model

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

Rutherford’s Gold Foil Experiment

Alpha () particles are helium nuclei Particles were fired at a thin sheet of

gold foil Particle hits on the detecting screen

(film) are recorded

Rutherford’s Findings

The nucleus is small The nucleus is dense 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:

Atomic Particles

Particle Charge

Mass # Location

Electron -1 0 Electron cloud

Proton +1 1 Nucleus

Neutron 0 1 Nucleus

Atomic NumberAtomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.

Element # of protons Atomic # (Z)

Carbon 6 6

Phosphorus 15 15

Gold 79 79

Mass NumberMass 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

IsotopesIsotopes 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 Masses

Isotope Symbol Composition of the nucleus

% in nature

Carbon-12

12C 6 protons6 neutrons

98.89%

Carbon-13

13C 6 protons7 neutrons

1.11%

Carbon-14

14C 6 protons8 neutrons

<0.01%

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

Carbon = 12.011

Electrons and Bonding

000

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Electrons

• For neutral atoms (just sitting around, not doing anything):

# electrons = # protons

Atomic number can tell you the # electrons WHEN NEUTRAL

+

-

Hydrogen

+

+

++

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-

-

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Beryllium

Valence Electrons

• In outer most energy level

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Beryllium

Responsible for chemical properties & how element reacts

How many valence electrons?• Look at the group number at the top

of each column.• This only works for the tall stacks. If it’s double digits, look at the one’s position only.

18888888888888

GroupsThe columns

GroupsGroupsGroups

Groups• All elements in a

group (column) have the same number of valence electrons.

+

-

Hydrogen

00

0

0+

++-

-

-

Lithium

They share common chemical properties; they react similarly.

Try determining the valence…

• Determine the valence electrons for…– C (carbon)– H (Hydrogen)– Ne (neon)

Periods

Peri

od

sP

eri

od

sP

eri

od

s

The rows

The Periods

• Each successive row has one more energy level than the previous one.

+

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Hydrogen

+

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-

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Beryllium

Row #1:1 energy level

Row #2:2 energylevels

Set up in rows because certain properties repeat periodically.

Compound

• Substance formed when atoms of different elements combine chemically

• Properties of a compound are different than the properties of the elements that form it

Why do atoms chemically combine?

• To become STABLE• For most atoms, this means obtaining 8

valence electrons• Exception- the first energy level is full

with only 2 electrons• But to be stable, atoms lose their

neutrality (they gain a positive or negative charge)

What do you mean they lose their neutrality?

• In its neutral state, Be looks like this.

000

00+

+

++

-

-

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Beryllium

It’s NOT STABLE

because it has only 2

valence electrons.

Finding 6 to fill the outer

shell takes too much

energy, SO it will ditch the

outer 2 leaving the full shell underneath.

Therefore…

• In giving away its two valence electrons, its new outer shell is full = STABLE

000

00+

+

++

-

-

Beryllium

BUT now it has 4 protons (+) and only 2 electrons (-) Has a +2 charge now = NOT NEUTRAL

000

00

-

-+

+

+++

+

+++

+

+++

+

++

-

-

-

-

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So how do atoms chemically combine?

• They can SHARE electrons• Or they can GIVE AWAY or TAKE electrons• Only the valence electrons are involved

Covalent Bonds

• Form when atoms SHARE their valence electrons

• These electrons circulate between the valence shells of both atoms

• Usually occurs between two nonmetals• Forms a molecule: a particle whose

atoms are covalently bonded

Ionic Bonds• Formed by atoms GIVING and TAKING

valence electrons• Each atom becomes a charged particle or

ION• Opposite charges attract, so the atoms

stick together• Usually forms between a metal and

nonmetal– Metals give away electrons– Nonmetals take electrons