Chapter 4

“Atomic Structure”

Subatomic particles of the atomAtoms are the building blocks of all

Matter.

Atoms are mostly empty space with

a small dense core known as the

Nucleus.

Atoms are made up of 3 smaller

particles (Subatomic Particles) :

Protons, Neutrons, and Electrons

Nucleus Makes up most of the

mass of the atom.

Made up of the following

subatomic particles:

Protons: positively

charged

Neutrons: neutral particles

Has an overall + charge.

Protons

positively charged;

Located inside nucleus;

gives the atom its identity

has the mass of 1atomic mass unit (amu)

Same size as a neutron (1amu)

Protons + Neutrons = the mass of the

atom (atomic mass)

Neutrons

neutral charge;

inside nucleus;

has the mass of 1atomic mass unit (amu)

Same size as a proton (1amu)

Protons + Neutrons = the mass of the

atom (atomic mass)

Electrons negative charged

has the mass of 1/1840

**note- electrons are extremely small compared to

Protons & Neutrons

Located in the electron cloud

Electron Cloud- Area outside the nucleus

where you are most likely able to find a

nucleus.

Subatomic ParticlesParticle Charge Mass Location

Electron

(e-) -1 1/1840 amu Electron

cloud

Proton

(p+) +1 1 amu Nucleus

Neutron

(no) 0 1 amu Nucleus

Amu= atomic mass unit= a unit used to measure the particles of an atom. = 1/12th the

mass of a carbon atom

Section 4.3

Distinguishing Among Atoms

Atomic Number

Elements are different because they contain

different numbers of PROTONS.

The number of Protons gives the atom its

identity.

The “atomic number” of an element is the

number of protons in the nucleus.

Atomic Number

(# of protons)

What makes an Atom Neutral? When an atom has the same number of protons

as electrons. (+) = (-)

if the atom is neutral then it has not lost or

gained electrons.

Atomic charge- an atom will have a charge when

the protons and the electrons are not equal in

number. Atoms will lose or gain electrons to

become charged.

Atoms that have a charge are called ions.

Example: A sodium atom has 11 protons and 10

electrons. What is the charge? +11 protons (+)

-10 electrons (-)

--------------------Subtract

+1 charge

2 Protons (+)

6 Electons (-)

------------------------

-4 charge

What is the Charge of these atoms?

Which one is an Ion?

6 Protons (+)

6 Electons (-)

------------------------

0 charge**** Ion ****

Isotopes Atoms of the same element can have

different numbers of neutrons.

Thus, different mass numbers.

These are called isotopes.

Isotopes are atoms of the same

element having different masses,

due to varying numbers of neutrons. Elements occur in nature as mixtures of

isotopes.

How to Name Isotopes

Write the name then put the

mass number after the name

of the element:

carbon-12 shorthand: C-12

carbon-14 shorthand: C-14

uranium-235 shorthand: U-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

The 3 Isotopes of Hydrogen

Mass number vs Atomic Mass Atomic Mass- Atomic mass is the

average of all the naturally

occurring isotopes of that element. the decimal you see on the periodic table

An average of all the isotopes of that element.

Mass number- the number of

protons and neutrons in the

nucleus of a specific isotope The mass of that specific isotope.

Atomic

Mass

Mass Number

Mass number is the number of

protons and neutrons in the nucleus

of an isotope:

Neutrons = Mass # - p+

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 Aka: Isotope Notation

Contain the symbol of the element,

the mass number and the atomic

number.

XMass

number

Atomic

numberSubscript →

Superscript →

Symbols

Find each of these:

a) number of protons

b) number of

neutrons

c) number of

electrons

d) Atomic number

e) Mass Number

Br80

35

Symbols

If an element has an atomic

number of 34 and a mass

number of 78, what is the:

a) number of protons

b) number of neutrons

c) number of electrons

d) complete symbol

Symbols

If an element has 91

protons and 140 neutrons

what is the

a) Atomic number

b) Mass number

c) number of electrons

d) complete symbol

Symbols

If an element has 78

electrons and 117 neutrons

what is the

a) Atomic number

b) Mass number

c) number of protons

d) complete symbol

Complete Symbols of an ION

If the symbol has a superscript on

the right side, it has lost or gained

electrons.

XMass

number

Atomic

numberSubscript →

Superscript → # of electrons

lost or gained

• Negative number = #

of electrons it has

gained

• Positive number = #

of electrons it has lost

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

Practice Problem:

Element X has 2 Natural occurring isotopes. Isotope 10X has the

mass of 10.012 and a relative abundance of 19.91% . The other

istotope 11X has the mass of 11.009 and the abundance of 80.09%.

Calculate the average atomic mass of this element.

Mass X Abundance (changed to a decimal)

10X: 10.012amu X 0.1991 = 1.993 amu11X: 11.009amu X 0.8009 = 8.817 amu

---------------------------------------------------Add--------------

Average Atomic Mass = 10.810 amu

The Periodic Table:

A Preview

A “periodic table” is an

arrangement of elements in which

the elements are separated into

groups based on a set of repeating

properties

The periodic table allows you to

easily compare the properties of

one element to another

The Periodic Table:

A Preview

Each horizontal row (there are 7 of

them) is called a period

Each vertical column is called a

group, or family

Elements in a group have similar

chemical and physical properties

Identified with a number and

either an “A” or “B”

More presented in Chapter 6