ATOMIC STRUCTURE Chapter 4. Section Overview 4.1: Defining the Atom 4.2: Structure of the Nuclear Atom 4.3: Distinguishing Among Atoms

ATOMIC STRUCTUREChapter 4

Section Overview• 4.1: Defining the Atom• 4.2: Structure of the Nuclear Atom• 4.3: Distinguishing Among Atoms

DEFINING THE ATOMSection 4.1

Early Models of the Atom• An atom is the smallest particle of an element that retains

its identity in a chemical reaction.• Democritus’s Atomic Philosophy: Believed that atoms were

indivisible and indestructible. However, his idea lacked explanation of chemical behavior and experimental support

• Dalton’s Atomic Theory: Transformed Democritus’s ideas on atoms into a scientific theory using experiments relying on the scientific method.• All elements are compose of atoms.• Atoms of the same element are identical. Atoms of different elements

are different.• Atoms of different elements can mix together or chemically combine

to form compounds.• Chemical reactions occur when atoms are separated, joined, or

rearranged.

Sizing up the Atom• Despite their small size, individual atoms are observable

with instruments such as scanning tunneling microscopes.• Individual atoms can even be moved around and

arranged into patterns.• This holds promise for thee creation of atomic-sized

electronic deices.• This is called the “nanoscale” and could become essential

to future applications in medicine, communications, solar energy, and space exploration.

STRUCTURE OF THE NUCLEAR ATOMSection 4.2

Subatomic Particles• One important change to Dalton’s theory today is that

atoms are now known to be divisible.• They can be broken down into three types of subatomic

particles: electrons, protons, and neutrons.• Electrons: Negatively charged subatomic particle

(discovered by J. J. Thomson). The mass (discovered by Robert A. Millikan) is 1/1840 the mass of hydrogen.

• Protons: Positively charged subatomic particles (discovered by Eugen Goldstein) with a mass about 1840 times that of an electron.

• Neutrons: No charge (discovered by James Chadwick) with a mass almost equal to a proton.

*The net charge of an atom is neutral*

The Atomic Nucleus• After the discovery of the subatomic particles, many scientists

wondered how these were arranged inside of an individual atom.

• Rutherford’s Gold-Foil Experiment: Set up an experiment where he directed a narrow beam of alpha particles at a thin sheet of gold foil. The hypothesis was that the alpha particles would go straight through with only a slight deflection. However, the great majority passed through without any deflection with some bouncing straight back.

• Rutherford’s Atomic Model: His experiment led to a new model of the atom in which the atom is mostly empty space (explaining lack of deflection) with a nucleus in the center.

* In the atomic atom, protons and neutrons are located in the nucleus with the electrons distributed around it occupying most of the volume*

DISTINGUISHING AMONG ATOMSSection 4.3

Atomic Number• Elements are different because they contain a different

number of protons.• The atomic number of an element is the number of

protons in the nucleus of that atom (ex. All hydrogen atoms have one proton, so their atomic number is one).

• Since the net charge of an atom is neutral, the number of protons are equal to the number of electrons.

Mass Number• The mass of an atom is concentrated in the nucleus and

therefore depends on the number of protons and neutrons.

• The total number of protons and neutrons an atom is called the mass number (ex. Helium has two protons and two neutrons and therefore a mass number of 4).

• The number of neutrons in an atom is the difference between the mass number and the atomic number.

• The composition of an atom is written using the symbol, the atomic number, and the mass number (ex. 197Au).

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Determining the Composition of an Atom

• Example Problem: How many protons, electrons, and neutrons are in each atom?

a. Beryllium (Be) atomic #=4 mass #=9

b. Neon (Ne) atomic #=10 mass #=20

c. Sodium NA) atomic#=11 mass#=23



a. Beryllium (Be) atomic #=4 mass #=9• Solution:

Knowns Unknowns

Atomic # # of protons # of neutrons

Mass # # of electrons

Remember: # of protons = atomic # and # electrons=# of protons

So: 4 protons and 4 electrons

Remember: # of neutrons = mass # - atomic #

So: 9-5 = 5 neutrons



b. Neon (Ne) atomic #=10 mass #=20• Solution:

Knowns Unknowns









c. Sodium NA) atomic#=11 mass#=23• Solution:

Knowns Unknowns







Isotopes• Sometimes, atoms of the same element do differ.• Isotopes are atoms that have the same number of protons

but different numbers of neutrons.• Because isotopes have a different number of neutrons,

they all have a different mass number.• Isotopes are still chemically alike because they still have

the same number of protons and electrons.

Atomic Mass• Scientists use a mass spectrometer to determine the mass of

atoms.• Although this can be useful, it is more useful to compare the

relative masses of atoms using a reference isotope as a standard.

• The standard used is carbon-12.• An atomic mass unit (amu) is defined as one twelfth of the

mass of a carbon-12 atom.• The atomic mass of an element is a weighted average mass of

the atoms in a naturally occurring sample of that element which takes into account isotopes.

• To calculate the atomic mass, multiple the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.

Atomic Mass• Example Problem: Element X has two natural isotopes.

The isotope with a mass of 10.012 amu has a relative abundance of 19.91%. The isotope with a mass of 11.009 amu has a relative abundance of 80.09%. Calculate the atomic mass of this element.

Atomic Mass• Example Problem: Element X has two natural isotopes. The

isotope with a mass of 10.012 amu has a relative abundance of 19.91%. The isotope with a mass of 11.009 amu has a relative abundance of 80.09%. Calculate the atomic mass of this element.

• Solution:

Knowns Unknown

Isotope X-10 Isotope X-11 atomic mass of X

Mass= 10.012 amu Mass= 11.009 amu

Abundance=80.09% Abundance=19.91%

10.012 amu x 0.1191 = 1.993 amu

11.009 amu x 0.8009 = 8.817 amu

1.993 amu + 8.817 amu = 10.810 amu

The Periodic Table - Preview• A periodic table is an arrangement of elements in which

the elements are separated into groups based on a set of repeating properties.

• A periodic table allows you to easily compare the properties of one element (or a group of elements) to another element (or group of elements).

• The atomic number is above each element symbol and the atomic mas is below each symbol.

• Each horizontal row is called a period.• Each vertical column is called a group. Elements in the

same group have similar chemical and physical properties.

The Periodic Table - Preview

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ATOMIC STRUCTURE Chapter 4. Section Overview 4.1: Defining the Atom 4.2: Structure of the Nuclear Atom 4.3: Distinguishing Among Atoms