Section 2

The Structure of an Atom

Key Concepts

• What are three subatomic particles?

• What properties can be used to compare protons, electrons, and neutrons?

• How are atoms of one element different from atoms of other elements?

• What is the difference between two isotopes of the same element?

Properties of Subatomic Particles

• By 1920, Rutherford had seen evidence for the existence of two subatomic particles and had predicted the existence of a third particle.

• Protons, electrons, and neutrons are subatomic particles.

Protons

• A proton is a positively charged subatomic particle that is found in the nucleus of an atom.

• Each proton is assigned a charge of +1.

Electrons

• An electron is a negatively charged subatomic particle that is found in the space outside the nucleus.

• Each electron has a charge of -1.

Neutrons

• In 1932, the English physicist James Chadwick designed an experiment to show that neutrons exist

• A neutron is a neutral subatomic particle that is found in the nucleus of an atom.

• It has a mass almost exactly equal to that of a proton.

Comparing Subatomic Particles

• Protons, electrons, and neutrons can be distinguished by mass, charge, and location in an atom.

• Atomic Number• The atomic number of an element equals

the number of protons in an atom of that element.

• Atoms of different elements have different numbers of protons.

• Each positive charge in an atom is balanced by a negative charge because atoms are neutral.

• So the atomic number of an element also equals the number of electrons in an atom.

Atomic Number and Mass Number

• What is the atomic number of the following elements.

• Potassium (K)

• Oxygen (O)

• Iron (Fe)

• Helium (He)

19

8

26

2

Atomic Number and Mass Number

• Mass Number• The mass number of an atom is the sum

of the protons and neutrons in the nucleus of that atom.

• An atom of aluminum with 13 protons and 14 neutrons has a mass number of 27.

• If you know the atomic number and the mass number of an atom, you can find the number of neutrons by subtracting.

Number of neutrons = Mass number – Atomic number

Atomic Number and Mass Number

• What is the Atomic Mass of the following elements

• Potassium (K) - Neutrons = 20

• Oxygen (O) – Neutrons = 8

• Iron (Fe) – Neutrons = 30

• Helium (He) – Neutrons = 2

+ 19 = 39

+ 8 = 16

+ 2 = 4

+ 26 = 56

Atomic Structure

Element Symbol Atomic Number

Mass Number

# of Protons

# of Neutron

# of Electron

Helium He 2 4 2 2 2

Magnesium Mg 12 24 12 12 12

Zinc Zn 30 65 30 35 30

Bromine Br 35 80 35 45 35

Aluminum Al 13 27 13 14 13

Uranium U 92 238 92 146 92

Sodium Na 11 23 11 12 11

Krypton Kr 36 84 36 48 36

Calcium Ca 20 40 20 20 20

Silver Ag 47 108 47 61 47

Isotopes

• Isotopes are atoms of the same element that have different numbers of neutrons and different mass numbers.

• Isotopes of an element have the same atomic number but different mass numbers because they have different numbers of neutrons.

Isotopes • Hydrogen atom and it’s 3 isotopes• Hydrogen isotopes are the only isotopes to

have special names.

Hydrogen Deuterium Tritium

Isotopes

IsotopesElement Protons Neutrons Mass Natural %Hydrogen 1 0 1 99.99Deuterium 1 1 2 0.01Helium-3 2 1 3 1.3*10-4

Helium 2 2 4 100Carbon 6 6 12 98.90Carbon-13 6 7 13 1.10Nitrogen 7 7 14 99.64Nitrogen-15 7 8 15 0.37Oxygen 8 8 16 99.76Oxygen-17 8 9 17 0.04Oxygen-18 8 10 18 0.20

Questions

• What is the difference between Oxygen-16 and Oxygen-18?

• Ans. Different numbers of neutrons• How are they the same?• Ans. The same number of protons• What part of the atom do isotopes have to

do with?• Ans. The nucleus

Reviewing Concepts• 1. Name three subatomic particles.• 2. Name three properties you could use to

distinguish a proton from an electron.• 3. Which characteristic of an atom always varies

among atoms of different elements?• 4. How are the isotopes of an element different

from one another?• 5. What do neutrons and protons have in

common? How are they different?• 6. How can atoms be neutral if they contain

charged particles?• 7. What is the difference between atoms of

oxygen-16 and oxygen-17?