Section 4–2:The Structure of an Atom

Coach KelsoePhysical SciencePages 108–112

Objectives

• Identify three subatomic particles and compare their properties.

• Distinguish the atomic number of an element from the mass number of an isotope, and use these numbers to describe the structure of atoms.

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.

• There are three subatomic particles – – Protons– Electrons– Neutrons

Protons

• Rutherford determined that the amount of positive charge varies among elements.

• Each nucleus must contain at least one particle with a positive charge. These particles are called 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

• The particles that Thomson detected were later named electrons.

• An electron is a negatively charged subatomic 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. He concluded that the particles he produced were neutral because a charged object did not deflect their paths.

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

• Protons and neutrons have almost the exact same mass, but electrons are 1/2000 the size of protons. Although an electron is smaller in mass, they have an equivalent negative charge to a proton’s positive charge.

Atomic Number

• The atoms of any given element always have the same number of protons. There are always 3 protons in lithium and 4 in beryllium.

• The atomic number of an element equals the number of protons in an atom of that element.

Atomic Number

• Atoms of different elements have different numbers of protons. This is what gives atoms their identity!

• 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. Lithium has 3 electrons; beryllium has 4.

Mass Number

• The atomic number tells you the number of protons in the nucleus of an atom, but doesn’t tell you how many neutrons there are.

• The mass number of an atom is the sum of the protons and neutrons in the nucleus of that atom.

• For example, an atom of carbon that has 6 protons and 7 neutrons would have a mass number of 13.

Mass Number

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

• Example: 23 (MN of Na) – 11 (AN of Na) = 12

Isotopes

• In Dalton’s atomic theory, he states that all atoms of a given element are identical. Every atom of a given element has the same number of protons as electrons.

• However, atoms of the same element can have different masses due to the number of neutrons!

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

• For example, there are three types of oxygen: oxygen-16, oxygen-17, and oxygen-18. The number at the end represents the mass number.

• All of these oxygen atoms have 8 protons (its atomic number is 8), but the number of neutrons will change:– Oxygen 16 16-8 = 8 neutrons– Oxygen 17 17-8 = 9 neutrons– Oxygen 18 18-8 = 10 neutrons

Vocabulary

• Proton• Electron• Neutron• Atomic number• Mass number• Isotopes