Section 4–2: The Structure of an Atom

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Section 4–2: The Structure of an Atom
Coach Kelsoe Physical Science Pages 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 – Atomic 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 Number of neutrons = Mass number – Atomic Number

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

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