General Arrangement of the Subatomic Particles 1 1896

General Arrangement of the Subatomic Particles 1

1896 • Radioactivity was discovered by Henri Becquerel. • Radioactive elements spontaneously emit alpha particles, beta particles, and gamma rays from their nuclei. 2

1907 • Ernest Rutherford found that alpha particles emitted by certain radioactive elements were helium nuclei. 3

• Rutherford in 1911 performed experiments that shot a stream of alpha particles at a gold foil. • Most of the alpha particles passed through the foil with little or no deflection. • He found that a few were deflected at large angles and some alpha particles even bounced back. 4

Rutherford’s alpha particle scattering experiment. 5 5. 5

• An electron with a mass of 1/1837 amu could not have deflected an alpha particle with a mass of 4 amu. • Rutherford knew that like charges repel. • Rutherford concluded that each gold atom contained a positively charged mass that occupied a tiny volume. He called this mass the nucleus. 6

• If a positive alpha particle approached close enough to the positive mass it was deflected. • Most of the alpha particles passed through the gold foil. 1 in 8000 deflected. This led Rutherford to conclude that a gold atom was mostly empty space. 7

• Because alpha particles (He nucleus, 2 protons and 2 neutrons) have relatively high masses, the extent of the reflections led Rutherford to conclude that the nucleus was very heavy and dense. 8

Deflection Scattering Deflection and scattering of alpha particles by positive gold nuclei. 9 5. 5

• Rutherford was astonished at the result: “It was quite the most incredible event that ever happened to me in my life. It was as incredible as if you fired a 15 -inch shell at a piece of tissue paper and it came back and hit you!” 10

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• Rutherford’s experiment showed that an atom had a dense, positively charged nucleus. • Chadwick’s work in 1932 demonstrated the atom contains neutrons. • Rutherford also noted that light, negatively charged electrons were present in an atom and offset the positive nuclear charge. 12

• Rutherford put forward a model of the atom in which a dense, positively charged nucleus is located at the atom’s center. • The negative electrons surround the nucleus. • The nucleus contains protons and neutrons 13

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Nucleus • contains protons and neutrons, but does not contain electrons. 15

Atomic Numbers of the Elements 16

• The atomic number of an element is equal to the number of protons in the nucleus of that element. • The atomic number of an atom determines which element the atom is. 17

Every atom with an atomic number of 1 is a hydrogen atom. Every hydrogen atom contains 1 proton in its nucleus. 18

Every atom with an atomic number of 6 is a carbon atom. Every carbon atom contains 6 protons in its nucleus. 19

atomic number U 92 Every atom with an atomic number of 92 is a uranium atom. 92 protons in the nucleus 20

Isotopes of the Elements 21

• Atoms of the same element can have different masses. • They always have the same number of protons, but they can have different numbers of neutrons in their nuclei. • The difference in the number of neutrons accounts for the difference in mass. • These are isotopes of the same element. 22

Isotopic Notation 23

Isotopic Notation 6 protons + 6 neutrons 12 C 6 6 protons 24

Isotopic Notation 6 protons + 8 neutrons 14 C 6 6 protons 25

Isotopic Notation 8 protons + 8 neutrons 16 O 8 8 protons 26

Isotopic Notation 8 protons + 9 neutrons 17 O 8 8 protons 27

Isotopic Notation 8 protons + 10 neutrons 18 O 8 8 protons 28

Hydrogen has three isotopes 1 proton 0 neutrons 1 neutron 2 neutrons 29

Examples of Isotopes Element Protons Electrons. Neutrons. Symbol Hydrogen Uranium Chlorine 1 1 1 92 92 17 17 0 1 2 143 146 18 20 1 1 H 1 2 H 3 H 92 1 235 U 92 238 U 17 35 Cl 17 37 Cl 30

Atomic Mass 31

• The mass of a single atom is too small to measure on a balance. • Using a mass spectrometer, the mass of the hydrogen atom was determined. 32

A Modern Mass Spectrometer Positive ions formed from sample. Electrical field From the intensity and positions at slits A mass of the lines Deflection on the mass of accelerates spectrogram, the different ions positive ions. positive is recorded. isotopes and their at relative occurs amounts can be determined. magnetic field. 5. 8 33

A typical reading from a mass spectrometer. The two principal isotopes of copper are shown with the abundance (%) given. 5. 9 34

Using a mass spectrometer, the mass of one hydrogen atom was determined to be 1. 673 x 10 -24 g 35

Relationship Between Mass Number and Atomic Number 36

The mass number minus the atomic number equals the number of neutrons in the nucleus. mass number atomic mass number 109 47 = = number of neutrons 62 37

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