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# Structure of an Atom ## Mass number Protons and neutrons have approximately the same mass. Electrons have a mass so small it can be ignored. The mass of an atom depends on how many protons and neutrons it has: Mass number = number of protons + number of neutrons ## The Arrangement of Electrons...

# Structure of an Atom ## Mass number Protons and neutrons have approximately the same mass. Electrons have a mass so small it can be ignored. The mass of an atom depends on how many protons and neutrons it has: Mass number = number of protons + number of neutrons ## The Arrangement of Electrons Bohr and Rutherford worked out a theoretical model for the atom in which the electrons are arranged in layers around the nucleus. Each layer or shell can hold a certain number of electrons. The first shell (called the K shell) can hold a maximum of two electrons. The second shell (called the L shell) can hold a maximum of eight electrons, and the third shell (called the M shell) can hold up to eighteen electrons but is stable at eight. The electrons build up from the first shell, filling each one until all the electrons are in place. This means all the electrons are as close to the nucleus as they can be. The table shows how the electrons are arranged for the first 18 elements. | Element | Symbol | Atomic Number | Number of First Shell Electrons (K) | Second Shell (L) | Third Shell (M) | Electric Configuration | |---|---|---|---|---|---|---| | Hydrogen | H | 1 | 1 | | | 0 | | Helium | He | 2 | 2 | | | | | Lithium | Li | 3 | 2 | 1 | | | | Beryllium | Be | 4 | 2 | 2 | | | | Boron | B | 5 | 2 | 3 | | | | Carbon | C | 6 | 2 | 4 | | | | Nitrogen | N | 7 | 2 | 5 | | | | Oxygen | O | 8 | 2 | 6 | | | | Fluorine | F | 9 | 2 | 7 | | | | Neon | Ne | 10 | 2 | 8 | | | | Sodium | Na | 11 | 2 | 8 | 1 | | | Magnesium | Mg | 12 | 2 | 8 | 2 | | | Aluminium | Al | 13 | 2 | 8 | 3 | |