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# Atomic Structure Is the Basis for Life's Chemistry ## 2.1 Atomic Structure Atoms are the fundamental building blocks of matter. Each atom consists of a dense, positively charged nucleus and negatively charged electrons orbiting around it. The nucleus contains protons and neutrons. * **Protons...

# Atomic Structure Is the Basis for Life's Chemistry ## 2.1 Atomic Structure Atoms are the fundamental building blocks of matter. Each atom consists of a dense, positively charged nucleus and negatively charged electrons orbiting around it. The nucleus contains protons and neutrons. * **Protons:** Have a positive charge and a mass of approximately 1 dalton (Da) * **Neutrons:** Have no charge and a mass of approximately 1 Da * **Electrons:** Have a negative charge and a negligible mass (about 1/2000th of a proton or neutron) Atoms are electrically neutral because the number of electrons equals the number of protons. ### Elements An element is a pure substance consisting only of one kind of atom. Different elements have unique characteristics. There are 94 naturally occurring elements and approximately 24 man-made elements. The six most abundant elements in living organisms are: * Carbon (C) * Hydrogen (H) * Oxygen (O) * Nitrogen (N) * Phosphorus (P) * Sulfur (S) ### Atomic Number and Mass Number * **Atomic number:** The number of protons in the nucleus of an atom. This is unique to each element. * **Mass number:** The total number of protons and neutrons in an atom. Carbon has an atomic number of 6, and an oxygen atom has an atomic number of 8. ### Electron Shells Electrons orbit the nucleus in different energy levels called electron shells. The first shell can hold a maximum of two electrons, and subsequent shells can hold up to eight electrons. An atom is most stable when its outermost electron shell is full (octet rule). Atoms with unfilled outer shells tend to react with other atoms to fill their outer shell, either by sharing electrons or gaining/losing electrons. These interactions often form molecules. ### Bohr Model The Bohr model illustrates an atom as a small, dense nucleus with electrons orbiting at fixed distances. While a simplified model, it helps visually represent electron distribution. ### Example: Oxygen Oxygen's atomic number is 8 and has 8 protons. To have a neutral charge, oxygen must contain 8 electrons. The diagram (included in the original image) shows how electrons fill the electron shells of an oxygen atom **Figure 2.1:** (The original image includes a diagram of the electron shell configurations for six major elements. The illustration is not included here.) Atoms with unfilled outer shells tend to react with other atoms, either by gaining/losing electrons, or sharing them, through chemical reactions. These reactions are vital for the formation of molecules and maintain the proper function of life processes.