BSCI 3234 Lecture Notes - Lecture 2: Electron Shell, Electron Configuration, Relative Atomic Mass

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Introduction to Microbiology
Lecture 2- Chemical Principles
Some Definitions:
An atom is
- The smallest unit of matter that reacts chemically.
• contains positively charged protons in the nucleus
• contains neutrally charged neutrons in the nucleus Fig. 2.1
• negatively charged electrons orbit the nucleus
• The number of electrons is equal to the number of protons.
Atomic number is the number of protons in the nucleus.
Atomic weight is approximately equal to the total number of protons plus neutrons.
Example: Sodium, atomic number = 11 and atomic weight = 22.99
Element: Definition: A substance that cannot be broken down by chemical reactions into
simpler substances.
• All the atoms of an element behave the same way chemically.
• All the atoms of an element have an identical number of protons.
• There are 92 naturally occurring elements.
• All elements are designated by a one or two letter symbol (e.g. H-hydrogen, O-
oxygen)
• Many elements have isotopes. An isotope is an atom with a different number of
neutrons in the nucleus. Isotopes are useful for tracking the movement of a
compound for medical or scientific purposes.
• Example: Carbon 14 is an unstable isotope of carbon used for dating of ancient
material and fossils.
• There are 26 elements commonly found in living organisms
• The four most abundant elements in living organisms are hydrogen, oxygen,
carbon, and nitrogen (H,O,C,N)
Electron configurations in Atomic Structure
Electrons move (orbit) in a very ordered manner
• 2 electrons in the innermost shell
• up to 8 electrons in the second shell
• up to 8 electrons in the third shell
• up to 18 electrons in the 4th , 5th, and 6th shells
Why is electron configuration important for understanding the behavior of elements?
• Atoms interact chemically with other atoms in a way that allows them to fill the outermost
electron shell.
Example: Helium--atomic number 2 = inert gas shell 1 is filled.
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When the outermost shell is not filled with electrons, the atom is unstable and reactive with
other molecules that have electrons to share.
An atom can give up, accept, or share electrons.
Valance: Combining capacity of an atom = the number of extra or missing electrons
Examples:
H--valance 1 (one unfilled, only one electron)
O--valance 2 (two unfilled spaces in outer shell)
C--valance 4 (four unfilled, four extra electrons in outer shell)
Mg--valance 2 (2 extra electrons in outer shell)
What is a chemical compound?
- A chemical compound is a substance composed of two or more different chemical
elements.
Example H2O is a compound of 2 hydrogen atoms and 1 oxygen atom.
Chemical Bonds
What is the definition of a chemical bond?
- A chemical bond is an attractive force between atoms forming a molecule.
• Energy is required for bond formation
Ionic Bond-- bond formed due to gain or loss of electrons. Weaker bond than covalent bond.
Example of an ionic bond
Figure 2.2
Na+ -- 11 protons, 11 electrons, electron donor
Cl- -- 17 protons, 17 electrons, electron acceptor
What is a covalent bond?
- A chemical bond formed when electrons are shared.
Example of Covalent Bond
Fig. 2.3
• Electrons are shared
• Bonds are strong
• Each pair of electrons shared = a single covalent bond
Hydrogen Bonds
• Very weak bonds compared to ionic or covalent bonds, but important in biological
materials
• Occur when a hydrogen atom that is covalently bonded to one oxygen or nitrogen
atom is attracted to another oxygen or nitrogen
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Document Summary

Atomic number is the number of protons in the nucleus. Atomic weight is approximately equal to the total number of protons plus neutrons. Element: definition: a substance that cannot be broken down by chemical reactions into simpler substances. An isotope is an atom with a different number of neutrons in the nucleus. 2 electrons in the innermost shell up to 8 electrons in the second shell up to 8 electrons in the third shell up to 18 electrons in the 4th , 5th, and 6th shells. Electrons move (orbit) in a very ordered manner. Why is electron configuration important for understanding the behavior of elements: atoms interact chemically with other atoms in a way that allows them to fill the outermost electron shell. Example: helium--atomic number 2 = inert gas shell 1 is filled. An atom can give up, accept, or share electrons.

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