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Chapter 2 The Chemistry of Life
Chemical Elements
-all matter is composed of atoms
-atom consists of a dense, positively charged nucleus & negatively charged electrons orbit
around the nucleus
-nucleus contains protons & neutrons
-atomic mass unit (amu) is mass of proton
-mass of electrons can be ignored because its mass is very low
-proton has positive electric charge, +1
-electron has negative charge equal to that of proton, -1
-neutron is neutral & has no charge
-when number of proton equals number of electron, atom is neutral
Element
-element is pure substance that contains only one kind of atom
-6 main elements found in organism: carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur
Protons
-atomic number is the number of proton
-mass number is the total number of proton & neutron in the nucleus
-atomic number is written at the lower left, mass number at the upper left
Neutrons
-isotope is element that has the same number of proton, but different number of neutron
-atomic mass/ atomic weight is the average mass of element based on the relative abundance of
various isotopes
-radioisotope is unstable & gives off energy in the form of alpha, beta or gamma radiation from
atomic nucleus
-can be used as tag and label to trace organism
-radioactive decay is the release of energy that transforms original atom
->can change number of protons, resulting in different elements
Electrons
-impossible to determine the exact location of electron at any given time
-can only estimate the volume of space electron is located in orbital
-1st electron shell contains 1 s-orbital, hold up to 2 electrons
-2nd electron shell contain 4 orbitals (1 s-orbital, 3 p-orbital), hold up to 8 electrons
-higher energy level means electron needs more energy to overcome the pull of positively
charged nucleus
-s-orbital (the lowest energy level) fills with electrons first
-valence (outermost electron shell) determines how atom behaves chemically
-atom is stable when valence shell reaches maximum number of electrons
-stable elements are helium neon, argon (noble gas)
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Document Summary

Atom consists of a dense, positively charged nucleus & negatively charged electrons orbit around the nucleus. Atomic mass unit (amu) is mass of proton. Mass of electrons can be ignored because its mass is very low. Electron has negative charge equal to that of proton, -1. When number of proton equals number of electron, atom is neutral. Element is pure substance that contains only one kind of atom. 6 main elements found in organism: carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur. Mass number is the total number of proton & neutron in the nucleus. Atomic number is written at the lower left, mass number at the upper left. Isotope is element that has the same number of proton, but different number of neutron. Atomic mass/ atomic weight is the average mass of element based on the relative abundance of various isotopes. Radioisotope is unstable & gives off energy in the form of alpha, beta or gamma radiation from atomic nucleus.

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