The Chemistry of Life Part 1 Composition of Matter!!

Matter  Everything in the universe is made of matter, which is anything that has mass and takes up space.  Mass is the amount of matter an object has, and is not the same as weight, which is the force of gravity acting on the mass.

Matter  Which would you have less of on the moon than on Earth: mass or weight?  This is because the moon exerts less gravity than the Earth!

Atoms  The most basic unit of matter is the atom. The Greek philosopher Democritus first used the word atomos, which means “unable to be cut” nearly 2500 years ago!  Atoms are incredibly small. Placed side by side, 100 million atoms would make a row only about 1cm long (about the width of your pinkie!). Despite their extremely small size, atoms are made up of subatomic particles that are even smaller!

Atoms  The subatomic particles that make up an atom are: Name of particle Mass of particle Charge of particle

Atoms  Strong forces bind protons and neutrons together to form the nucleus, which is at the center of the atom.  Electrons are in constant motion in the space surrounding the nucleus. They are attracted to the positively charged nucleus but remain outside the nucleus because of the energy of their motion.

Atoms  Atoms have equal numbers of protons and electrons, so they have equal positive and negative charges. Therefore, atoms are said to be neutral.

Atoms  Picture of a helium atom:

Elements  If all the atoms in a substance are the same kind of atom, that substance is an element. More than 100 elements have been identified, with about 30 elements important to living things.

Elements  Living things are mostly made of these 4 elements: 1) oxygen 2) carbon 3) hydrogen 4) nitrogen

Elements  Information about the elements is summarized on a chart known as the Periodic Table.  What can you learn about an element from the Periodic Table?

Isotopes  All atoms of an element have the same number of protons. However, all atoms of an element do NOT necessarily have the same number of neutrons.  Atoms of the same element that have a different number of neutrons are called isotopes.

Isotopes  The more neutrons an atom has, the greater its atomic mass.  Most elements have a mixture of different isotopes, so the atomic mass you see on the Periodic Table is the average atomic mass of all the different isotopes of an element.

Isotopes  Some isotopes are radioactive, meaning that their nuclei are unstable and break down at a constant rate over time. The radiation these isotopes give off can be dangerous, but also has a number of important uses: 1)Determining ages of rocks and fossils 2)Treat cancer 3)Kill bacteria that contaminate food 4)Tracers to follow movement of substances in organisms

Compounds  In nature, atoms of most elements can readily combine with atoms of the same or different elements. Compounds are made up of atoms of two or more different elements in fixed proportions.

Compounds  A chemical formula shows the kinds and numbers of atoms of each element in a particular compound.  Example: water- made of 2 atoms of hydrogen and one atom of oxygen: H 2 O

Compounds  The physical and chemical properties of a compound are usually very different from those of the element from which it was formed.  (example: hydrogen and oxygen are both gases, but when they combine at room temperature, they form liquid H 2 O)

Compounds  Why do atoms form compounds? Short answer: to become stable  Ok, so what does THAT mean?

Compounds  Longer answer: an atom is stable when it’s outer energy level is filled with the maximum number of electrons it can hold. Some elements, such as helium and neon consist of atoms that already have the maximum number of electrons in their outer energy levels. These elements do not form compounds with other elements under normal circumstances.

Compounds  BUT…most atoms are not stable in their natural state, so they react with other atoms in different ways to fill their outer energy levels and become stable.

Compounds  Well, how do we know if an atom’s outer energy level is filled? Answer: The first energy level of any atom can hold up to 2 electrons. The second and third energy levels can hold up to 8 electrons.

Compounds  When atoms form compounds, the attractive forces that hold the atoms together are called chemical bonds.  The two main types of chemical bonds are covalent bonds and ionic bonds.

Covalent Bonds  A covalent bond forms when two atoms share one or more pairs of electrons in order to fill their outer energy levels.  This sharing of electrons is the attractive force that holds the two atoms together.

Covalent Bonds  Example: Hydrogen has 1 electron in its outer energy level and needs 2 to be stable. Oxygen has 6 electrons in its outer energy level and needs 8 to be stable.

Covalent Bonds  If two hydrogen atoms each share their one electron with an oxygen atom, oxygen now has 8 electrons in its outer energy level and is stable.  Oxygen will return the favor, sharing one of its electrons with each of the two hydrogen atoms.  Now they each have 2 electrons in their outer energy levels and they are stable too!

Ionic Bonds  An ionic bond is formed when one or more electrons is transferred from one atom to another. (no sharing here…this time, one atom completely gives up electrons, and another atom takes them for itself!)

Ionic Bonds  Remember that atoms are neutral in their natural state, with equal numbers of protons (+) and electrons (-).  When a neutral atom gives up one or more electrons, it now has more protons than electrons.  This gives the atom an overall positive charge.

Ionic Bonds  When a neutral atom gains one or more new electrons, it now has more electrons than protons.  This gives the atom an overall negative charge.

Ionic Bonds  Positively and negatively charged atoms are known as ions.  Oppositely charged ions have a strong attraction to each other.  This attraction between a positively charged ion and a negatively charged ion is called an ionic bond.

Ionic Bonds  Example: Sodium Chloride (NaCl) Sodium (Na) has 1 electron in its outer energy level. If it loses this electron, the second energy level becomes the outer energy level and it is filled with 8 electrons. Sodium is now stable. However, since it has lost an electron, the sodium atom now has a positive charge (it is a positive ion, Na + )

Ionic Bonds  Chlorine (Cl) has 7 electrons in its outer energy level. If it picks up the electron given up by sodium, it now has 8 electrons in its outer energy level and is stable.  However, since it gained an extra electron, the chlorine atom now has a negative charge (it is a negative ion, Cl - )

Ionic Bonds  Since sodium is now positive and chlorine is now negative, they are suddenly attracted to each other! This attraction is the ionic bond that holds these two atoms together! (You can be thankful for ionic bonds if you like salty foods!)