Parts of an Atom

History of Atoms Atomos He was suggesting the Greek word meaning “Unable to be cut” Used by Democritus over 2,500 years ago He was suggesting the existence of what we call an atom.

First off, what is an atom? Atoms are the smallest parts to which an element can be divided yet still be that element. Atoms are the building blocks of all matter.

How small is an atom? An atom is so small that a penny contains about 2 x 1022 or 20,000,000,000,000,000,000,000 atoms!!!

What Makes Up All Matter? Element Something that can’t be broken down into simpler pieces. Exp: Gold Can be melted, divided into small pieces, hammered flat….still gold 92 Naturally Occurring 20 Man-made

But there’s more… Atoms consist of even smaller particles. These include: Protons Neutrons Electrons

Parts of an Atom The protons and neutrons make up the center or nucleus.

The Nucleus The center of an atom Small, dense (packed tightly) Contains protons and neutrons: most of the atom’s mass So dense, if it were the size of a grape, it would weigh over 9 million tons!

Protons Located in the nucleus Positive charge (+) Mass of 1 atomic mass unit (amu) protons = positive

Protons These are the elements identity so the periodic table is organized by them If you change number of protons in an atom you change the element you have.

How to Find Protons You use the atomic number on the periodic table. Atomic Number: The number of protons in an atom.

Neutrons Located in the nucleus Have no charge - neutral Mass of 1 amu neutrons = neutral

How to Find Neutrons… You need to do a little math. You are going to round the atomic mass (average number of protons and neutrons found in all atoms of that type).

How to Find Neutrons Continued… Once you rounded the atomic mass to a whole number you have created a mass number. Mass Number = The number of protons and neutrons found in a single atom.

How to Find Neutrons… Now do a little math Mass Number (p + n) Atomic Number (p) = Neutrons in that atom

Electrons Found outside of the nucleus in electron clouds (think of rings around a target) Negatively charged (-) Mass of almost zero Usually, the number of electrons equals the numbers of protons to cancel out the charges bob (think of -1+1=0) electrons = negative

Review: Parts of an Atom Charge Location Mass Proton Positive (+) Nucleus 1 amu Neutron No charge Electron Negative (-) Electron Clouds Almost zero

Valence Electrons Recall that an atom has core and valence electrons. Core electrons are found close to the nucleus. Valence electrons are found farthest away from the nucleus. It is valence electrons that are responsible for holding two or more atoms together in a chemical bond.

Valence Electrons The number of valence electrons corresponds to the group number of the representative elements. Lewis symbols are used to show the valence electrons (dots represent the valence electrons.)

When Atoms Combine Two or more different atoms held together by chemical bonds. Can be any bond type Ionic Covalent Hydrogen Compound Formed when atoms are joined together by covalent bonds. Molecule

Why do chemical bonds form? Chemical bonds form to allow atoms to lower their energy and become more stable. G. N. Lewis (1916) suggested that atoms become stable when they have a full valence shell of electrons (noble gas configuration). (The octet rule)

Octet Rule The octet rule states that atoms bond in such a way so that each atom acquires eight electrons in its outer shell. There are two ways in which an atom may achieve an octet. (a) By transfer of electrons from one atom to another (b) By sharing one or more pairs of electrons

How do chemical bonds form? In order to fill their valence shell, atoms donate, gain or share their valence electrons. Atoms with a few (1-3) valence electrons (mostly metals) tend to donate their valence electrons to become stable. Atoms with many (4-7) valence electrons (mostly nonmetals) tend to gain or share electrons to become stable.

How do ionic bonds form? Ionic bonding occurs when one atom gives and the other gains electrons.  Ionic bonding occurs because the atoms in the compound have opposite tendency to gain or lose electrons.  The elements will gain or lose electrons in an effort to acquire a full valence shell. Ionic bonding most commonly occurs when a metal and a non-metal bond together. 

Ionic Bonds An ionic bond is formed by the attraction between positively charged cations and negatively charged anions. This “electrostatic attraction” is similar to the attraction between opposite poles on two magnets.

Ionic Bonds The ionic bonds formed from the combination of anions and cations are very strong and result in the formation of a rigid, crystalline structure. The structure for NaCl, ordinary table salt, is shown here.

Ionic Bonding Atoms will either give up or take electrons to get to have eight in their highest energy level Na Cl

Hey, can you help me out and gimme an electron? Na Cl

Why certainly, I have one I don’t want anyway. Na Cl

Give up/take away electrons Na Cl

Charges Sodium now has a +1 charge since it has lost an electron Chlorine now has a -1 charge since it gained an electron - + Na Cl

Charges Sodium now has a +1 charge since it has lost an electron Chlorine a minus charge since it gained an electron Na Cl + -

Ionic Bond - NaCl 4Eva + Na Cl Opposites attract, so a bond is formed between the two of them. Na Cl + - (or until water breaks us apart) NaCl 4Eva

How do covalent bonds form? Covalent bonding occurs when two atoms share electrons.  Covalent bonding occurs because the atoms in the compound have a similar tendency to gain electrons.  The elements will share electrons in an effort to fill their valence shells. Covalent bonding most commonly occurs when two non-metals bond together. 

Covalent Bonds Covalent bonds are formed when two nonmetal atoms share electrons and the shared electrons in the covalent bond belong to both atoms. When hydrogen chloride, HCl, is formed, the hydrogen atom shares its one valence electron with the chlorine giving the chlorine atom eight electrons in its valence shell making it isoelectronic with argon. The chlorine atom shares one of its valence electrons with the hydrogen, giving it two electrons in its valence shell making it isoelectronic with helium.

Covalent Bonds Cl Cl Compounds formed by two non-metals More of a sharing of electrons rather than a give-take relationship Cl Cl

Can you spare an electron? Cl Cl

JINX! Cl Cl

Why don’t you share an electron? You know sharing is caring! Cl Cl

Valent=valence electrons… therefore, covalent is sharing electrons! Co=together Valent=valence electrons… therefore, covalent is sharing electrons! Shared pair of electrons = Covalent Bond Cl Cl

Covalent Bonds

2. Covalent bonds- Two atoms share one or more pairs of outer-shell electrons. Oxygen Atom Oxygen Atom Oxygen Molecule (O2)

- water is a polar molecule because oxygen is more electronegative than hydrogen, and therefore electrons are pulled closer to oxygen.

Polar Covalent Bonds Covalent bonds result from the sharing of valence electrons. Often, the two atoms do not share the electrons equally…One of the atoms holds on to the electrons more tightly than the other. When one of the atoms holds the shared electrons more tightly, the bond is polarized. A polar covalent bond is one in which the electrons are not shared equally.

Electronegativity Each element has an innate ability to attract valence electrons. Electronegativity is the ability of an atom to attract electrons in a chemical bond. Linus Pauling devised a method for measuring the electronegativity of each of the elements. Fluorine is the most electronegative element.

Electronegativity Electronegativity increases as you go left to right across a period. Electronegativity increases as you go from bottom to top in a family.

Electronegativity Differences The electronegativity of H is 2.1 and of Cl is 3.0. Since there is a difference in electronegativity between the two elements (3.0 – 2.1 = 0.9), the bond in H – Cl is polar. Since Cl is more electronegative, the bonding electrons are attracted toward the Cl atom and away from the H atom. This will give the Cl atom a slightly negative charge and the H atom a slightly positive charge.

Nonpolar Covalent Bonds What if two atoms in a covalent bond have the same or similar electronegativities? The bond is not polarized and it is a nonpolar covalent bond. If the electronegativity difference is less than 0.5, it is usually considered a nonpolar bond. The diatomic halogen molecules have nonpolar covalent bonds.

Water Chemistry All living organisms are dependent on water. The structure of water is the basis for its unique properties. The most important property of water is the ability to form hydrogen bonds.

Hydrogen bonding

Water Chemistry Within a water molecule, the bonds between oxygen and hydrogen are highly polar. Polar = Atoms pull on electrons unevenly. Partial electrical charges develop: - oxygen is partially negative - hydrogen is partially positive

Water Chemistry Hydrogen bonds are weak attractions between the partially negative oxygen of one water molecule and the partially positive hydrogen of a different water molecule. Hydrogen bonds can form between water molecules or between water and another charged molecule. These Bonds NOT Bonds Between Atoms These are bonds between Separate Molecules

Water Chemistry

Water Chemistry The polarity of water causes it to be cohesive and adhesive. cohesion: water molecules stick to other water molecules by hydrogen bonding adhesion: water molecules stick to other polar molecules by hydrogen bonding

Water Chemistry

Surface tension

Properties of Water 1. Water has a high specific heat. - A large amount of energy is required to change the temperature of water. 2. Water has a high heat of vaporization. - The evaporation of water from a surface causes cooling of that surface.

Properties of Water 3. Solid water is less dense than liquid water. - Bodies of water freeze from the top down. 4. Water is a good solvent. - Water dissolves polar molecules and ions.

Properties of Water

Properties of Water 5. Water organizes nonpolar molecules. - hydrophilic: “water-loving” -hydrophobic: “water-fearing” - Water causes hydrophobic molecules to aggregate or assume specific shapes. 6. Water can form ions. H2O  OH-1 + H+1 hydroxide ion hydrogen ion