Atoms and Molecules:The Chemical Basis of Life

Elements

– Substances that cannot be broken down into simpler substances by ordinary chemical reactions

– Each has a chemical symbol

Proton Neutron Electron

Hydrogen1 proton1 electron

Oxygen8 protons8 neutrons8 electrons

+ 0 –

Everything is made of matter Matter is made of atoms

The World of Elements

CC

Different kinds of atoms = elementsDifferent kinds of atoms = elementsDifferent kinds of atoms = elementsDifferent kinds of atoms = elements

HHOONN

PP SSNaNaKKMgMgCaCa

C,H,N,O,P,S

• The most abundant elements in organisms

Life requires ~25 chemical elements

• About 25 elements are essential for life– Four elements make up 96% of living matter:

• carbon (C) • hydrogen (H)

• oxygen (O) • nitrogen (N)

– Four elements make up most of remaining 4%: • phosphorus (P) • calcium (Ca)

• sulfur (S) • potassium (K)

AtomThe smallest portion of an

element that retains its chemical properties

Structure of the Atom

The periodic table is arranged by atomic number

Every element hasA fixed number of protons in the atomic

nucleus, known as the atomic number

Atomic mass – add up the number of protons and neutrons

The average atomic mass (atomic weight) of an atom:

– Takes into account the mass of each isotope and their relative abundance in nature.

– Expressed by in atomic mass units (amu)– Usually not a whole number since it is an average!

Contain the same number of protons and electrons, but the number of neutrons varies

Isotopes Two or more forms of atoms of the same element

• The chemical behavior of an atom is determined by the number and arrangement of its valence electrons

• When the valence shell is not full, the atom tends to lose, gain, or share electrons

Chemical Bonds

–Forces of attraction that hold atoms of a compound together–The two principal types are• Covalent bonds• Ionic bonds

Covalent Bonds

– Share electrons between atoms– Each atom has a filled valence shell– Bond can be single, double, or triple

Covalent bonds

Carbon can form up to 4 covalent bonds!

Covalent bonds can be Polar or Nonpolar

Polar Covalent Bonds – Electrons are not shared equally

Nonpolar Covalent Bonds

Electrons shared equallyNo areas of partial charge

Ions– Particle with one or more units of electrical charge– Results when an atom gains or loses electrons• Cations—positively charged ions• Anions—negatively charged ions

Ionic Bonds

– Formed due to attraction between a cation and an anion

– An ionic compound is a substance consisting of cations and anions bonded together

– An example of ionic bond is the attraction between sodium ions and chloride ions

Ionic bonding

Hydrogen bonds

– Tend to form between an atom with partial negative charge and a hydrogen atom covalently bonded to oxygen or nitrogen

– Readily formed and broken– While individually weak, hydrogen bonds are

strong when present in large numbers

Hydrogen bonding

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