Cellular Chemistry (1)

Unit 2

THE PERIODIC TABLE

THE PERIODIC TABLE

• A table of chemical elements.• Invented by Russian chemist: Dmitri

Mendeleev in 1869– To show the recurring periodic trends in the

properties of the elements.

THE PERIODIC TABLE

• Widely used in chemistry, biology, physics, engineering– to classify, systemize, and compare chemical

behavior• There have been many models. • The current standard form has 118 elements.

Start With Atoms

• All substances consist of atoms• An element is a substance that contains only

one type of atom

Start With Atoms

• Atoms– Are the fundamental building-block particle of

matter

• Life’s unique characteristics start with the properties of different atoms

Subatomic Particles and Their Charge

• Atoms differ in numbers of subatomic particles– Atoms consist of electrons moving around a

nucleus of protons and neutrons

Subatomic Particles and Their Charge

• Charge– Electrical property of some subatomic

particles– Opposite charges attract; like charges repel

• Electron (e-) – Negatively charged subatomic particle that

occupies orbitals around the atomic nucleus

Subatomic Particles in the Nucleus

• Nucleus– Core of an atom, occupied by protons and

neutrons

• Proton (p+)– Positively charged subatomic particle found in

the nucleus of all atoms

• Neutron– Uncharged subatomic particle found in the

atomic nucleus

Different Elements: Different Types of Atoms

• Element– A pure substance that consists only of atoms with

the same number of protons– The element oxygen contains only oxygen atoms

etc.

• Atomic number– Number of protons in the atomic nucleus– Determines the element

Elements in Living Things

• The proportions of different elements differ between living and nonliving things

• Some atoms, such as carbon, are found in greater proportions in molecules made only by living things – the molecules of life

Why Electrons Matter

• Electrons travel around the nucleus in different orbitals (shells) – atoms with vacancies in their outer shells tend to interact with other atoms– Atoms get rid of vacancies by gaining or

losing electrons, or sharing electrons with other atoms

• Shell model– Model of electron distribution in an atom

Fig. 2-3 (top), p. 22

Shell Models

Fig. 2-3 (a-c), p. 22

1 proton 1 21 electron

first shell hydrogen (H) helium (He)

A) The first shell corresponds to the first energy level, and it can hold up to 2 electrons. Hydrogen has one proton, so it has one vacancy. A helium atom has 2 protons, and no vacancies. The number of protons in each shell model is shown.

6 8 10

second shell carbon (C) oxygen (O) neon (Ne)

B) The second shell corresponds to the second energy level, and it can hold up to 8 electrons. Carbon has 6 protons, so its first shell is full. Its second shell has 4 electrons, and four vacancies.Oxygen has 8 protons and two vacancies. Neon has 10 protons and no vacancies.

11 17 18

third shell sodium (Na) chlorine (Cl) argon (Ar)

C) The third shell, which corresponds to the third energy level, can hold up to 8 electrons, for a total of 18. A sodium atom has 11 protons, so its first twoshells are full; the third shell has one electron. Thus, sodium has seven vacancies. Chlorine has 17 protons and one vacancy. Argon has 18 protons and no vacancies.

Shell Models

Ions

• The negative charge of an electron balances the positive charge of a proton in the nucleus

• Changing the number of electrons may fill its outer shell, but changes the charge of the atom

• Ion– Atom that carries a charge because it has an

unequal number of protons and electrons

Fig. 2-4, p. 23

Sodium atom

1111p+

11e–

charge: 0

electron loss

Sodium ion

11p+11

charge: +110e–

Chlorine atom

17 17p+

17e–

charge: 0

electron gain

Chloride ion

18e–

17 17p+

charge: –1

Ion Formation

From Atoms to Molecules

• Atoms can also fill their vacancies by sharing electrons with other atoms

• A chemical bond forms when the electrons of two atoms interact

• Chemical bond– An attractive force that arises between two

atoms when their electrons interact

From Atoms to Molecules

• Molecule– Group of two or more atoms joined by chemical

bonds

• Compound– Type of molecule that has atoms of more than one

element

• All compounds are molecules; but not all molecules are compounds!

Referring to a Molecule

Ionic Bonds and Covalent Bonds

• Depending on the atoms, a chemical bond may be ionic or covalent

• Ionic bond– A strong mutual attraction formed between

ions of opposite charge

• Covalent bond– Two atoms sharing a pair of electrons

An Ionic Bond: Sodium Chloride

Covalent Bonds

• Molecular hydrogen (H—H) and molecular oxygen (O=O)

Hydrogen Bonds

• Hydrogen bond– Attraction that forms between a covalently

bonded hydrogen atom and another atom taking part in a separate covalent bond

Importance of Hydrogen Bonds

• Hydrogen bonds form and break more easily than covalent or ionic bonds – they do not form molecules

• Hydrogen bonds impart unique properties to substances such as water, and hold molecules such as DNA in their characteristic shapes