Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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 Shell Models Fig. 2-3 (top), p. 22 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. 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. Shell Models first shell 6 second shell carbon (C) 11 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 two shells 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. third shell 2 1 1 proton 1 electron hydrogen (H) sodium (Na) helium (He) 8 oxygen (O) 17 chlorine (Cl) 10 neon (Ne) 18 argon (Ar) Fig. 2-3 (a-c), p. 22 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 Ion Formation electron gain 17 Chlorine atom 17p+ 17e– charge: 0 Chloride ion 17 electron loss 11 11 17p+ 18e– charge: –1 Sodium atom 11p+ 11e– charge: 0 Sodium ion 11p+ 10e– charge: +1 Fig. 2-4, p. 23 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