Textbook Ch.2: Atoms, Molecules, and Ions

• An element is composed of tiny particles called atoms. All atoms of a given element show the same chemical properties. Atoms of different elements show different properties • In an ordinary chemical reaction, atoms move from one substance to another, but no atom of any element disappears or is changed into an atom of another element • Compounds are formed when atoms of two or more elements combine. In a given compound, the relative numbers of atoms of each kind are definite and constant. In general, these relative numbers can be expressed as integers or simple fractions

the smallest particle of an element that can enter into a chemical reaction

negatively charged particles that are common to all atoms, and carry a unit negative charge (-1), and have a very small mass, roughly 1/2000 that of the lightest atom

has a mass nearly equal to that of an ordinary hydrogen atom. The proton carries a unit positive charge (+1), equal in magnitude to that of the electron (-1)

uncharged particles with a mass slightly greater than that of a proton

there is no detectable change in mass in an ordinary chemical reaction. If atoms are conserved in a reaction, the atomic mass will also be conserved

a compound always contains the same elements in the same proportions by mass. If the atom ratio of the elements in a compound is fixed, their proportions by mass must also be fixed

Applies in situations in which two elements form more than one compound The law states that in these compounds, the masses of one element that combine with a fixed mass of the second element are in a ratio of small whole numbers

• All the atoms of a particular element have the same number of protons in the nucleus • Given the symbol Z Z = number of protons

the number of protons = number of electrons

• Found by adding up the number of protons and neutrons in the nucleus • Given the symbol A A = number of protons + number of neutrons

all atoms of a given element have the same atomic number (Z), but they differ from one another in mass number (A)

atoms that contain the same number of protons but a different number of neutrons

A symbol that shows the composition of a nucleus • The atomic number (Z) appears as a subscript on the lower left of the symbol of the element • The mass number (A) is written as a superscript in the the upper left

when unstable isotopes decompose (decay)

• The result of radioactivity • The atomic number of the product nucleus differs from that of the reactant • Can be beta particles identical in their properties to electrons • Can be alpha particles, which are The particles Z=2, A=4 carrying a +2 charge • Can be gamma rays, which consist of high-energy radiation

an arrangement of elements, in order of increasing atomic number, in horizontal rows of such a length that elements with similar chemical properties fall directly beneath one another in vertical groups

the horizontal rows in the periodic table

the vertical columns in the periodic table

elements falling in Groups 1,2,13,14,15,16,17, and 18

elements falling in Groups 3-12

the metals in groups 13, 14, and 15

a special name for the elements in group 1 • Lithium (Li), sodium (Na), and potassium (K) in this group all react vigorously with water to produce hydrogen gas

a special name for the elements in group 2