Atoms, bonds and reacting amounts

Identify what stays with an element

Read an atom's identity from its number of protons. The atomic number (numero atomico) counts these positive charges in the nucleus. A neutral atom has the same number of electrons outside the nucleus. The mass number counts protons and neutrons together, so subtract the atomic number to find the neutron count. In the notation $^{23}_{11}\mathrm{Na}$ the lower number identifies sodium and the upper number gives the total of 23 nucleons. This nucleus contains 11 protons and 12 neutrons.

An isotope has the same number of protons as another atom of that element but a different number of neutrons. An ion has gained or lost electrons. Removing one electron from neutral sodium gives a positive ion with ten electrons, while its nucleus remains the same. This accounting is valuable when two different ions have the same electron count. Each is still identified by the charge of its nucleus. The decimal atomic mass on a periodic table can be a weighted average over isotopes. Use the integer mass number of the specified isotope for a neutron count.

Translate an outer shell into a formula

For simple main-group atoms, the outer-shell electrons help explain chemical behaviour. Elements in the same group often have similar outer-electron arrangements and similar bonding patterns. Sodium has one outer electron, whereas chlorine has seven. Transfer of an electron can produce a positive sodium ion and a negative chloride ion. Their electrical attraction acts throughout an ionic lattice. The written formula gives the smallest whole-number ratio of ions consistent with electrical neutrality.

Consider magnesium ions carrying two positive charges each and chloride ions carrying one negative charge each. Two chloride ions balance one magnesium ion. This gives $\mathrm{MgCl_2}$. The subscript next to chlorine describes the composition of the compound. Altering it changes the formula. Covalent bonding instead involves a shared pair of electrons between atoms. A molecule of hydrogen contains two hydrogen atoms sharing one pair. Counting atoms in one formula unit or molecule is a separate operation from counting how many such units participate in a reaction.

Balance atoms before comparing supplies

A chemical equation records the rearrangement of atoms. Change coefficients in front of formulas to balance it. Each coefficient multiplies every atom in its formula, including any subscripts. In $2\mathrm{H_2}+\mathrm{O_2}\rightarrow2\mathrm{H_2O}$, four hydrogen atoms and two oxygen atoms appear on each side. The coefficients give the ratio of molecules. They also give the ratio of moles because each mole contains the same number of specified particles. Grams measure mass, so taking a coefficient ratio directly between masses usually gives the wrong prediction.

Convert a sample's mass to amount by dividing by molar mass in grams per mole. Build a compound's molar mass by multiplying each element's molar mass by its subscript and adding. If you have a balanced equation and the amount of each reactant, divide each available amount by its coefficient. Think of each quotient as the possible number of complete reaction batches measured in moles of batches. The smallest quotient gives the limiting reactant (reagente limitante). When the reaction runs to completion, that supply runs out first.

Multiply the smallest quotient by a product coefficient to obtain the amount of that product. Multiply again by its molar mass only if the question asks for grams. For an excess reactant, subtract the amount consumed from its starting amount. You can check your result by accounting for the atoms on each side or by adding the mass left over to the product mass. If this accounting includes all products and reactants in a closed system, the final mass must equal the starting mass. When comparing two candidate answers, ask whether the larger one would require more of any reactant than you actually possess.