How can you calculate the mass of an atom?

Answer 1

As you have probably learned, an atom is a very, very small object.

However, we can easily handle a very

large group of atoms and weigh that. If we have an idea of how to

figure out how many atoms are in that particular group, we can

simply divide the mass of the group into the individual atoms in

that group. For example, let us say for the moment that we know that

there are 1000 atoms of carbon for every 1g of charcoal, if I get

the mass of a piece of charcoal (which is pure carbon) and it turns

out to be 2g in mass, then I know that there are 2000 atoms of carbon in there.

The tricky part is knowing how many atoms of carbon are in 1g of

charcoal. Here is the problem, as you already noted, we can say that

1 oxygen atom is equivalent in mass to 16 hydrogen atoms, this would

be good if we knew what the mass was of either oxygen or hydrogen.

Let us say, we did not, but we knew that 12 atoms of hydrogen were

equivalent to the mass of 1 carbon atom, then if we knew the mass of

carbon, then we could know the mass of hydrogen and oxygen. But let

us say we did not know the actual mass of any atom, what then? All

we have are relative masses, the mass of an atom in relation to

another atom. What we need is a starting point.

Scientists simply agreed that 12g of carbon atoms would contain a

"mole" of carbon atoms. At this point, it does not matter how many

atoms are in a mole, let us say for the moment that it turns out

that there are 12 atoms in a mole (a mole in this case is just like

"dozen") then each atom of carbon must have a mass of 1g (12g

divided by 12atoms). Then we can say that the mass of a hydrogen is

1/12g (since 1 carbon is to 12 hydrogen) and the mass of oxygen is

16/12g (or 4/3g, since 16 hydrogens is 1 oxygen). The point here is

that once we agree on a grouping of atoms (the mole) and agree on

the mass of that grouping for one of the atoms, then we would know

the mass of all the other atoms. Once we have our starting point, we

know all the relative masses.

Answer 2

3 Ways To Find Atomic Mass

1) Look Up Atomic Mass on the Periodic Table

If it's your first encounter with chemistry, your instructor will want you to learn how to use the Periodic Table to find the Atomic Mass of an element. This number usually is given below an element's symbol. Look for the decimal number, which is a weighted average of all the natural isotopes of an element.

Example: If you are asked to give the atomic mass of carbon, you first need to know its element symbol, C. Look for C on the periodic table.

One number is carbons element number or atomic number. Atomic number increase as you go across the table. This is not the value you want. The atomic mass or atomic weight is the decimal number, The number of significant figures varies according to the table, but the value is around 12.01.

This value on the periodic table is given in atomic mass units or amu, but for chemistry calculations you usually write atomic mass in terms of grams per mole or g/mol. The atomic mass of carbon would be 12.01 grams per mole of carbon atoms.

2) Sum of Protons and Neutrons for a Single Atom

To calculate the atomic mass of a single atom of an element, add up the mass of protons and neutrons.

Example: Find the atomic mass of an isotope of carbon that has 7 neutrons. You can see from the periodic table that carbon has an atomic number of 6, which is its number of protons. The atomic mass of the atom is the mass of the protons plus the mass of the neutrons, 6 + 7, or 13.

3) Weighted Average for All Atoms of an Element

The atomic mass of an element is a weighted average of all the element's isotopes based on their natural abundance. It is simple to calculate the atomic mass of an element with these steps.

Typically, in these problems, you are provided with a list of isotopes with their mass and their natural abundance either as a decimal or percent value.

1. Multiply each isotope's mass by its abundance. If your abundance is a percent, divide your answer by 100.
2. Add these values together.

The answer is the total atomic mass or atomic weight of the element.

Example: You are given a sample containing 98% Carbon 12, and 2% Carbon 13. What is the relative atomic mass of the element?

First convert the percentages to decimal values by dividing each percentage by 100. The sample becomes 0.98 carbon-12 and 0.02 carbon-13. (Tip: You can check your math by making certain the decimals add up to 1. 0.98 + 0.02 = 1.00)

Next, multiply the atomic mass of each isotope by the proportion of the element in the sample:

0.98 x 12 = 11.76

0.02 x 13 = 0.26

For the final answer, add these together:

11.76 + 0.26 = 12.02 g/mol

Advanced Note: This atomic mass is slightly higher than the value given in the periodic table for the element Carbon. What does this tell you? The sample you were given to analyze contained more carbon-13 than average. You know this because your relative atomic mass is higher than the periodic table value, even though the periodic table number includes heavier isotopes, such as carbon-14. Also, note the numbers given on the periodic table apply to the Earth's crust/atmosphere and may have little bearing on the expected isotope ratio in the mantle or core or on other worlds.

-All credit goes to chemistry.about.com

Answer 3

number of protons and neutrons in the atoms nueclus number of protons and neutrons in the atoms nueclus

Answer 4

mass number= no. of protons + no. of neutron

Answer 5

Atomic mass the the mass of the protons plus the mass of the neutrons.

Answer 6

Protons and neutrons= mass