Ingles 1

FIRST PRACTICE

People have known about elements like carbon and gold since ancient time. The elements couldn’t be changed using any chemical method. Each element has a unique number of protons.If you examine samples of iron and silver, you can’t tell how many protons the atoms have. However, you can tell the elements apart because they have different properties. You might notice there aremore similarities between iron and silver than between iron and oxygen. Could there be a way to organize the elements so you could tell at a glance which ones had similar properties?
DmitriMendeleev was the first scientist to create a periodic table of the elements similar to the one we use today. You can see Mendeleev’s original table (1869). This table showed that when the elements wereordered by increasing atomic weight, a pattern appeared where properties of the elements repeated periodically. This periodic table is a chart that groups the elements according to their similarproperties.
Why do you think Mendeleev made a periodic table? Many elements remained to be discovered in Mendeleev’s time. The periodic table helped predict the properties of new elements.
Compare themodern periodic table with Mendeleev’s table. What do you notice? Mendeleev’s table didn’t have very many elements, did it? He had question marks and spaces between elements, where he predictedundiscovered elements would fit.
Remember changing the number of protons changes the atomic number, which is the number of the element. When you look at the modern periodic table, do you see anyskipped atomic numbers that would be undiscovered elements? New elements today aren’t discovered. They are made. You can still use the periodic table to predict the properties of these new elements.
Theperiodic table helps predict some properties of the elements compared to each other. Atom size decreases as you move from left to right across the table and increases as you move down a column....