4 - Atoms & Elements Flashcards
(78 cards)
How many elements are in nature?
91
Who were the first people to believe that matter was composed of tiny particles?
Democritus and Leucippus
Who formed the atomic theory?
John Dalton
What three points does the atomic theory state?
1) All atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements
2) Each element is composed of tiny, indestructible particles called atoms
3) Atoms combine themselves in simple, whole-number ratios to form compounds
Describe the characteristics of electrons
Negative charge, smaller and lighter than atoms, uniformly present in a variety o substances
Who discovered the electron?
J.J. Thomson
Describe the plum pudding model created by J.J. Thomson
The negatively charged electrons were small particles held within a positively charged sphere.
Who tried to confirm J.J. Thomson’s plum pudding model but actually disproved it?
Ernest Rutherford
What charge do atoms have? Why?
Neutral charge; the number of protons is always equal to the number of electrons, so the charges cancel out
Explain Rutherford’s experiment
He fired alpha particles (that had a positive charge) at an ultrathin sheet of gold foil, expecting the particles to pass right through the foil
What did the plum pudding model suggest?
The positive charge and negatively charged electrons were evenly dispersed throughout the atom.
How did Rutherford’s experiment disprove the plum pudding model?
Some alpha particles passed through while some were deflected. Since they were deflected, that means there was a positive charge concentrated in tiny spots throughout the foil, disproving Thomson since the charge was in fact, not evenly spread out.
Who developed the nuclear theory of the atom?
Ernest Rutherford
What 3 points does the nuclear theory state?
1) Most of the atom’s mass and all of its positive charge are contained in a small core called the nucleus
2) Most of the volume of the atom is empty space through which the tiny, negatively charged electrons are dispersed
3) The number of protons and electrons are equal, so the atom is charge-neutral
What are the atomic nuclei made of?
Protons and neutrons
Where is most of the atom’s mass located?
The nucleus
Which two subatomic particles weigh almost the same?
Protons and neutrons
What is amu? How is it defined?
Atomic mass unit; it is defined as 1/12 of the mass of a carbon-12 atom
What is the charge of the proton? The electron?
Proton is positive (1+) and the electron is negative (1-)
What do positive and negative charges do?
They attract each other
What do positive and positive, or negative and negative charges do?
They repel each other
Why is matter usually charge-neutral?
The atoms have an equal number of protons and electrons, so the charges cancel, making matter charge-neutral
What is an element defined by?
The number of protons in its atoms
What is the atomic number?
The number of protons in the nucleus of an atom, represented by Z
